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Machining a bar of metal on a lathe.

Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewellery. It therefore includes a correspondingly wide range of skills, processes, and tools.

Metalworking is a science, art, hobby, industry and trade. Its historical roots span cultures, civilizations, and millennia. Metalworking has evolved from the discovery of smelting various ores, producing malleable and ductile metal useful for tools and adornments. Modern metalworking processes, though diverse and specialized, can be categorized as forming, cutting or joining processes. Today's machine shop includes a number of machine tools capable of creating a precise, useful workpiece.

Contents

Prehistory

Metalworking predates history. No one knows with any certainty where or when metalworking began. The earliest technologies were impermanent to say the least and were unlikely to leave any evidence for long. The advance that brought metal into focus was the connection of fire and metals. Who accomplished this is as unknown as the when and where, but the Egyptians are thought to have been one of the first civilizations to work gold.

Not all metal required fire to obtain it or work it. Isaac Asimov speculated that gold was the "first metal."[1] His reasoning is that gold by its chemistry is found in nature as nuggets of pure gold. In other words, gold, as rare as it is, is always found in nature as the metal that it is. There are a few other metals that sometimes occur natively, and as a result of meteors. Almost all other metals are found in ores, a mineral bearing rock, that require heat or some other process to liberate the metal. Another feature of gold is that it is workable as it is found, meaning that no technology beyond eyes to find a nugget and a hammer and an anvil to work the metal is needed. Stone hammer and stone anvil will suffice for technology. This is the result of gold's properties of malleability and ductility. The earliest tools were stone, bone, wood, and sinew. They sufficed to work gold.

At some unknown point the connection between heat and the liberation of metals from rock became clear, rocks rich in copper, tin, and lead came into demand. These ores were mined wherever they were recognized. Remnants of such ancient mines have been found all over what is today the Middle East.[2] Metalworking was being carried out by the South Asian inhabitants of Mehrgarh between 7000–3300 BCE.[3] The end of the beginning of metalworking occurs sometime around 6000 BCE when copper smelting became common in the Middle East.

The ancients knew of seven metals. Here they are arranged in order of their oxidation potential:

  • Iron +0.44,
  • Tin +0.14
  • Lead +0.13
  • Copper -0.34
  • Mercury -0.79
  • Silver -0.80
  • Gold -1.50

The oxidation potential is important because it is one indicator of how tightly bound to the ore the metal is likely to be. As can be seen, iron is significantly higher than the other six metals while gold is dramatically lower than the six above it. Gold's low oxidation is one of the main reasons that gold is found in nuggets. These nuggets are relatively pure gold and are workable as they are found.

Copper ore, being relatively abundant, and tin ore became the next important players in the story of metalworking. Using heat to smelt copper from ore, a great deal of copper was produced. It was used for both jewelry and simple tools. However, copper by itself was too soft for tools requiring edges and stiffness. At some point tin was added into the molten copper and bronze was born. Bronze is an alloy of copper and tin. Bronze was an important advance because it had the edge-durability and stiffness that pure copper lacked. Until the advent of iron, bronze was the most advanced metal for tools and weapons in common use (see Bronze Age for more detail).

Looking beyond the Middle East, these same advances and materials were being discovered and used the world around. China and Britain jumped into the use of bronze with little time being devoted to copper. Japan began the use of bronze and iron almost simultaneously. In the Americas things were different. Although the peoples of the Americas knew of metals, it wasn't until the arrival of Europeans that metal for tools and weapons took off. Jewelry and art were the principal uses of metals in the Americas prior to European influence.

Around the date 2700 BCE, production of bronze was common in locales where the necessary materials could be assembled for smelting, heating, and working the metal. Iron was beginning to be smelted. Iron began its emergence as an important metal for tools and weapons. The Iron Age was dawning.

History

A turret lathe operator machining parts for transport planes at the Consolidated Aircraft Corporation plant, Fort Worth, Texas, USA in the 1940s.

By the historical periods of the Pharaohs in Egypt, the Vedic Kings in India, the Tribes of Israel, and the Mayan Civilization in North America, among other ancient populations, precious metals began to have value attached to them. In some cases rules for ownership, distribution, and trade were created, enforced, and agreed upon by the respective peoples. By the above periods metalworkers were very skilled at creating objects of adornment, religious artifacts, and trade instruments of precious metals (non-ferrous), as well as weaponry usually of ferrous metals and/or alloys. These skills were finely honed and well executed. The techniques were practiced by artisans, blacksmiths, atharvavedic practitioners, alchemists, and other categories of metalworkers around the globe. For example, the ancient technique of granulation is found around the world in numerous ancient cultures before the historic record shows people traveled seas or overland to far regions of the earth to share this process that still being used by metalsmiths today.

As time progressed metal objects became more common, and ever more complex. The need to further acquire and work metals grew in importance. Skills related to extracting metal ores from the earth began to evolve, and metalsmiths became more knowledgeable. Metalsmiths became important members of society. Fates and economies of entire civilizations were greatly affected by the availability of metals and metalsmiths. The metalworker depends on the extraction of precious metals to make jewellery, build more efficient electronics, and for industrial and technological applications from construction to shipping containers to rail, and air transport. Without metals, goods and services would cease to move around the globe on the scale we know today.

More individuals than ever before are learning metalworking as a creative outlet in the forms of jewellery making, hobby restoration of aircraft and cars, blacksmithing, tinsmithing, tinkering, and in other art and craft pursuits. Trade schools continue to teach welding in all of its forms, and there is a proliferation of schools of Lapidary and Jewelers arts and sciences at this- the beginning of the 21st Century AD.

General metalworking processes

A combination square used for transferring designs.
A caliper is used to precisely measure a short length.

Metalworking generally is divided into the following categories, forming, cutting, and, joining. Each of these categories contain various processes.

Compatibility chart of materials versus processes[4]
Material
Process Iron Steel Aluminium Copper Magnesium Nickel Refractory metals Titanium Zinc
Sand casting X X X X X X 0
Permanent mold casting X 0 X 0 X 0 0
Die casting X 0 X X
Investment casting X X X 0 0
Closed-die forging X 0 0 0 0 0 0
Extrusion 0 X X X 0 0 0
Cold heading X X X 0
Stamping & deep drawing X X X 0 X 0 0
Screw machine 0 X X X 0 X 0 0 0
Powder metallurgy X X 0 X 0 X 0
Key: X = Routinely performed, 0 = Performed with difficulty, caution, or some sacrifice, blank = Not recommended

Prior to most operations, the metal must be marked out and/or measured, depending on the desired finished product.

Marking out (also known as layout) is the process of transferring a design or pattern to a workpiece and is the first step in the handcraft of metalworking. It is performed in many industries or hobbies, although in the repetition industries the need to mark out every individual piece is eliminated. In the metal trades area, marking out consists of transferring the engineer's plan to the workpiece in preparation for the next step, machining or manufacture.

Calipers are handtools designed to precisely measure the distance between two points. Most calipers have two sets of flat, perpendicular edges used for inner or outer diameter. These calipers can be accurate to within one-thousandth of an inch (25.4μm). Different types of calipers have different mechanisms for displaying the distance measured. Where larger objects need to be measured with less precision, a tape measure is often used.

Forming processes

These forming processes modify metal or workpiece by deforming the object, that is, without removing any material. Forming is done with heat and pressure, or with mechanical force, or both.

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Casting

A sand casting mold

Casting achieves a specific form by pouring molten metal into a mold and allowing it to cool, with no mechanical force. Forms of casting include:

Plastic deforming

A red-hot metal workpiece is inserted into a forging press.

Plastic deformation involves using heat or pressure to make a workpiece more conductive to mechanical force. Historically, this and casting were done by blacksmiths, though today the process has been industrialized.

Sheet metal forming

A metal spun brass vase

These types of forming process involve the application of mechanical force at room temperature.

Cutting processes

A CNC plasma cutting machining

Cutting is a collection of processes wherein material is brought to a specified geometry by removing excess material using various kinds of tooling to leave a finished part that meets specifications. The net result of cutting is two products, the waste or excess material, and the finished part. If this were a discussion of woodworking, the waste would be sawdust and excess wood. In cutting metals the waste is chips or swarf and excess metal. These processes can be divided into chip producing cutting, generally known as machining. Burning or cutting with an oxyfuel torch is a welding process not machining. There are also miscellaneous specialty processes such as chemical milling.

Cutting is nearly fully represented by:

  • Chip producing processes most commonly known as machining
  • Burning, a set of processes which cut by oxidizing a kerf to separate pieces of metal
  • Specialty processes

Drilling a hole in a metal part is the most common example of a chip producing process. Using an oxy-fuel cutting torch to separate a plate of steel into smaller pieces is an example of burning. Chemical milling is an example of a specialty process that removes excess material by the use of etching chemicals and masking chemicals.

There are many technologies available to cut metal, including:

Cutting fluid or coolant is used where there is significant friction and heat at the cutting interface between a cutter such as a drill or an end mill and the workpiece. Coolant is generally introduced by a spray across the face of the tool and workpiece to decrease friction and temperature at the cutting tool/workpiece interface to prevent excessive tool wear. In practice there are many methods of delivering coolant.

Machining

A milling machine

Milling is the complex shaping of metal or other materials by removing material to form the final shape. It is generally done on a milling machine, a power-driven machine that in its basic form consists of a milling cutter that rotates about the spindle axis (like a drill), and a worktable that can move in multiple directions (usually two dimensions [x and y axis] relative to the workpiece. The spindle usually moves in the z axis. It is possible to raise the table (where the workpiece rests). Milling machines may be operated manually or under computer numerical control (CNC), and can perform a vast number of complex operations, such as slot cutting, planing, drilling and threading, rabbeting, routing, etc. Two common types of mills are the horizontal mill and vertical mill.

The pieces produced are usually complex 3D objects that are converted into x, y, and z coordinates that are then fed into the CNC machine and allow it to complete the tasks required. The milling machine can produce most parts in 3D, but some require the objects to be rotated around the x, y, or z coordinate axis (depending on the need). Tolerances are usually in the thousandths of an inch (Unit known as Thou), depending on the specific machine.

In order to keep both the bit and material cool, a high temperature coolant is used. In most cases the coolant is sprayed from a hose directly onto the bit and material. This coolant can either be machine or user controlled, depending on the machine.

Materials that can be milled range from aluminium to stainless steel and most everything in between. Each material requires a different speed on the milling tool and varies in the amount of material that can be removed in one pass of the tool. Harder materials are usually milled at slower speeds with small amounts of material removed. Softer materials vary, but usually are milled with a high bit speed.

The use of a milling machine adds costs that are factored into the manufacturing process. Each time the machine is used coolant is also used, which must be periodically added in order to prevent breaking bits. A milling bit must also be changed as needed in order to prevent damage to the material. Time is the biggest factor for costs. Complex parts can require hours to complete, while very simple parts take only minutes. This in turn varies the production time as well, as each part will require different amounts of time.

Safety is key with these machines. The bits are traveling at high speeds and removing pieces of usually scalding hot metal. The advantage of having a CNC milling machine is that it protects the machine operator.

Turning

A lathe cutting material from a workpiece.

Turning is a metalcutting process for producing a cylindrical surface with a single point tool. The workpiece is rotated on a spindle and the cutting tool is fed into it radially, axially or both. Producing surfaces perpendicular to the workpiece axis is called facing. Producing surfaces using both radial and axial feeds is called profiling.[5]

A lathe is a machine tool which spins a block or cylinder of material so that when abrasive, cutting, or deformation tools are applied to the workpiece, it can be shaped to produce an object which has rotational symmetry about an axis of rotation. Examples of objects that can be produced on a lathe include candlestick holders, table legs, bowls, baseball bats, crankshafts, camshafts, and bearing mounts.

Lathes have three main components: the headstock, the carriage, and the tailstock. The headstock's spindle secures the workpiece with a chuck, whose jaws (usually three or four) are tightened around the piece. The spindle rotates at high speed, providing the energy to cut the material. While historic lathes were powered by belts from the ceiling, modern examples uses electric motors. The workpiece extends out of the spindle along the axis of rotation above the flat bed. The carriage is platform that can be moved horizontally along and perpendicular to the axis of rotation. A hardened cutting tool is held at the desired height (usually the middle of the workpiece) by the toolpost. The carriage is then moved around the rotating workpiece, and the cutting tool gradually shaves material from the workpiece. The tailstock can be sided along the axis of rotation and then locked in place as necessary. It may hold centers to further secure the workpiece, or cutting tools driven into the end of the workpiece, .

Other operation that can be performed with a single point tool on a lathe are[6]:

Chamfering: Cutting an angle on the comer of a cylinder.
Parting: The tool is fed radially into the workpiece to cut off the end of a part.
Threading: A tool is fed along and across the outside or inside surface of rotating parts to produce external or internal threads.
Boring: A single-point tool is fed linearly and parallel to the axis of rotation.
Drilling: Feeding the drill into the workpiece axially.
Knurling: Produces a regular cross-hatched pattern in work surfaces.

Modern computer numerical control (CNC) lathes can do secondary operations like milling by using driven tools. When driven tools are used the work piece stops rotating and the driven tool executes the machining operation with a rotating cutting tool. The CNC machines use x, y, and z coordinates in order to control the turning tools and produce the product. Most modern day CNC lathes are able to produce most turned objects in 3D.

Materials appropriate for turning used are softer metals, although harder metals can be turned with a bit more time and effort.

The turning tool material must be harder than the material being turned in order for the process to work. Production rates for this process depend on the object being turned and the speed at which it can be done. More complex materials, therefore, will take more time.

Threading

Three different types and sizes of taps.

There are many threading processes including: cutting threads with a tap or die, thread milling, single-point thread cutting, thread rolling and forming, and thread grinding. A tap is used to cut a female thread on the inside surface of a pre-drilled hole, while a die cuts a male thread on a preformed cylindrical rod.

Grinding

A surface grinder

Grinding uses an abrasive process to remove material from the workpiece. A grinding machine is a machine tool used for producing very fine finishes, making very light cuts, or high precision forms using a abrasive wheel as the cutting device. This wheel can be made up of various sizes and types of stones, diamonds or inorganic materials.

The simplest grinder is a bench grinder or a hand-held angle grinder, for deburring parts or cutting metal with a zip-disc.

Grinders have increased in size and complexity with advances in time and technology. From the old days of a manual toolroom grinder sharpening endmills for a production shop, to today's 30000 RPM CNC auto-loading manufacturing cell producing jet turbines, grinding processes vary greatly.

Grinders need to be very rigid machines to produce the required finish. Some grinders are even used to produce glass scales for positioning CNC machine axis. The common rule is the machines used to produce scales be 10 times more accurate than the machines the parts are produced for.

In the past grinders were used for finishing operations only because of limitations of tooling. Modern grinding wheel materials and the use of industrial diamonds or other man-made coatings (cubic boron nitride) on wheel forms have allowed grinders to achieve excellent results in production environments instead of being relegated to the back of the shop.

Modern technology has advanced grinding operations to include CNC controls, high material removal rates with high precision, lending itself well to aerospace applications and high volume production runs of precision components.

Filing

A file is an abrasive surface like this one that allows machinists to remove small, imprecise amounts of metal.

Filing is combination of grinding and saw tooth cutting using a file. Prior to the development of modern machining equipment it provided a relatively accurate means for the production of small parts, especially those with flat surfaces. The skilled use of a file allowed a machinist to work to fine tolerances and was the hallmark of the craft. Today filing is rarely used as a production technique in industry, though it remains as a common method of deburring.

Other

Broaching is a machining operation used to cut keyways into shafts. Electron beam machining (EBM) is a machining process where high-velocity electrons are directed toward a work piece, creating heat and vaporizing the material. Ultrasonic machining uses ultrasonic vibrations to machine very hard or brittle materials.

Joining processes

Mig welding

Welding

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, but sometimes pressure is used in conjunction with heat, or by itself, to produce the weld.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, underwater and in space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, poisonous fumes, and overexposure to ultraviolet light.

Brazing

Brazing is a joining process in which a filler metal is melted and drawn into a capillary formed by the assembly of two or more work pieces. The filler metal reacts metallurgically with the workpiece(s) and solidifies in the capillary, forming a strong joint. Unlike welding, the work piece is not melted. Brazing is similar to soldering, but occurs at temperatures in excess of 450 °C (842 °F). Brazing has the advantage of producing less thermal stresses than welding, and brazed assemblies tend to be more ductile than weldments because alloying elements can not segregate and precipitate.

Brazing techniques include, flame brazing, resistance brazing, furnace brazing, diffusion brazing, and inductive brazing.

Soldering

Soldering a printed circuit board.

Soldering is a joining process that occurs at temperatures below 450 °C (842 °F). It is similar to brazing in the fact that a filler is melted and drawn into a capillary to form a join, although at a lower temperature. Because of this lower temperature and different alloys used as fillers, the metallurgical reaction between filler and work piece is minimal, resulting in a weaker joint.

Associated processes

While these processes are not primary metalworking processes, they are often performed before or after metalworking processes.

Heat treatment

Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. Common heat treatment processes include annealing, precipitation strengthening, quenching, and tempering. The annealing process softens the metal by allowing recovery of cold work and grain growth. Quenching can be used to harden alloy steels, or in precipitation hardenable alloys, to trap dissolved solute atoms in solution. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.

Often, mechanical and thermal treatments are combined in what is known as thermo-mechanical treatments for better properties and more efficient processing of materials. These processes are common to high alloy special steels, super alloys and titanium alloys.

Plating

Electroplating is a common surface-treatment technique. It involves bonding a thin layer of another metal such as gold, silver, chromium or zinc to the surface of the product. It is used to reduce corrosion as well as to improve the product's aesthetic appearance.

Thermal spraying

Thermal spraying techniques are another popular finishing option, and often have better high temperature properties than electroplated coatings.

See also

References

  1. ^ Asimov, Isaac: "The Solar System and Back", page 151 ff. Doubleday and Company,Inc. 1969.
  2. ^ Percy Knauth et al.: "The Emergence of Man, The Metalsmiths", page 10-11 ff. Time-Life Books, 1974.
  3. ^ Possehl, Gregory L. (1996).
  4. ^ Degarmo, p. 183.
  5. ^ Schneider, George. Turning Tools and Operations, American Machinist, January 2010
  6. ^ Schneider, George. Turning Tools and Operations, American Machinist, January 2010

Bibliography

  • Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4 .

Further reading

  • Possehl, Gregory L. (1996). Mehrgarh in Oxford Companion to Archaeology, edited by Brian Fagan. Oxford University Press.

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

METAL-WORK. Among the many stages in the development of primeval man, none can have been of greater moment in his struggle for existence than the discovery of the metals, and the means of working them. The names generally given to the three prehistoric periods of man's life on the earth - the Stone, the Bronze and the Iron age - imply the vast importance of the progressive steps from the flint knife to the bronze celt, and lastly to the keen-edged elastic iron weapon or tool.

The metals chiefly used in the arts have been gold, silver, copper and tin (the last two generally mixed, forming an alloy called bronze), iron and lead (see the separate articles on these metals). Their peculiarities have naturally marked out each of them for special uses and methods of treatment. The durability and the extraordinary ductility and pliancy of gold, its power of being subdivided, drawn out or flattened into wire or leaf of almost infinite fineness, have led to its being used for works where great minuteness and delicacy of execution were required; while its beauty and rarity have, for the most part, limited its use to objects of adornment and luxury, as distinct from those of utility. In a lesser degree most of the qualities of gold are shared by silver, and consequently the treatment of these two metals has always been very similar, though the greater abundance of the latter metal has allowed it to be used on a larger scale and for a greater variety of purposes. The great fluidity of bronze when melted, the slightness of its contraction on solidifying, together with its density and hardness, make it especially suitable for casting, and allow of its taking the impress of the mould with extreme sharpness and delicacy. In the form of plate it can be tempered and annealed till its elasticity and toughness are much increased, and it can then be formed into almost any shape under the hammer and punch. By other methods of treatment, known to the ancient Egyptians, Greeks and others, but now forgotten, it could be hardened and formed into knife and razor edges of the utmost keenness. In many specimens of ancient bronze, small quantities of silver, lead and zinc have been found, but their presence is probably accidental. In modern times brass has been much used, chiefly for the sake of its cheapness as compared with bronze. In beauty, durability and delicacy of surface it is very inferior to bronze, and, though of some commercial importance, has been of but little use in the production of works of art. To some extent copper was used in an almost pure state during medieval times, especially from the 12th to the 15th century, mainly for objects of ecclesiastical use, such as pyxes, monstrances, reliquaries and croziers, partly on account of its sof tness under the tool, and also because it was slightly easier to apply enamel and gilding to pure copper than to bronze (see fig. I). In the medieval period it was used to some extent in the shape of thin sheeting for roofs, as at St Mark's, Venice; while during the 16th and 17th centuries it was largely employed for ornamental domestic vessels of various sorts.

Table of contents

Iron.

The abundance in which iron is found in so many places, its great strength, its remarkable ductility and malleability in a red-hot state, and the ease with which two heated surfaces of iron can be welded together under the hammer combine to make it specially suitable for works on a large scale where strength with lightness are required - things such as screens, window-grills, ornamental hinges and the like. In its hot plastic state iron can be formed and modelled under the hammer to almost any degree of refinement, while its great strength allows it to be beaten out into leaves and ornaments of almost paperlike thinness and delicacy. With repeated hammering, drawing out and annealing, it gains much in strength and toughness, and the addition of a very minute quantity of carbon converts it into steel, less tough, but of the keenest hardness. The large employment of cast iron is comparatively modern, in England at least only dating from the i 6th century; it is not, however, incapable of artistic treatment if due regard be paid to the necessities of casting, and if no attempt is made to imitate the fine-drawn lightness to which wrought iron so readily lends itself. At the best, however, it is not generally suited for the finest work, as the great contraction of iron in passing from the fluid to the solid state renders the cast somewhat blunt and spiritless.

Among the Assyrians, Egyptians and Greeks the use of iron, either cast or wrought, was very limited, bronze being the favourite metal almost for all purposes. The difficulty of smelting the ore was probably one reason for this, as well as the now forgotten skill which enabled bronze to be tempered to a steel-like edge. It had, however, its value, of which a proof occurs in Homer (Il. xxiii.), where a mass of iron is mentioned as being one of the prizes at the funeral games of Patroclus.

Methods of Manipulation in Metal-Work

Gold, silver and bronze may be treated in various ways, the chief of which are (1) casting in a mould,' e and (2) treatment by hammering and punching (Fr. repousse). The first of these, casting is chiefly adapted for bronze, or ' Analyses of the iron of prehistoric weapons have brought to light the interesting fact that many of these earliest specimens of iron manufacture contain a considerable percentage of nickel. This special alloy does not occur in any known iron ores, but is invariably found in meteoric iron. It thus appears that iron was manufactured from meteorolites which had fallen to the earth in an almost pure metallic state, possibly long before prehistoric man had learnt how to dig for and smelt iron in any of the forms of ore which are found on this planet.

in the case of the more precious metals only if they are used on a very small scale. The reason of this is that a repousse relief is of much thinner substance than if the same design were cast, even by the most skilful metal-worker, and so a large surface may be produced with a very small expenditure of valuable metal. Casting is probably the most primitive method of. metal-work. This has passed through three stages, the first being represented by solid castings, such as are most celts and other implements of the prehistoric time; the mould was formed of clay, sand or stone, and the fluid metal was poured in till the hollow was full. The next stage was, in the case of bronze, to introduce an iron core, probably to save needless expenditure of the more valuable metal. The British Museum possesses an interesting Etruscan or Archaic Italian example of this primitive device. It is a bronze statuette from Sessa on the Volturno, about 2 ft. high, of a female standing, robed in a close-fitting chiton. The presence of the iron core has been made visible by the splitting of the figure, owing to the unequal contraction of the two metals. The forearms, which are extended, have been cast separately and soldered or brazed on to the elbows. The third and last stage in the progress of the art of casting was the employment of a core, generally of clay, round which the metal was cast in a mere skin, only thick enough for strength, without waste of metal. The Greeks and Romans attained to the greatest possible skill in this process. Their exact method is not certainly known, but it appears probable that they were acquainted with the process now called a cire perdue - the same as that employed by the great Italian artists in bronze. Cellini, the great Florentine artist of the 16th century, has described it fully in his Trattato della Scultura. If a statue was to be cast, the figure was first roughly modelled in clay - only rather smaller in all its dimensions than the future bronze; all over this a skin of wax was laid, and worked by the sculptor with modelling tools to the required form and finish. A mixture of pounded brick, clay and ashes was then ground finely in water to the consistence of cream, and successive coats of this mixture were then applied with a brush, till a second skin was formed all over the wax, fitting closely into every line and depression of the modelling. Soft clay was then carefully laid on to strengthen the mould, in considerable thickness, till the whole statue appeared like a shapeless mass of clay, round which iron hoops were bound to hold it all together. The whole was then thoroughly dried, and placed in a hot oven, which baked the clay, both of the core and the outside mould, and melted the wax, which was allowed to run out from small holes made for the purpose. Thus a hollow was left, corresponding to the skin of wax between the core and the mould, the relative positions of which were preserved by various small rods of bronze, which had previously been driven through from the outer mould to the rough core. The mould was now ready, and melted bronze was poured in till the whole space between the core and the outer mould was full. After slowly cooling, the outer mould was broken away from outside the statue and the inner core as much as possible broken up and raked out through a hole in the foot or some other part of the statue. The projecting rods of bronze were then cut away, and the whole finished by rubbing down and polishing over any roughness or defective places. The most skilful sculptors, however, had but little of this after-touching to do, the final modelling and even polish which they had put upon the wax being faithfully reproduced in the bronze casting. The further enrichment of the object by enamels and inlay of other metals was practised at a very early period by Assyrian, Egyptian and Greek metal-workers, as well as by the artists of Persia and medieval Europe.

The second chief process, that of hammered work (Gr. a41vpiiXaros; Fr. repousse), was probably adopted for bronze-work on a large scale before the art of forming large castings was discovered. In the most primitive method thin plates of bronze were hammered over a wooden core, rudely cut into the required shape, the core serving the double purpose of giving shape to and strengthening the thin metal. A further development in the art of hammered work consisted in laying the metal plate on a soft FIG. 1. - Monstrance of Copper Gilt; Italian work of the 15th century.

and elastic bed of cement made of pitch and pounded brick. The design was then beaten into relief from the back with hammers and punches, the pitch bed yielding to the protuberances which were thus formed, and serving to prevent the punch from breaking the metal into holes. The pitch was then melted away from the front of the embossed relief, and applied in a similar way to the back, so that the modelling could be completed on the face of the relief, the final touches being given by the graver. This process was chiefly applied by medieval artists to the precious metals, but by the Assyrians, Greeks and other early nations it was largely used for bronze. The great gates of Shalmaneser II., 858-823 B.C., from Balawat, now in the British Museum, are a remarkable example of this sort of work on a large scale, though FIG. 2. - One of the Siris Bronzes.

the treatment of the reliefs is minute and delicate. The "Siris bronzes," in the same museum, are a most astonishing example of the skill, attained by:.Greek artists in this repousse work (see Bronsted's Bronzes of Siris, 1836). They are a pair of shoulderpieces from a suit of bronze armour, and each has in very high relief a combat between a Greek warrior and an Amazon. No work of art in metal has probably ever surpassed these little figures for beauty, vigour and expression, while the skill with which the artist has beaten these high reliefs out of a flat plate of metal appears almost miraculous. The heads of the figures are nearly detached from the ground, their substance is little thicker than paper, and yet in no place has the metal been broken through by the punch. They are probably of the school of Praxiteles, and date from the 4th century B.C. (see fig. 2).

Copper and tin have been but little used separately. Copper in its pure state may be worked by the same methods as bronze, but it is inferior to it in hardness, strength and beauty of surface. Tin is too weak and brittle a metal to be employed alone for any but small objects. Some considerable number of tin drinkingcups and bowls of the Celtic period have been found in Cornwall in the neighbourhood of the celebrated tin and copper mines, which have been worked from a very early period. The use of lead has been more extended. In sheets it forms the best of all coverings for roofs and even spires. In the Roman and medieval periods it was largely used for coffins, which were often richly ornamented with cast work in relief. Though fusible at a very low temperature, and very soft, it has great power of resisting decay from damp or exposure. Its most important use in an artistic form has been in the shape of baptismal fonts, chiefly between the lrth and the 14th centuries. The superior beauty of colour and durability of old specimens of lead is owing to the natural presence of a small proportion of silver. Modern smelters carefully extract this silver from the lead ore, thereby greatly impairing the durability and beauty of the metal.

As in almost all the arts, the ancient Egyptians excelled in their metal-work, especially in the use of bronze and the precious metals. These were worked by casting and hammering, and ornamented by inlay, gilding and enamels with the greatest possible skill. From Egypt perhaps was derived the early skill of the Hebrews. Further instruction in the art of metal-working came probably to the Jews from the neighbouring country of Tyre. The description of the great gold lions of Solomon's throne, and the laver of cast bronze supported on figures of oxen, shows that the artificers of that time had overcome the difficulties of metal-working and founding on a large scale. The Assyrians were perhaps the most remarkable of all ancient nations for the colossal size and splendour of their works in metal; whole circuit walls of great cities, such as Ecbatana, are said to have been covered with metal plates, gilt or silvered. Herodotus, Athenaeus and other Greek and Roman writers have recorded the enormous number of colossal statues and other works of art for which Babylon and Nineveh were so famed. The numerous objects of bronze and other metals brought to light by the excavations in the Tigris and Euphrates valleys, though mostly on a small scale, bear witness to the great skill and artistic power of the people who produced them; while the discovery of some bronze statuettes, shown by inscriptions on them to be not later than 2200 B.C., proves how early was the development of this branch of art among the people of Assyria.

The Metal-Work of Greece

The early history of metal-working in Greece is extremely obscure, and archaeologists are divided in opinion even on so important a question as the relative use of bronze and iron in the Homeric age. The evidence of Mycenaean remains, as compared with the literary evidence of Homer, is both inadequate and inconclusive (see Aegean Civilization; Greek Art; Arms And Armour, Ancient; Plate; &c.). The poems of Homer are full of descriptions of elaborate works in bronze, gold and silver, which, even when full allowance is made for poetic fancy, show clearly enough very advanced skill in the working and ornamenting of these metals. Homer's description of the shield of Achilles, made of bronze, enriched with bands of figure reliefs in gold, silver and tin, could hardly have been written by a man who had not some personal acquaintance with works in metal of a very elaborate kind. Again, the accuracy of his descriptions of brazen houses - such as that of Alcinous, Od. vii. 8r - is borne witness to by Pausanias's mention of the bronze temple of Athena X aXKioucos in Sparta, and the bronze chamber dedicated to Myron in 648 B.C., as well as by the discovery of the stains and bronze nails, which show that the whole interior of the so-called treasury of Atreus at Mycenae was once covered with a lining of bronze plates. Of the two chief methods of working bronze, gold and silver, it is probable that the hammer process was first practised, at least for statues, among the Greeks, who themselves attributed the invention of the art of hollow casting to Theodorus and Rhoecus, both Samian sculptors, about the middle of the 6th century B.C. Pausanias specially mentions that one of the oldest statues he had ever seen was a large figure of Zeus in Sparta, made of hammered bronze plates riveted together. With increased skill in large castings, and the discovery of the use of cores, by which the fluid bronze was poured into a mere skin-like cavity, hammered or repousse work was only used in the case of small objects in which lightness was desirable, or for the precious metals in order to avoid large expenditure of metal. The colossal statues of ivory and gold by Pheidias were the most notable examples of this use of gold, especially his statue of Athena in the Parthenon, and the one of Zeus at Olympia. The nude parts, such as face and hands, were of ivory, while the armour and drapery were of beaten gold. The comparatively small weight of gold used by Pheidias is very remarkable when the great size of the statues is considered.

A graphic representation of the workshop of a Greek sculptor in bronze is given on a fictile vase in the Berlin Museum (see Gerhard's Trinkschalen, plates xii., xiii.). One man is raking out the fire in a high furnace, while another behind is blowing the bellows. Two others are smoothing the surface of a statue with scraping tools, formed like a strigil. A fourth is beating the arm of an unfinished figure, the head of which lies at the workman's feet. Perhaps the most important of early Greek works in cast bronze, both from its size and great historical interest, is the bronze pillar (now in the Hippodrome at Constantinople) which was erected to commemorate the victory of the allied Greek states over the Persians at Plataea in 479 B.C. (see Newton's Travels in the Levant). It is in the form of three serpents twisted together, and before the heads were broken off was at least ft. high. It is cast hollow, all in one piece, and has the names of the allied states engraved on the lower part of the coils. Its size and the beauty of its surface show great technical skill in the founder's art. On it once stood the gold tripod dedicated to Apollo as a tenth of the spoils. It is described by both Herodotus and Pausanias.

Marble was comparatively but little used by the earlier Greek sculptors, and even Myron, a rather older man than Pheidias, FIG. from the Milanese Candelabrum.

seems to have executed nearly all his most important statues in metal. Additional richness was given to Greek bronze-work by gold or silver inlay on lips, eyes and borders of the dress; one remarkable statuette in the British Museum has eyes inlaid with diamonds and fret-work inlay in silver on the border of the chiton. The mirrors of the Greeks are among the most important specimens of their artistic metal-work. These are bronze disks, one side polished to serve as a reflector, and the back ornamented with engraved outline drawings, often of great beauty (see Gerhard, Etruskische Spiegel, 1843-1867). In metal-work, as in other arts, the Romans were pupils and imitators of the Greeks. Owing to the growth of the spirit of luxury, a considerable demand arose for magnificent articles of gold and silver plate. The finest specimens of these that still exist are the very beautiful set of silver plate found buried near Hildesheim in 1869, now in the Berlin Museum. They consist of drinking vessels, bowls, vases, ladles and other objects of silver, parcel-gilt, and exquisitely decorated with figures in relief, both cast and repousse. There are electrotypes of these in the Victoria and Albert Museum. When the seat of the empire was changed, Byzantium became the chief centre for the production of artistic metal-work. From Byzantium the special skill in this art was transmitted in the 9th and 10th centuries to the Rhenish provinces of Germany and to Italy, and thence to the whole of western Europe; in this way the 18th century smith who wrought the Hampton Court iron gates was the heir to the mechanical skill of the ancient metalworkers of Phoenicia and Greece. In that period of extreme degradation into which all the higher arts fell after the destruction of the Roman Empire, though true feeling for beauty and knowledge of the subtleties of the human form remained for centuries almost dormant, yet at Byzantium at least there still survived great technical skill and power in the production of all sorts of metal-work. In the age of Justinian (first half of the 6th century) the great church of St Sophia at Constantinople was adorned with an almost incredible amount of wealth and splendour in the form of screens, altars, candlesticks and other ecclesiastical furniture made of massive gold and silver.

Metal-Work in Italy

It was therefore to Byzantium that Italy turned for metal-workers, and especially for goldsmiths, when, in the 6th to the 8th centuries, the basilica of St Peter's in Rome was enriched with masses of gold and silver for decorations and fittings, the gifts of many donors from Belisarius to Leo III., the mere catalogue of which reads like a tale from the Arabian Nights. The gorgeous Pala d'oro, still in St Mark's at Venice, a gold retable covered with delicate reliefs and enriched with enamels and jewels, was the work of Byzantine artists during the 11th century. This work was in progress for more than a hundred years, and was set in its place in though still unfinished (see Bellomo, Pala d'oro di St Marco, 1847). It was, however, especially for the production of bronze doors for churches, .ornamented with panels of cast work in high relief, that Italy obtained the services of Byzantine workmen (see Garrucci, Arte cristiana, 1872-1882). One artist, named Staurachios, produced many works of this class, some of which still exist, such as the bronze doors of the cathedral at Amalfi, dated 1066 A.D. Probably by the same artist, though his name was spelled differently, were the bronze doors of San Paolo fuori le Mura, Rome, careful drawings of which exist, though the originals were destroyed in the fire of 1824. Other important examples exist at Ravello (1197), Salerno (1099), Amalfi (1062), Atrani (1087); and doors at Monreale in Sicily and at Trani, signed by an artist named Barisanos (end of the 12th century); the reliefs on these last are remarkable for expression and dignity, in spite of their early rudeness of modelling and ignorance of the human figure. Most of these works in bronze were enriched with fine lines inlaid in silver, and in some cases with a kind of niello or enamel. The technical skill of these Byzantine metal-workers was soon acquired by native Italian artists, who produced many important works in bronze similar in style and execution to those of the Byzantine Greeks. Such, for example, are the bronze doors of San Zenone at Verona (unlike the others, of repoussel not cast work); those of the Duomo of Pisa, cast in 1180 by Bonannus, and of the Duomo of Troia, the last made in the beginning of the 12th century by Oderisius of Benevento. Another artist, named Roger of Amalfi, worked in the same way; and in the year 1219 the brothers Hubertus and Petrus of Piacenza cast the bronze door for one of the side chapels in San Giovanni in Laterano. One of the most important early specimens of metal-work is the gold and silver altar of Sant' Ambrogio [in Milan. In character of work and design it resembles the Venice Pala d'Oro, but is still earlier in date, being a gift to the church from Archbishop Angilbert II. in 835 A.D. (see Du Sommerard, and D'Agincourt, Moyen Age). It signed Wolvinivs Magister Phaber; nothing is known of the artist, but he probably belonged to the semiByzantine school of the Rhine provinces; according to Dr Rock he was an Anglo-Saxon goldsmith. It is a very sumptuous work, the front of the altar being entirely of gold, with repousse reliefs and cloisonné enamels; the back and ends are of silver, with gold ornaments. On the front are figures of Christ and the twelve apostles; the ends and back have reliefs illustrating the life of St Ambrose.

The most important existing work of art in metal of the 13th century is the great candelabrum now in Milan Cathedral. It is of gilt bronze, more than 14 ft. high; it has seven branches for n candles, and its upright stem is supported on four winged dragons. For delicate and spirited execution, together with refined gracefulness of design, it is unsurpassed by any similar work of art. Every one of the numerous little figures with which it is adorned is worthy of study for the beauty and expression of the face, and the dignified arrangement of the drapery (see fig. 3). The semiconventional open scroll-work of branches and fruit which wind around and frame each figure or group is devised with the most perfect taste and richness of fancy, while each minute part of this great piece of metal-work is finished with all the care that could have been bestowed on the smallest article of gold jewellery. Though something in the grotesque dragons of the base recalls the Byzantine school, yet the beauty of the figures and the keen feeling for graceful curves and folds in the drapery point to a native Italian as being the artist who produced this wonderful work of art. There is a cast in the Victoria and Albert Museum.

FIG. 4. - Silver Repousse Reliefs from the Pistoia Retable. During the 13th and 14th centuries in Italy the widespread influence of Niccola Pisano and his school encouraged the sculptor to use marble rather than bronze for his work. At this period wrought iron came into general use in the form of screens for chapels and tombs, and grills for windows. These are mostly of great beauty, and show remarkable skill in the use of the hammer, as well as power in adapting the design to the requirements of the material. Among the finest examples of this sort of work are the screens round the tombs of the Scala family at Verona, 1 35 0 - 1 375, - a sort of network of light cusped quatrefoils, each filled up with a small ladder (scala) in allusion to the name of the family. The most elaborate specimen of this wrought work is the screen to the Rinuccini chapel in Santa Croce, Florence, of 1371, in which moulded pillars and window-like tracery have been wrought and modelled by the hammer with extraordinary skill (see Wyatt, Metal-Work of Middle Ages). Of about the same date are the almost equally magnificent screens in Sta Trinita, Florence, and at Siena across the chapel in the Palazzo Pubblico. The main part of most of these screens is filled in with quatrefoils, and at the top is an open frieze formed of plate iron pierced, repousse, and enriched with engraving. In the 14th century great quantities of objects for ecclesiastical use were produced in Italy. The silver altar of the Florence baptistery was begun in the first half of the 14th century, and not completed till after 1477 (see Gaz. des beaux-arts, Jan. 1883). The greatest artists in metal laboured on it in succession, among them Orcagna, Ghiberti, Verrocchio, Ant. Pollaiuolo and many others. It has elaborate reliefs in repousse work, cast canopies and minute statuettes, with the further enrichment of translucent coloured enamels. The silver altar and retable of Pistoia Cathedral (see fig. 4), and the great shrine at Orvieto, are works of the same class, and of equal importance.

Whole volumes might be devoted to the magnificent works in bronze produced by the Florentine artists of this century, works such as the baptistery gates by Ghiberti, the statues of Verrocchio, Donatello and many others, the bronze screen in Prato cathedral by Simone, brother of Donatello, in 1444-1461, and the screen and bronze ornaments of the tomb of Piero and Giovanni dei Medici in San Lorenzo, Florence, by Verrocchio, in 1472. At the latter part of the 15th century and the beginning of the 16th the Pollaiuoli, Ricci and other artists devoted much labour and artistic skill to the production of candlesticks and smaller objects of bronze, such as door-knockers, many of which are works of the greatest beauty. The candlesticks in the Certosa near Pavia, and in the cathedrals of Venice and Padua, are the finest examples of these. Niccolo Grossi, who worked in wrought iron under the patronage of Lorenzo dei Medici, produced some wonderful specimens of metal-work, such as the candlesticks, lanterns, and rings fixed at intervals round the outside of the great palaces (see fig. 5). The Strozzi palace in Florence and the Palazzo del Magnifico at Siena have fine specimens of these - the former of wrought iron, the latter in cast bronze. At Venice fine work in metal, such as salvers and vases, was being produced, of almost Oriental design, and in some cases the work of resident Arab artificers. In the 16th century Benvenuto Cellini was supreme for skill in the production of enamelled jewellery, plate and even larger works of sculpture (see Plon's Ben. Cellini, 1882), and Giovanni de Bologna in the latter part of the same century inherited to some extent the skill and artistic power of the great 15th-century artists.

Spain

From a very early period the metal-workers of Spain have been distinguished for their skill, especially in the use of the precious metals. A very remarkable set of specimens of goldsmith's work of the 7th century are the eleven votive crowns, two crosses and other objects found in 1858 at Guarrazar, and now preserved at Madrid and in Paris in the Cluny Museum (see Du Sommerard, Musa de Cluny, 1852). Magnificent works in silver, such as shrines, altar crosses and church vessels of all kinds, were produced in Spain from the 14th to the 16th century - especially a number of sumptuous tabernacles (custodia) for the host, magnificent examples of which still exist in the cathedrals of Toledo and Seville. The bronze and wrought-iron screens - rejas, mostly of the 15th and 16th centuries - to be found in almost every important church in Spain are very fine examples of metal-work. They generally have moulded rails or balusters, and rich friezes of pierced and repousse work, the whole being often thickly plated with silver. The common use of metal for pulpits is a peculiarity FIG. 5. - Wrought-iron Candle Pricket; late 15th-century. Florentine work.

of Spain; they are sometimes of bronze, as the pairs in Burgos and Toledo cathedrals, or in wrought iron, like those at Zamora and in the church of San Gil, Burgos. The great candelabrum or tenebrarium in Seville Cathedral is the finest specimen of 16th-century metal-work in Spain; it was mainly the work of Bart. Morel in 1562. It is of cast bronze enriched with delicate scroll-work foliage, and with numbers of well-modelled statuettes. Especially in the art of metal-work Spain was much influenced in the 15th and 16th centuries by both Italy and Germany, so that numberless Spanish objects produced at that time owe little or nothing to native designers. At an earlier period Arab and Moorish influence is no less apparent.

1 FIG. 6. - Part of the "Eleanor Grill." England. - In Saxon times the English metal-workers, especially of the precious metals, possessed great skill, and appear to have produced shrines, altar-frontals, retables and other ecclesiastical furniture of considerable size and magnificence. Dunstan, archbishop of Canterbury (925-988), like Bernward, bishop of Hildesheim a few years later, and St Eloi of France three centuries earlier, was himself a skilful worker in all kinds of metal. The description of the gold and silver retable given to the high altar of Ely by Abbot Theodwin in the 11th century, shows it to have been a large and elaborate piece of work decorated with many reliefs and figures in the round. In 1241 Henry III. gave the order for the great gold shrine to contain the bones of Edward the Confessor. It was the work of members of the Otho family, among whom the goldsmith's and coiner's crafts appear to have been long hereditary. Countless other imporant works in the precious metals adorned every abbey and cathedral church in the kingdom. In the 13th century the English workers in wrought iron were especially skilful. The grill over the tomb of Queen Eleanor at Westminster, by Thomas de Leghton, made about 1294, is a remarkable example of skill in welding and modelling with the hammer (see fig. 6). The rich and graceful iron hinges, made often for small and out-of-the-way country churches, are a large and important class in the list of English wroughtiron work. Those on the refectory door of Merton College, Oxford, are a beautiful and well-preserved example dating from the 14th century. More mechanical in execution, though still very rich in effect, is that sort of iron tracery work produced by cutting out patterns in plate, and superimposing one plate over the other, so as to give richness of effect by the shadows produced by these varying planes. The screen by Henry V.'s tomb at Westminster is a good early specimen of this kind of work. The screen to Bishop West's chapel at Ely, and that round Edward VI.'s tomb at Windsor, both made towards the end of the i 5th century, are the most magnificent English examples of wrought iron; and much wrought-iron work of great beauty was produced at the beginning of the 18th century, especially under the superintendence of Sir Christopher Wren (see Ebbetts, Iron Work of 17th and 18th Centuries, 1880). Large flowing leaves of acanthus and other plants were beaten out with wonderful spirit and beauty of curve. The gates from Hampton Court are the finest examples of this class of work (see fig. 7).


From an early period bronze and latten (a variety of brass) were much used in England for the smaller objects both of ecclesiastical and domestic use, but except for tombs and lecterns were but little used on a large scale till the r6th century. The full-length recumbent effigies of Henry III. and Queen Eleanor at Westminster, cast in bronze by the "cire perdue" process, and thickly gilt, are equal, if not superior, in artistic beauty to any sculptor's work of the same period (end of the 13th century) that was produced in Italy or elsewhere. These FIG. 7. - Part of one of the Hampton Court Gates.

effigies are the work of an Englishman named William Torel. The gates to Henry VII.'s chapel, and the screen round his tomb at Westminster (see fig. 8), are very elaborate and beautiful examples of "latten" work, showing the greatest technical skill in the founder's art. In latten also were produced the numerous monumental brasses of which a large number still exist in England (see Brasses, Monumental).

In addition to its chief use as a roof covering, lead was sometimes used in England for making fonts, generally tub-shaped, with figures cast in relief. Many examples exist: e.g. at Tidenham, Gloucestershire; Warborough and Dorchester Oxon; Chirton, Wilts; and other places.

Germany

Unlike England, Germany in the 10th and iith centuries produced large and elaborate works in cast bronze, especially doors for churches, much resembling the contemporary doors made in Italy under Byzantine influence. Bernward, bishop of Hildesheim, 992-1022, was especially skilled in this FIG. 8. - Part of Henry VII.'s Bronze Screen.

work, and was much influenced in design by a visit to Rome in the suite of Otho III. The bronze column with winding reliefs now at Hildesheim was the result of his study of Trajan's column, and the bronze door which he made for his own cathedral shows classical influence, especially in the composition of the drapery of the figures in the panels. The bronze doors of Augsburg (1047-1072) are similar in style. The bronze tomb of Rudolph of Swabia in Merseburg Cathedral (1080) is another fine work of the same school. The production of works in gold and silver was also carried on vigorously in Germany. The shrine of the three kings at Cologne is the finest surviving example. At a later time Augsburg and Nuremberg were the chief centres for the production of artistic works in the various metals. Hermann Vischer, in the 15th century, and his son and grandsons were very remarkable as bronze founders. The font at Wittenberg, decorated with reliefs of the apostles, was the work of the elder Vischer, while Peter and his son produced, among other important works, the shrine of St Sebald at Nuremberg, a work of great finish and of astonishing richness of fancy in its design. The tomb of Maximilian I., and the statues round it, at Innsbruck, begun in 1521, are perhaps the most meritorious German work of this class in the 16th century, and show considerable Italian influence. In wrought iron the German smiths, especially during the 15th century, greatly excelled. Almost peculiar to Germany is the use of wrought iron for grave-crosses and sepulchral monuments, of which the Nuremberg and other cemeteries contain fine examples. Many elaborate well-canopies were made in wrought iron, and gave FIG. 9. - Brass Vase, pierced and gilt; 17th century Persian work.

full play to the fancy and invention of the smith. The celebrated 5th-century example over the well at Antwerp, attributed to Quintin Matsys, is the finest of these.

France

From the time of the Romans the city of Limoges has been celebrated for all sorts of metal-work, and especially for brass enriched with enamel. In the 13th and 14th centuries many life-size sepulchral effigies were made of beaten copper or bronze, and ornamented by various-coloured "champleve" enamels. The beauty of these effigies led to their being imported into England; most are now destroyed, but a fine specimen still exists at Westminster on the tomb of William de Valence (1296). In the ornamental iron-work for doors the French smiths were pre-eminent for the richness of design and skilful treatment of their metal. Probably no examples surpass those on the west doors of Notre Dame in Paris - unhappily much falsified by restoration. The crockets and finials on the fleches of Amiens and Rheims are beautiful specimens of a highly ornamental treatment of cast lead, for which France was especially celebrated. In most respects, however, the development of ,? ?. ,-„ the various kinds of metal-working went through much the same stages as in England.

Persia and Damascus.

The metal-workers of the East, especially in brass and steel, were renowned for their skill even in the time of Theophilus, the monkish writer on the subject in the 13th century. But it was during the reign of Shah Abbas I. (d. 1628) that the greatest amount of skill both in design and execution was reached by the Persian workmen. Delicate pierced vessels of gilt brass, enriched by tooling and inlay of gold and silver, were among the chief specialties of the Persians (see fig. 9). A process called by Europeans "damascening" (from Damascus, the chief seat of the export) was used to produce very delicate and rich surface ornament. A pattern was incised with a graver in iron or steel, and then gold wire was beaten into the sunk lines, the whole surface being then smoothed and polished. In the time of Cellini this process was copied in Italy, and largely used, especially for the decoration of weapons and armour. The repousse process both for brass and silver was much used by Oriental workers, and even now fine works of this class are produced in the East, old designs still being adhered to. (J. H. M.) Modern Art Metal-Work. - The term "art metal-work" is applied to those works in metal in which beauty of form or decorative effect is the first consideration, irrespective of whether the object is intended for use or is merely ornamental; and it embraces any article from a Birmingham brass bedstead to works of the highest artistic merit. The term, as definitely distinguishing one branch of metal-working from another, is objected to by many on the ground that no such prefix was required in the best periods of art, and that allied crafts continue to do without it to the present day. Indeed, as long as metalworking remained a handicraft - in other words, until the introduction of steam machinery - every article, however humble its purpose, seems to have been endowed with some traditional beauty of form. The robust, florid and distinctly Roman rendering of the classic, which followed the refined and attenuated treatment associated with the architecture of the brothers Adam, who died in 1792 and 1794, is the last development in England which can be regarded as a national style. The massively moulded ormolu stair balustrade of Northumberland House, now at 49 Prince's Gate; the candelabra at Windsor and Buckingham Palace, produced in Birmingham by the firm of Messenger; the cast-iron railings with javelin heads and lictors' fasces, the tripods, Corinthian column standard lamps and candelabra, boat-shaped oil lamps and tent-shaped lustres with classic mountings, are examples of the metal-work of a style which, outside the eccentric Brighton Pavilion and excursions into Gothic and Elizabethan, was universally accepted in the United Kingdom from the days of the Regency until after the accession of Victoria. Except perhaps the silversmiths, no one was conscious of being engaged in "art metal-working," yet the average is neither vulgar nor in bad taste, and the larger works are both dignified and suited to their architectural surroundings.

The introduction of gas as an illuminant, about 1816, at once induced a large demand and a novel description of metal fitting; and the craft fell under the control of a new commercial class, intent on breaking with past traditions, and utilizing steam power, electro-deposition, and every mechanical and scientific invention tending to economize metal or labour. But when all artistic perception in Great Britain appeared lost in admiration of the triumphs of machinery and the expansion of trade, a new influence in art matters, that of the prince consort, began to make itself felt. The Great Exhibition, state-aided schools of design, the South Kensington Museum, and the establishment of a Science and Art Department under Government, were among the results of the important art revival which he inaugurated. He is credited with having himself designed candelabra and other objects in metal, and he directly encouraged the production of the sumptuous treatise on metal-work by Digby Wyatt, which laid the foundations of the revival. To this work, and that of Owen Jones, can be traced the origin of the eclecticism which has laid all past styles of art under contribution. The Gothic revival also helped the recognition of art, without very directly affecting the movement. It was valuable in teaching how to work within definite limitations, but without slavish copying; it also emancipated a considerable body of craftsmen from the tyranny of manufacturers whose sole idea was that machine-work should supersede handicraft. Its greatest efforts were the metal chancel-screens designed by Sir G. G. Scott, that for Hereford Cathedral having been exhibited in 1862. It does not appear that the influence either of Owen Jones or Digby Wyatt on metal-working extended beyond bringing the variety and beauty of past styles to the direct notice of designers. Neither can the London silversmiths, though they employed the best talent available, particularly in the decade following the Great Exhibition of 1851, be credited with much influencing the art metal revival. They were rivalled by Elkington of Birmingham, who secured the permanent assistance of at least one fine artist, Morel Ladeuil, the producer of the Elcho Challenge Shield. Perhaps the first actual designer to make a lasting impression on the crafts was Thomas Jeckyll, some of whose work, including gates for Sandringham, was exhibited in 1862. Infinitely greater as a designer was Alfred Stevens, whose influence on English craftsmen might be regarded as almost comparable to that of Michelangelo on that of his Italian contemporaries. Stevens's designs certainly directly raised the standard of production in several metal-working firms by whom he was employed; whilst in the Wellington Memorial in St Paul's Cathedral, and in Dorchester House, his work is seen unfettered by commercial considerations. Omitting many whose occasional designs have had little influence on the development of the metal crafts, we come to Alfred Gilbert, whose influence for a time was scarcely less than that of Stevens himself. Monumental works, such as his statue of Queen Victoria at Winchester and his work at Windsor, may be handed down as his greatest achievements, but judged as art metal-work, his smaller productions, such as the centrepiece presented by the army and navy to Queen Victoria on her Jubilee, have been more important.

The charming bronze statuettes of Onslow Ford, the most representative of which are in the Tate Gallery; the work of George Frampton, as seen in the Mitchell Memorial; and the beautiful bas-reliefs of W. Stirling Lee, examples of which are the bronze gates of the Adelphi Bank at Liverpool, have all contributed, especially when applied to architectural decoration, to a high standard of excellence. Painters also have frequently designed and modelled for metal-work, for example, Lord Leighton, who produced bronze statuettes of most refined character; and Sir L. Alma-Tadema, who designed the grilles for his studio and entrance hall; but none so conspicuously as Professor H. von Herkomer, who, whether working in gold and enamel, iron, or his favourite alloy, pewter, infuses a freshness into his designs and methods which displays an unusual mastery over materials.

The gift of reproducing effects of nature or art by brush or chisel is not necessarily accompanied by power to design; but a noteworthy exponent of the dual faculty is G. C. Haite, whose designs are widely applied.

It is chiefly to architecture that metal-work owes its permanent artistic improvement. In England buildings of Norman Shaw and Ernest George demanded quiet and harmonious metalwork; and the custom of these architects of superintending and designing every detail, even for interiors, created the supply. The work of every worthy architect raises the standard of the crafts; but beyond others Messrs Ashbee, Lethaby and Wilson have taken an active personal interest in schools of metalwork. The technical schools have also been of immense service in creating a class of self-respecting craftsmen, whose wages enable them to regard their work as worthy occupation abounding in interest. Home industries such as the metal-working round Keswick (founded in 1884 by Canon and Mrs Rawnsley), executed during hours of idleness by field labourers and railway porters, educate the passer-by as well as the worker.

Cast Bronze Gates, Adelphi Bank, Liverpool. Designed by W. D. CAR6E, the figures by Stirling Lee, executed by Starkie Gardner And Co.

XVIII. 212.

A Rain-Water Rain-Water Head, In Lead, For The Victoria Law Courts, Birmingham. Designed by Aston Webb and Ingres Bell, and executed by Dent And Hellier.

Covered Bridge Of Iron, Sheathed In Cast Lead, Grand Hotel, London. Designed by William Woodward, and executed by Starkie Gardner And Co.

British architects and artists who design for the principal decorating firms are to-day as conversant with the Renaissance and succeeding styles of France and Italy as medieval revivalists were familiar with the Gothic styles with which they made us so well acquainted. Metal-work more or less based upon every kind of past style is produced in vast quantities, and in some cases so skilful are the workers that modern forgeries and reproductions are almost beyond the power of experts to detect. This large class of designers and craftsmen, to whom a thorough knowledge of the history of design is a necessity, follows and develops traditional lines. The new art school, on the contrary, breaks wholly with tradition, unless unconsciously influenced by the Japanese, and awards the highest place to originality in design. It is not to be expected that an art-revival following on, and in possession of, all the results of a period of unprecedented activity in scientific research should proceed with the same restraint as heretofore; but the unfettered activity, and the general encouragement to abandon the traditions of art, have no exact parallel in the past, and may yet prove a danger. It is perhaps the very rapidity of the movement that is likely to retard its progress, and to fail to carry with it the wealthy clients and the decorators they employ, or perhaps even to increase the disposition to cling to the reproductions of the styles of the i i th and i 8th centuries. The multiplication of art periodicals, lectures, books, photographs, meetings of societies and gilds, museums, schools of arts and crafts, polytechnics, scholarships, facilities for travel, exhibitions, even those of the Royal Academy, to which objects of applied art are now admitted, not only encourages many persons to become workers and designers in the applied arts, but exposes everything to the plagiarist, who travesties the freshest idea before it has well left the hands of its originator. Thus the inspirations of genius, appropriated by those who imperfectly appreciate their subtle beauty and quality, become hackneyed and lose their charm and interest. The keen desire to be unconventional in applied art has spread from Great Britain and the United States to Germany, Austria and other countries, but without well-defined first principles, or limitations It seems agreed in a general way that the completed work in metal is to be wholly the conception and, as far as possible, the actual handiwork of the designer: casting by the tire-perdue process, left practically untouched from the mould, and embossing, being the two most favoured processes. The female figure is largely made use of, and rich and harmonious colours are sought, the glitter of metal being invariably subdued by deadening its lustre, or by patinas and oxides. Gilding, stains and lacquers, electro-plating, chasing, "matting," frosting, burnishing, mechanically produced mouldings and enrichments, and the other processes esteemed in the 10th century, are disused and avoided. New contrasts are formed by the juxtaposition of differently toned metals; or these with an inlay of haliotis shell, introduced by Alfred Gilbert; or of coloured wax, favoured by Onslow Ford; or enamelling, perfected by Professor von Herkomer; or stained ivory, pearls, or semi-precious stones. The quality of .the surface left by the skilled artist or artisan is more regarded than symmetry of design, or even than correct modelling. Frequently only the important parts in a design are carefully finished and the rest merely sketched: the mode of working, whether by modelling-tools or hammer, being always left apparent.

The newer kinds of art metal-work have, until recently, reached the purchaser direct from the producer's workshop; but they may now also be seen in the shops of silversmiths, jewellers, and general dealers, who are thus helping to transfer production from large commercial manufactories to smaller ateliers under artistic control. The production of the larger household accessories, such as bedsteads, fenders, gas and electric fittings, clocks, &c., has hardly as yet come under the influence of the art movement. The services rendered by Mr W. A. S. Benson of Chiswick, who commenced about 1886 to revolutionize the production of sheet-brass and copper utensils, cannot be passed over. The average ecclesiastical metal-work has rather receded than progressed in merit, except when designed by architects and executed under their supervision. Though the demand for good domestic wrought-iron work has enormously increased, adaptations from the beautiful work of the 17th and 18th centuries have been found so suited to their architectural surroundings, that new departures have been relatively uncommon. Of such the gates for Sandringham, by Jeckyll; for Crewe Hall, by Charles Barry; and for the Victoria and Albert Museum, by Gamble, are the earliest and best known. Of the vast number designed upon traditional lines may be cited those for Lambton Castle, Welbeck, Eaton Hall, Twickenham, Clieveden, and the Astor Estate Office on the Victoria Embankment. Cast iron, brought to perfection by the Coalbrookdale Company about 1860, but now little esteemed, owing to the poverty of design which so often counterfeits smiths' work, presents great opportunities to founders possessing taste or willing to submit to artistic control. A very large field is also opening for cast-lead work, whether associated with architecture, as in the leaden covered-way over Northumberland Street, in London (see Plate), and the fine rain-water heads of the Birmingham Law Courts (see Plate), or with the revival of the use of metal statuary and vases in gardens. The subdued colour and soft contours of pewter render it once more a favoured material, peculiarly adapted to the methods of the art revival, and perhaps destined to supersede electro-plate for household purposes. In silver-work the proportion of new art designs exhibited by dealers and others is still relatively small; but jewellers, except when setting pure brilliants and pearls, are becoming more inclined to make their jewels of finely modelled gold and enamel enriched with precious and semi-precious stones, than of gems merely held together by wholly subordinate settings.

On the continent of Europe, France was the first to recognize the merits of its bygone designers and craftsmen, and even antecedent to the Exhibition of 1851, when art in Great Britain was dormant, it was possible to obtain in Paris faithful reproductions of the finest ormolu work of the 18th century. At the same time a most active production of modern designs was proceeding, stimulated by rewards, with the result that the supply of clocks, lamps, candelabra, statuettes, and other ornaments in bronze and zinc to the rest of Europe became a monopoly of Paris for nearly half a century. In all connected with their own homes the French adhere to their traditions far more than other nations, and the attempt at originality in the introduction of metal-work into the scheme of decoration of a room is almost unknown. In the domain of bronze and imitation bronze statuary the originality of the French is absolutely unrivalled. And not only in bronze, but in Paris jewellery, enamels, silver, pewter and iron work a cultured refinement is apparent, beside which other productions, even the most finished, appear crude. The French artist attains his ideal, and it is difficult to imagine, from his standpoint, that the metal-work of the present can be surpassed. The best English metal-worker, on the contrary, is probably not often quite satisfied with the results he attains, perhaps because in Great Britain the pursuit of art has for centuries been fitful and individual, while in France art traditions are hereditary. The metal-work of Belgium is based at present entirely on that of France, without attaining the same standard, unless designed for ecclesiastical uses. In Holland these crafts have not progressed. Italian metal-workers are mainly employed in reproduction; but traditions linger in some remote parts, while the sporadic appearance of craftsmen of a high order is evidence that the ancient artistic spirit is not wholly extinct. Similarly, the surprising damascening by Messrs Zuluaga of Madrid in the monument to General Prim, and that of Alvarez of Toledo, give hope that the Spanish craftsman only needs to be properly directed. German and Austrian workers had for years shown more energy than originality, but they have recently embraced the newest English developments and carried them to extremes of exaggeration. For really fresh and progressive indigenous art we may perhaps have, in the near future, to turn to America and to Russia, where, having little artistic past to refer to, designers and craftsmen display unequalled individuality and force. It is from the Far East, however, that the most serious rivalry may be anticipated. The metal-work of China and Japan, so pleasantly naïve and inexpensive, though becoming undesirably modified as to design through contact with European buyers, is losing none of its matchless technique, which indeed in Japan is still being developed. In any history of the art revival the influence of such firms as Barbedienne and Christofle in Paris and Tiffany in New York cannot be ignored.

(J. S. G.) Industrial Metal-Work. The malleability and ductility of metals lie at the basis of the work of the goldand silver-smiths at one extreme, and of the boiler-maker at the other. Sheet metals can be made to assume almost any shape under the hammer, or by pressure, provided they are subjected to annealing to restore the property of malleability. The most awkward shapes, involving excessive extensions of metal, are produced by drawing processes between dies of iron and steel in power presses. All the common domestic utensils in tinned and enamelled ware, and all the ordinary patterns of the silversmiths, are similarly done. Frequent annealings are necessary to prevent fracture of the metal; but with these and the observance of certain other precautions of a practical character the degree of extension possible is enormous. Another illustration of the malleability of metal is afforded by metal spinning. A sheet of metal set revolving at a high speed in a lathe is bent over into cup-shaped forms, with numerous mouldings, by a blunt hardened tool. A great deal of work is done in this way, though this sphere has also been invaded by the draw presses, whose output would seem incredible to those not familiar with the work. Objects that do not require annealing are produced by dozens per minute, and all the movements of feeding and stamping and removal are often automatic. The ductility of metals and alloys is utilized in wire and tube-drawing through dies on long benches. This work also requires frequent annealing, for otherwise the wires or tubes would rupture. Even hard steel is treated in this way to form tubes for the highest hydraulic and steam pressures.

Platers' Work (see Boiler) is distinguished from work in sheet metals by the fact that plates have considerable thickness, which sheets have not. Plates range in thickness from 4 in. to 2 in., but for most purposes they do not go beyond 4 in. or I in. Over these thicknesses they are used chiefly for the largest marine boilers. Armour plates which are several inches in thickness do not come in this group, being a special article of manufacture. Sheets are of thicknesses of less than 4 in. This distinction of thickness is of importance in its bearing on workshop practice. A thin sheet requires a very different kind of treatment from a thick plate. Not only is more powerful machinery required for the latter, but in bending it allowance has to be made for the difference in radius of outer and inner layers, which increases with increase of thickness. Short, sharp bends which are readily made in thin sheets cannot be done in thick plates, as the metal would be stressed too much in the outer layers. The methods of union also differ, riveting being adopted for thick plates, and soldering or brazing generally for thin.

Coppersmiths' Work is an important section of sheet-metal working. It is divided into two great departments: the domestic utensil side, on which the brazier's craft is exercised; and the engineering side, which is concerned in some engine-work, locomotive and marine, and in the manufacture of brewers' utensils. The methods of the first are allied to those of the tinman, those of the second to the methods of the plater. Tinsmiths' work resembles the lighter part of the work of the coppersmith. There is no essential difference in dealing with tin (i.e. sheets of iron or steel coated with tin) and copper of the same thickness. Hence the craft of tinmen and braziers is carried on by the same individuals. There are, however, differences of treatment in detail, because copper is more malleable and softer than tin plate. The geometry of sheet-metal work and of platers' and boiler-makers' work is identical up to a certain stage. The divergence appears when plates are substituted for sheets. A thin sheet has for all practical purposes no thickness - that is, the geometrical pattern marked on it will develop the object required after it is bent. Nearly all patterns are the developments of the envelopes of geometrical solids of regular or irregular outlines, few of plane faces; when they are made up of combinations of plane faces, or of faces curved in one plane only, there is no difference in dealing with thin sheets or thick plates. But when curving occurs In different planes at right or other angles (hollowing), the metal has to be drawn or extended on the outside, and important differences arise. A typical form is the hemisphere, from which many modified forms are derived. The production of this is always a tedious task. It involves details of "wrinkling" and "razing," if done by hand-work in copper. In thick plates it is not attempted by hand, but pressing is done between dies, or segments of the sphere are prepared separately and riveted together. In tin it is effected by stamping. In all work done in thick plates the dimensions marked out must have reference to the final shape of the article. Generally the dimensions are taken as in the middle of the plate, but they may be on the inside or outside according to circumstances. But in any case the thickness must enter into the calculations, whereas in thin sheets no account is taken of thickness.

Raised Work

All the works in sheet metal that are bent in one plane only are easily made. The shapes of all polygonal and all cylindrical and conical forms are obtained by simple development - that is, the envelopments of these bodies are marked out on a flat plane, and when cut, are bent or folded to give the required envelopes. Only common geometrical problems are involved in the case of sheets of sensible thickness, and allowances are made for thickness. But in those forms where curving must take place in different directions the layers or fibres of metal are made to glide over one another, extension taking place in some layers but not in others, and this goes on without producing much reduction in the thickness. This is only possible with malleable and ductile metals and alloys. As a general rule it is restricted to metals which are not cast, for, with some slight exceptions, it is impossible to produce relative movements of the layers in cast iron, steel or cast brass. But most rolled metals and alloys can be so treated, copper being the best for the purpose. The methods employed are "raising" by the hammer, and pressing in dies. But the severity of the treatment would tear the material asunder if rearrangement of the particles were not obtained by frequent annealing (q.v.).

If an object has to be beaten into concave form from a flat thin sheet, the outer portions must be hammered until they occupy smaller dimensions than on the flat sheet. If a circular disk is wrought into a hemisphere and the attempt is made to hammer the edges round, crumpling must occur. This in fact is the first operation, termed wrinkling, the edge showing a series of flutes. These flutes have to be obliterated by another series of hammerings termed razing. The result is that the object assumes a smooth concave and convex shape, without the thickness of the metal becoming reduced.

Cast Work. - The metals and alloys which are neither malleable nor ductile can only be worked into required shapes by melting and casting in moulds. Abundance of remains which date from the Neolithic period testify to the high antiquity of this class of work, and also to the great skill which the ancient founders had acquired. Statue-founding is a highly specialized department of metal-work, in which the artists of the middle ages excelled. Two methods have been employed, the cire-perdue, or wax process (see above), and the present, or all sand method. In the latter the artist provides a model in plaster from which the founder takes a mould within an encircling box. This mould must obviously be made in scores of little separate sections (false cores or drawbacks) to permit of their removal from the model without causing fracture of the sand. These are subsequently replaced piece by piece in the encircling frame, and a core made within it, leaving a space of 4 in. or thereabouts into which the metal is poured. The advantage of this process is that the artist's model is not destroyed as in the cire-perdue, and if a "waster" results, a second mould can be taken. A large statue occupies from one to three months in the moulding.

The extreme tenuity of objects which are hammered, drawn or rolled cannot for obvious reasons be attained by casting. Casting also is complicated by the shrinkage which occurs in cooling down from the molten state, and in some alloys by the formation of eutectics, and the liquation of some constituents. The temperature of pouring is now known to be of more importance than was formerly suspected. The after-treatment of castings by annealing exercises great influence on results in malleable cast iron and steel.

There are many metals and alloys which are malleable and ductile, and also readily fused and cast. This is the case with gold, silver, copper, tin, lead and others, and especially with low carbon steel, which is first cast as an ingot, then annealed and rolled into plates as well as the thinnest sheets. The ancient wootz, and the products of the native furnaces of Africa are first cast, then hammered out thin. Many of the patent bronzes are by slight variations in the proportions of the constituents made suitable for casting, for forging, and for rolling into sheets. But in all the great modern manufacturing processes it is true that metals and alloys, though of the same name, have a different composition according as they are intended for casting on the one hand, or for forging, rolling and drawing on the other. Wrought or malleable iron has less of carbon and other elements in its composition than has cast iron. Steel intended for castings has slightly more carbon and other elements than the cast-steel ingot intended for rolling into plates. So also with the numerous bronzes, the phosphor, the delta, the aluminium and other alloys of copper; each is made in several grades to render it suitable for different kinds of treatment.

There are no materials used in manufacture of which the craftsman is able to vary the composition and physical qualities so extensively as the metals and their alloys. Much light has been thrown on facts which have long been known in a practical way, by the labours of the Alloys Research Committee of the Institution of Mechanical Engineers (England). These, together with independent researches into the heat treatment of steel and iron, have opened up many unsolved problems fraught with deepest interest and importance.

One of the most difficult problems with which the metal-worker who handles constructional forms has to deal is the maintenance of a due relation between absolute strength and a useful degree of elasticity. Only after many failures has the fact been grasped that a very high degree of strength is inconsistent with a trustworthy degree of elasticity. The reasons were not understood until the researches of Wailer demonstrated the difference between the effects of merely dead loads and of live loads, and between repetitions of stress of one kind only, and the vastly more destructive effects of both kinds alternating.

The texture of metals and alloys is related to the character of the operations which can be done upon them. Broadly the malleable and ductile metals and alloys show a fibrous character when ruptured, the fusible ones a crystalline fracture. The difference is seen both in the workshop and in the specimens ruptured in testing-machines. A piece of wrought iron, or mild steel or copper, if torn asunder shows long lustrous fibres, resembling a bundle of threads in appearance. A piece of cast iron, or steel or bronze, shows on rupture a granular, crystalline surface destitute of any fibre. The ductile metals and alloys also extend from Jo to 30 with reduction of area before they fracture, the crystalline ones snap shortly without warning. In some instances, however, the method of application of stress exercises an influence. Wrought iron and mild steel may be made to show a short and crystalline fracture by a sudden application of stress, while if drawn asunder slowly they develop the silky, fibrous appearance. The men who design and work in metals have to take account of these vital differences and characteristics, and must be careful not to apply treatment suitable to one kind to another of a dissimilar character. Tools, appliances and methods have little in common. Between the work of the smith, the sheet-metal worker and the founder, there is a great gulf. An artistic taste will recognize the essential differences, and not endeavour, apart from questions of strength, to graft a design suitable for one on another. It is bad taste to imitate the tracery of the ductile wrought iron in cast designs, the foliations of ancient wrought-iron grilles and screens in heavy cast iron. Severe simplicity is also most in harmony with constructional designs in plated work, where stresses occur in straight lines. From this point of view the lattice-girder bridge is an ideal design in steel.

One of the most valuable characteristics of the iron alloys is their capacity for hardening, which they owe in the main to the presence of certain small percentages of carbon relatively to minute quantities of other elements: as manganese, tungsten, nickel and others of less importance. The capacity for hardening is an invaluable property not only in regard to cutting-tools, but also in prolonging the life of parts subjected to severe friction. Great advances have been made in the utilization of this property as a result of the growth of the precision grinding-machines, which are able to correct the inaccuracies of hardened work as effectually as those of soft materials. It is utilized in the spindles of machinetools, in the balls and rollers for high-speed bearings, slides, pivots and such like.

Methods of Union

The methods of union of works in metal are extremely varied. An advantage in casting is that the most complicated shapes are made in one piece. But all other complicated forms have to be united by other means - as welding, soldering, riveting or bolting. The two first-named are trustworthy, but are evidently unsuitable for the greater portion of engineers' work, for which .riveting and bolting are the methods adopted. Even the simple elements of rivets and bolts have produced immense developments since the days when bolts were made by hand, holes cored or hand-drilled, and rivets formed and closed by hand labour. Nutand bolt-making machinery, both for forging and screw cutting, operates automatically, and drilling machinery is highly specialized. Hand-riveting on large contracts has been wholly displaced by power-riveting machines. The methods of union adopted are not allowed to impair the strength of structures, which is calculated on the weakest sections through the rivet or bolt holes. Hence much ingenuity is exercised in order to obtain the strongest joint which is consistent with security of union. This is the explanation of all the varied forms of riveted joints, which to casual observers often appear to be of a fanciful character.

Protection of Surfaces

The protection and coloration of metals and alloys includes a large number of industries. The engineer uses paints for his iron and steel. A small amount of work is treated by the Bower-Barff and allied processes, by which a coating of magnetic oxide is left on the metal. Hot tar - Angus Smith's process - is used for water-pipes. Boiled linseed-oil is employed as a non-corrosive coating preceding the application of the lead and iron oxide paints. In steam boilers artificial galvanic couples are often set up by the suspension of zinc plates in the boiler, so that the corrosion of the zinc may preserve the steel boiler plates from waste. Various artificial protective coatings are applied to the plates of steel ships. Bright surfaces are protected with oil or with lacquer. The ornamental bronzes and brasses are generally lacquered, though in engineers' machinery they are as a rule not protected with any coating. For ornamental work lacquering divides favour with colouring - sometimes done with coloured lacquers, but often with chemical colourings, of which the copper and iron salts are the chief basis. (J. G. H.) Literature. - Prehistoric: Worsaae, Nordiske Oldsager i Kjobenhavn (1854); Perrin, Etude prehistorique - Age du bronze (1870). Classical: Layard, Nineveh and Babylon (1853); Pliny, Natural History, bk. xxxiv.; Br6ndsted, Den Fikoroniske Cista (1847); Ggrhard, various monographs (1843-1867); Muller, Etrusker, &c., and other works; Ciampi, Dell' Antica toreutica (1815); Von Bibra, Die Bronzen and Kupfer-Legirungen der alten and dltesten Volker (1869); C. Bischoff, Das Kupfer in der vorchristlichen Zeit (1865) Medieval, &c.: Digby Wyatt, Metal-Work of the Middle Ages (1849); Shaw, Ornamental Metal-Work (1836); Drury Fortnum, S.K.M. Handbook of Bronzes (1877); King, Orfevrerie et ouvrages en metal du moyen age (1852-1854); Hefner-Alteneck, Serrurerie du moyen age (1869); Viollet-le-duc, Dict. du mobilier, "Serrurerie" and "Orfevrerie," (1858 &c.); Lacroix, Tresor de S. Denis, and L'Art du moyen age (various dates); Karch, Die Rdthselbilder an der Broncethiire zu Augsburg (1869); Krug, Entwiirfe fiir Gold-, Silber-, and Bronze-Arbeiten; Linas, Orfevrerie merovingienne (1864), and Orfevrerie du XIII me siecle (1856); Bordeaux, Serrurerie du moyen age (1858); Didron, Manuel des oeuvres de bronze et d'orfevrerie du moyen age (1859); Du Sommerard, Arts au moyen age (1838-1846), and Musee de Cluny (1852); Rico y Sinobas, Trabajos de metales (1871); Bock, Die Goldschmiedekunst des Mittelalters (1855), and Kleinodien des heil.-romischen Reiches; Jouy, Les gemmes et les joyaux (1865); Texier, Dictionnaire d'orfevrerie (1857); Virgil Solis, Designs for Goldand Silversmiths (1512), (facsimile reproduction, 1862); Molinier, Les Bronzes de la Renaissance (1886); Servant, Les bronzes d'art (1880); Wilhelm Bode, Italian Bronze Statuettes of the Renaissance (Eng. trans. by W. Gretor, first 2 vols., 1909). Practical Treatises: Theophilus, Diversarum artium schedula (12th cent.), (see Quellenschriften fiir Kunstgeschichte, VII., Vienna 1877); Cellini, Trattati dell' oreficeria e della scultura (ed. Milanesi, Florence, 1856); Vasari, Tre arti del disegno, pt. ii. (Milanesi's ed., 1882); Garnier, Manuel du ciseleur (1859); Haas, Der Metallarbeiter (1902).


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