Clay: Wikis


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From Wikipedia, the free encyclopedia

The Gay Head cliffs in Martha's Vineyard consist almost entirely of clay.

Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired. Clay deposits are mostly composed of clay minerals (phyllosilicate minerals), minerals which impart plasticity and harden when fired and/or dried, and variable amounts of water trapped in the mineral structure by polar attraction. Organic materials which do not impart plasticity may also be a part of clay deposits.[1]



Deforestation for clay extraction in Rio de Janeiro city - Brazil. The picture is of Morro da Covanca - Jacarepaguá.

Clay minerals are typically formed over long periods of time by the gradual chemical weathering of rocks, usually silicate-bearing, by low concentrations of carbonic acid and other diluted solvents. These solvents, usually acidic, migrate through the weathering rock after leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed by hydrothermal activity. Clay deposits may be formed in place as residual deposits in soil, but thick deposits usually are formed as the result of a secondary sedimentary deposition process after they have been eroded and transported from their original location of formation. Clay deposits are typically associated with very low energy depositional environments such as large lakes and marine deposits.

Primary clays, also known as kaolins, are located at the site of formation. Secondary clay deposits have been moved by erosion and water from their primary location[2].


Clays are distinguished from other fine-grained soils by differences in size and/or mineralogy. Silts, which are fine-grained soils which do not include clay minerals, tend to have larger particle sizes than clays, but there is some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include both silts and clays. The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 µm (clays being finer than silts), sedimentologists often use 4-5 μm, and colloid chemists use 1 μm.[1] Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg Limits. ISO 14688 grades clay particles as being smaller than 2 μm and silts larger.


Depending on the academic source, there are three or four main groups of clays: kaolinite, montmorillonite-smectite, illite and chlorite. Chlorites are not always considered a clay, sometimes being classified as a separate group within the phyllosilicates. There are approximately 30 different types of "pure" clays in these categories, but most "natural" clays are mixtures of these different types, along with other weathered minerals.

Varve (or varved clay) is clay with visible annual layers, formed by seasonal differences in erosion and organic content. This type of deposit is common in former glacial lakes. When glacial lakes are formed there is very little movement of the water that makes the lake, and these eroded soils settle on the lake bed. This allows such an even distribution on the different layers of clay.[3].

Quick clay is a unique type of marine clay indigenous to the glaciated terrains of Norway, Canada, Northern Ireland and Sweden. It is a highly sensitive clay, prone to liquefaction, which has been involved in several deadly landslides.

Historical and modern uses

Clay layers in a construction site. Dry clay is normally much more stable than sand with regard to excavations.

Clays exhibit plasticity when mixed with water in certain proportions. When dry, clay becomes firm and when fired in a kiln, permanent physical and chemical reactions occur. These reactions, among other changes, cause the clay to be converted into a ceramic material. Because of these properties, clay is used for making pottery items, both utilitarian and decorative. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware, stoneware and porcelain. Prehistoric humans discovered the useful properties of clay, and one of the earliest artifacts ever uncovered is a drinking vessel made of sun-dried clay.[citation needed] Depending on the content of the soil, clay can appear in various colors, from a dull gray to a deep orange-red.

Clay tablets were used as the first writing medium, inscribed with cuneiform script through the use of a blunt reed called a stylus.

Clays sintered in fire were the first form of ceramic. Bricks, cooking pots, art objects, dishware and even musical instruments such as the ocarina can all be shaped from clay before being fired. Clay is also used in many industrial processes, such as paper making, cement production and chemical filtering. Clay is also often used in the manufacture of pipes for smoking tobacco. Until the late 20th century bentonite clay was widely used as a mold binder in the manufacture of sand castings.

Clay, being relatively impermeable to water, is also used where natural seals are needed, such as in the cores of dams, or as a barrier in landfills against toxic seepage ('lining' the landfill, preferably in combination with geotextiles).[4]

Recent studies have investigated clay's absorption capacities in various applications, such as the removal of heavy metals from waste water and air purification.

Medical and agricultural uses

A traditional use of clay, such as Armenian bole, is to soothe an upset stomach, similar to the way parrots (and later, humans) in South America originally used it.[5]

A more recent, and more limited, use is as a specially formulated spray applied to fruits, vegetables and other vegetation to repel or deter codling moth damage, and at least for apples, to prevent sun scald.

A recent article in The Journal of Antimicrobial Chemotherapy found that certain iron-rich clay was effective in killing bacteria.[6]

See also


  1. ^ a b Guggenheim & Martin 1995, pp. 255–256
  2. ^ Environmental Characteristics of Clays and Clay Mineral Deposits
  3. ^ Environmental Characteristics of Clays and Clay Mineral Deposits
  4. ^ Preliminary evaluation of a compacted bentonite / sand mixture as a landfill liner material (Abstract) - Koçkar, Mustafa K.; Akgün, Haluk; Aktürk, Özgür; Department of Geological Engineering, Middle East Technical University, Ankara, Turkey
  5. ^ Evolutionary biology: Dirty eating for healthy living by Jared M. Diamond
  6. ^ Broad-spectrum in vitro antibacterial activities of clay minerals against antibiotic-susceptible and antibiotic-resistant bacterial pathogens


  • Ehlers, Ernest G. and Blatt, Harvey (1982). 'Petrology, Igneous, Sedimentary, and Metamorphic' San Francisco: W.H. Freeman and Company. ISBN 0-7167-1279-2.
  • Hillier S. (2003) Clay Mineralogy. pp 139–142 In: Middleton G.V., Church M.J., Coniglio M., Hardie L.A. and Longstaffe F.J.(Editors) Encyclopedia of sediments and sedimentary rocks. Kluwer Academic Publishers, Dordrecht.

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Up to date as of January 14, 2010

From Wikiquote

Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired.


  • Do they believe that they themselves are made of a finer clay than the rest of mankind?
  • All that happens to us, including our humiliations, our misfortunes, our embarrassments, all is given to us as raw material, as clay, so that we may shape our art.
    • Jorge Luis Borges, Twenty Conversations with Borges, Including a Selection of Poems: Interviews by Roberto Alifano (1984)
  • Time's wheel runs back or stops: Potter and clay endure.
  • The heart -- which may be broken: happy they!
    Thrice fortunate! who of that fragile mould,
    The precious porcelain of human clay,
    Break with the first fall.
  • How shall we celebrate the day,
    When God appeared in mortal clay,
    The mark of worldly scorn.
  • If we could once understand the common clay of earth we should understand everything.
  • Clay is embedded in our subconscious. It has been there for at least 50,000 years.
    • Art Clokey, quoted by Mike Antonucci (Knight Ridder), "Gumby's creator formed a spirit in clay", The Milwaukee Journal Sentinel, 1 January 1998, p. 6E
  • A vase of unbaked clay, when broken, may be remoulded, but not a baked one.
  • These look like the workmanship of heaven;
    This is the porcelain clay of human kind,
    And therefore cast into these noble moulds.
  • This Great God,
    Like a mammy bending over her baby,
    Kneeled down in the dust
    Toiling over a lump of clay
    Till He shaped it in His own image.
  • Why, if the Soul can fling the Dust aside,
    And naked on the Air of Heaven ride,
    Were't not a Shame — were't not a Shame for him
    In this clay carcase crippled to abide?
  • Shall the clay say to him that fashioneth it, What makest thou?
  • Who are they who speak evil of the clay? The very stars are made of clay like mine!
    • Jack London,letter to Charles Warren Stoddard (11 August 1905)
  • Nobody grasped you by the shoulder while there was still time. Now the clay of which you were shaped has dried and hardened, and naught in you will ever awaken the sleeping musician, the poet, the astronomer that possibly inhabited you in the beginning.
  • Imperial Caesar, dead and turned to clay,
    Might stop a hole to keep the wind away.
  • If they find some stain or blemish in a name of note, not grieving that their greatest are so small, inflate themselves with some insane delight, and judge all nature from her feet of clay.
  • What is man but a mass of thawing clay? The ball of the human finger is but a drop congealed. The fingers and toes flow to their extent from the thawing mass of the body. Who knows what the human body would expand and flow out to under a more genial heaven?
  • To get wealth and security by guile
    Is like one who pours water into a pot of unbaked clay.

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Up to date as of January 22, 2010

From Wikisource

by James Joyce in the year 1914
A short story for the anthology Dubliners published in 1914

THE matron had given her leave to go out as soon as the women's tea was over and Maria looked forward to her evening out. The kitchen was spick and span: the cook said you could see yourself in the big copper boilers. The fire was nice and bright and on one of the side-tables were four very big barmbracks. These barmbracks seemed uncut; but if you went closer you would see that they had been cut into long thick even slices and were ready to be handed round at tea. Maria had cut them herself.

Maria was a very, very small person indeed but she had a very long nose and a very long chin. She talked a little through her nose, always soothingly: "Yes, my dear," and "No, my dear." She was always sent for when the women quarrelled Over their tubs and always succeeded in making peace. One day the matron had said to her:

"Maria, you are a veritable peace-maker!"

And the sub-matron and two of the Board ladies had heard the compliment. And Ginger Mooney was always saying what she wouldn't do to the dummy who had charge of the irons if it wasn't for Maria. Everyone was so fond of Maria.

The women would have their tea at six o'clock and she would be able to get away before seven. From Ballsbridge to the Pillar, twenty minutes; from the Pillar to Drumcondra, twenty minutes; and twenty minutes to buy the things. She would be there before eight. She took out her purse with the silver clasps and read again the words A Present from Belfast. She was very fond of that purse because Joe had brought it to her five years before when he and Alphy had gone to Belfast on a Whit-Monday trip. In the purse were two half-crowns and some coppers. She would have five shillings clear after paying tram fare. What a nice evening they would have, all the children singing! Only she hoped that Joe wouldn't come in drunk. He was so different when he took any drink.

Often he had wanted her to go and live with them;-but she would have felt herself in the way (though Joe's wife was ever so nice with her) and she had become accustomed to the life of the laundry. Joe was a good fellow. She had nursed him and Alphy too; and Joe used often say:

"Mamma is mamma but Maria is my proper mother."

After the break-up at home the boys had got her that position in the Dublin by Lamplight laundry, and she liked it. She used to have such a bad opinion of Protestants but now she thought they were very nice people, a little quiet and serious, but still very nice people to live with. Then she had her plants in the conservatory and she liked looking after them. She had lovely ferns and wax-plants and, whenever anyone came to visit her, she always gave the visitor one or two slips from her conservatory. There was one thing she didn't like and that was the tracts on the walks; but the matron was such a nice person to deal with, so genteel.

When the cook told her everything was ready she went into the women's room and began to pull the big bell. In a few minutes the women began to come in by twos and threes, wiping their steaming hands in their petticoats and pulling down the sleeves of their blouses over their red steaming arms. They settled down before their huge mugs which the cook and the dummy filled up with hot tea, already mixed with milk and sugar in huge tin cans. Maria superintended the distribution of the barmbrack and saw that every woman got her four slices. There was a great deal of laughing and joking during the meal. Lizzie Fleming said Maria was sure to get the ring and, though Fleming had said that for so many Hallow Eves, Maria had to laugh and say she didn't want any ring or man either; and when she laughed her grey-green eyes sparkled with disappointed shyness and the tip of her nose nearly met the tip of her chin. Then Ginger Mooney lifted her mug of tea and proposed Maria's health while all the other women clattered with their mugs on the table, and said she was sorry she hadn't a sup of porter to drink it in. And Maria laughed again till the tip of her nose nearly met the tip of her chin and till her minute body nearly shook itself asunder because she knew that Mooney meant well though, of course, she had the notions of a common woman.

But wasn't Maria glad when the women had finished their tea and the cook and the dummy had begun to clear away the tea- things! She went into her little bedroom and, remembering that the next morning was a mass morning, changed the hand of the alarm from seven to six. Then she took off her working skirt and her house-boots and laid her best skirt out on the bed and her tiny dress-boots beside the foot of the bed. She changed her blouse too and, as she stood before the mirror, she thought of how she used to dress for mass on Sunday morning when she was a young girl; and she looked with quaint affection at the diminutive body which she had so often adorned, In spite of its years she found it a nice tidy little body.

When she got outside the streets were shining with rain and she was glad of her old brown waterproof. The tram was full and she had to sit on the little stool at the end of the car, facing all the people, with her toes barely touching the floor. She arranged in her mind all she was going to do and thought how much better it was to be independent and to have your own money in your pocket. She hoped they would have a nice evening. She was sure they would but she could not help thinking what a pity it was Alphy and Joe were not speaking. They were always falling out now but when they were boys together they used to be the best of friends: but such was life.

She got out of her tram at the Pillar and ferreted her way quickly among the crowds. She went into Downes's cake-shop but the shop was so full of people that it was a long time before she could get herself attended to. She bought a dozen of mixed penny cakes, and at last came out of the shop laden with a big bag. Then she thought what else would she buy: she wanted to buy something really nice. They would be sure to have plenty of apples and nuts. It was hard to know what to buy and all she could think of was cake. She decided to buy some plumcake but Downes's plumcake had not enough almond icing on top of it so she went over to a shop in Henry Street. Here she was a long time in suiting herself and the stylish young lady behind the counter, who was evidently a little annoyed by her, asked her was it wedding-cake she wanted to buy. That made Maria blush and smile at the young lady; but the young lady took it all very seriously and finally cut a thick slice of plumcake, parcelled it up and said:

"Two-and-four, please."

She thought she would have to stand in the Drumcondra tram because none of the young men seemed to notice her but an elderly gentleman made room for her. He was a stout gentleman and he wore a brown hard hat; he had a square red face and a greyish moustache. Maria thought he was a colonel-looking gentleman and she reflected how much more polite he was than the young men who simply stared straight before them. The gentleman began to chat with her about Hallow Eve and the rainy weather. He supposed the bag was full of good things for the little ones and said it was only right that the youngsters should enjoy themselves while they were young. Maria agreed with him and favoured him with demure nods and hems. He was very nice with her, and when she was getting out at the Canal Bridge she thanked him and bowed, and he bowed to her and raised his hat and smiled agreeably, and while she was going up along the terrace, bending her tiny head under the rain, she thought how easy it was to know a gentleman even when he has a drop taken.

Everybody said: "0, here's Maria!" when she came to Joe's house. Joe was there, having come home from business, and all the children had their Sunday dresses on. There were two big girls in from next door and games were going on. Maria gave the bag of cakes to the eldest boy, Alphy, to divide and Mrs. Donnelly said it was too good of her to bring such a big bag of cakes and made all the children say:

"Thanks, Maria."

But Maria said she had brought something special for papa and mamma, something they would be sure to like, and she began to look for her plumcake. She tried in Downes's bag and then in the pockets of her waterproof and then on the hallstand but nowhere could she find it. Then she asked all the children had any of them eaten it -- by mistake, of course -- but the children all said no and looked as if they did not like to eat cakes if they were to be accused of stealing. Everybody had a solution for the mystery and Mrs. Donnelly said it was plain that Maria had left it behind her in the tram. Maria, remembering how confused the gentleman with the greyish moustache had made her, coloured with shame and vexation and disappointment. At the thought of the failure of her little surprise and of the two and fourpence she had thrown away for nothing she nearly cried outright.

But Joe said it didn't matter and made her sit down by the fire. He was very nice with her. He told her all that went on in his office, repeating for her a smart answer which he had made to the manager. Maria did not understand why Joe laughed so much over the answer he had made but she said that the manager must have been a very overbearing person to deal with. Joe said he wasn't so bad when you knew how to take him, that he was a decent sort so long as you didn't rub him the wrong way. Mrs. Donnelly played the piano for the children and they danced and sang. Then the two next-door girls handed round the nuts. Nobody could find the nutcrackers and Joe was nearly getting cross over it and asked how did they expect Maria to crack nuts without a nutcracker. But Maria said she didn't like nuts and that they weren't to bother about her. Then Joe asked would she take a bottle of stout and Mrs. Donnelly said there was port wine too in the house if she would prefer that. Maria said she would rather they didn't ask her to take anything: but Joe insisted.

So Maria let him have his way and they sat by the fire talking over old times and Maria thought she would put in a good word for Alphy. But Joe cried that God might strike him stone dead if ever he spoke a word to his brother again and Maria said she was sorry she had mentioned the matter. Mrs. Donnelly told her husband it was a great shame for him to speak that way of his own flesh and blood but Joe said that Alphy was no brother of his and there was nearly being a row on the head of it. But Joe said he would not lose his temper on account of the night it was and asked his wife to open some more stout. The two next-door girls had arranged some Hallow Eve games and soon everything was merry again. Maria was delighted to see the children so merry and Joe and his wife in such good spirits. The next-door girls put some saucers on the table and then led the children up to the table, blindfold. One got the prayer-book and the other three got the water; and when one of the next-door girls got the ring Mrs. Donnelly shook her finger at the blushing girl as much as to say: 0, I know all about it! They insisted then on blindfolding Maria and leading her up to the table to see what she would get; and, while they were putting on the bandage, Maria laughed and laughed again till the tip of her nose nearly met the tip of her chin.

They led her up to the table amid laughing and joking and she put her hand out in the air as she was told to do. She moved her hand about here and there in the air and descended on one of the saucers. She felt a soft wet substance with her fingers and was surprised that nobody spoke or took off her bandage. There was a pause for a few seconds; and then a great deal of scuffling and whispering. Somebody said something about the garden, and at last Mrs. Donnelly said something very cross to one of the next-door girls and told her to throw it out at once: that was no play. Maria understood that it was wrong that time and so she had to do it over again: and this time she got the prayer-book.

After that Mrs. Donnelly played Miss McCloud's Reel for the children and Joe made Maria take a glass of wine. Soon they were all quite merry again and Mrs. Donnelly said Maria would enter a convent before the year was out because she had got the prayer-book. Maria had never seen Joe so nice to her as he was that night, so full of pleasant talk and reminiscences. She said they were all very good to her.

At last the children grew tired and sleepy and Joe asked Maria would she not sing some little song before she went, one of the old songs. Mrs. Donnelly said "Do, please, Maria!" and so Maria had to get up and stand beside the piano. Mrs. Donnelly bade the children be quiet and listen to Maria's song. Then she played the prelude and said "Now, Maria!" and Maria, blushing very much began to sing in a tiny quavering voice. She sang I Dreamt that I Dwelt, and when she came to the second verse she sang again:

I dreamt that I dwelt in marble halls
  With vassals and serfs at my side,
And of all who assembled within those walls
  That I was the hope and the pride.

I had riches too great to count; could boast
  Of a high ancestral name,
But I also dreamt, which pleased me most,
  That you loved me still the same.

But no one tried to show her her mistake; and when she had ended her song Joe was very much moved. He said that there was no time like the long ago and no music for him like poor old Balfe, whatever other people might say; and his eyes filled up so much with tears that he could not find what he was looking for and in the end he had to ask his wife to tell him where the corkscrew was.

PD-icon.svg This work is in the public domain in the United States because it was published before January 1, 1923.

The author died in 1941, so this work is also in the public domain in countries and areas where the copyright term is the author's life plus 60 years or less. This work may also be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works.

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

CLAY (from O. Eng. claeg, a word common in various forms to Teutonic languages, cf. Ger. Klei), commonly defined as a fine-grained, almost impalpable substance, very soft, more or less coherent when dry, plastic and retentive of water when wet; it has an "earthy" odour when breathed upon or moistened, and consists essentially of hydrous aluminium silicate with various impurities. Of clay are formed a great number of rocks, which collectively are known as "clay-rocks" or "pelitic rocks" (from Gr. irnX6s, clay), e.g. mudstone, shale, slate: these exhibit in greater or less perfection the properties above described according to their freedom from impurities. In nature, clays are rarely free from foreign ingredients, many of which can be detected with the unaided eye, while others may be observed by means of the microscope. The commonest impurities are: - (1) organic matter, humus, &c. (exemplified by clay-soils with an admixture of peat, oil shales, carbonaceous shales); (2) fossils (such as plants in the shales of the Lias and Coal Measures, shells in clays of all geological periods and in fresh water marls); (3) carbonate of lime (rarely altogether absent, but abundant. in marls, cement-stones and argillaceous limestones); (4) sulphide of iron, as pyrite or marcasite (when finely diffused, giving the clay a dark grey-blue colour, which weathers to brown - e.g. London Clay; also as nodules and concretions, e.g. Gault); (5) oxides of iron (staining the clay bright red when ferric oxide, red ochre; yellow when hydrous, e.g. yellow ochre); (6) sand or detrital silica (forming loams, arenaceous clays, argillaceous sandstones, &c.). Less frequently present are the following: - rock salt (Triassic clays, and marls of Cheshire, &c.); gypsum (London Clay, Triassic clays); dolomite, phosphate of lime, vivianite (phosphate of iron), oxides of manganese, copper ores (e.g. Kupferschiefer), wavellite and amber. As the impurities increase in amount the clay rocks pass gradually into argillaceous sands and sandstones, argillaceous limestones and dolomites, shaly coals and clay ironstones.

Natural clays, even when most pure, show a considerable range of composition, and hence cannot be regarded as consisting of a single mineral; clay is a rock, and has that variability which characterizes all rocks. Of the essential properties of clay some are merely physical, and depend on the minute size of the particles. If any rock be taken (even a piece of pure quartz) and crushed to a very fine powder, it will show some of the peculiarities of clays; for example, it will be plastic, retentive of moisture, impermeable to water, and will shrink to some extent if the moist mass be kneaded, and then allowed to dry. It happens, however, that many rocks are not disintegrated to this extreme degree by natural processes, and weathering invariably accompanies disintegration. Quartz, for example, has little or no cleavage, and is not attacked by the atmosphere. It breaks up into fragments, which become rounded by attrition, but after they reach a certain minuteness are borne along by currents of water or air in a state of suspension, and are not further reduced in size. Hence sands are more coarse grained than clays. A great number of rock-forming minerals, however, possess a good cleavage, so that when bruised they split into thin fragments;. many of these minerals decompose somewhat readily, yielding secondary minerals, which are comparatively soft and have a scaly character, with eminently perfect cleavages, which facilitate splitting into exceedingly thin plates. The principal substances of this description are kaolin, muscovite and chlorite. Kaolin and muscovite are formed principally after felspar (and the felspars are the commonest minerals of all crystalline rocks); also from nepheline, leucite, scapolite and a variety of other rock-forming minerals. Chlorite arises from biotite, augite and hornblende. Serpentine, which may be fibrous or scaly, is a. secondary product of olivine and certain pyroxenes. Clays consist essentially of the above ingredients (although serpentine is not known to take part in them to any extent, it is closely allied to chlorite). At the same time other substances are produced as decomposition goes on. They are principally finely divided quartz, epidote, zoisite, rutile, limonite, calcite, pyrites,. and very small particles of these are rarely absent from natural clays. These fine-grained materials are at first mixed with broken and more or less weathered rock fragments and coarser mineral particles in the soil and subsoil, but by the action of wind and rain they are swept away and deposited in distant situations. "Loess" is a fine calcareous clay, which has been wind-borne, and subsequently laid down on the margins of dry steppes and deserts. Most clays are waterborne, having been carried from the surface of the land by rain and transported by the brooks and rivers into lakes or the sea. In this state the fine particles are known as "mud." They are deposited where the currents are checked and the water becomes very still. If temporarily laid down in other situations they are ultimately lifted again and removed. A little clay, stirred up with water in a glass vessel, takes hours to settle, and even after two or three days some remains in suspension; in fact, it has been suggested that in such cases the clay forms a sort of "colloidal solution" in the water. Traces of dissolved salts, such as common salt, gypsum or alum, greatly accelerate deposition. For these reasons the principal gathering places of fine pure clays are deep, still lakes, and the sea bottom at considerable distances from the shore. The coarser materials settle nearer the land, and the shallower portions of the sea floor are strewn with gravel and sand, except in occasional depressions and near the mouths of rivers where mud may gather. Farther out the great mud deposits begin, extending from 50 to 200 m. from the land, according to the amount of sediment brought in, and the rate at which the water deepens. A girdle of mud accumulations encircles all the continents. These sediments are fine and tenacious; their principal components, in addition to clay, being small grains of quartz, zircon, tourmaline, hornblende, felspar and iron compounds. Their typical colour is blackishblue, owing to the abundance of sulphuretted hydrogen; when fresh they have a sulphurous odour, when weathered they are brown, as their iron is present as hydrous oxides (limonite, &c.). These deposits are tenanted by numerous forms of marine life, and the sulphur they contain is derived from decomposing organic matter. Occasionally water-logged plant debris is mingled with the mud. In a few places a red colour prevails, the iron being mostly oxidized; elsewhere the muds are green owing to abundant glauconite. Traced landwards the muds become more sandy, while on their outer margins they grade into the abysmal deposits, such as the globigerina ooze (see Ocean And Oceanography). Near volcanoes they contain many volcanic minerals, and around coral islands they are often in large part calcareous.

Microscopic sections of some of the more coherent clays and shales may be prepared by saturating them with Canada balsam by long boiling, and slicing the resultant mass in the same manner as one of the harder rocks. They show that clay rocks contain abundant very small grains of quartz (about o oi to o 05 mm. in diameter), with often felspar, tourmaline, zircon, epidote, rutile and more or less calcite. These may form more than one-third of an ordinary shale; the greater part, however, consists of still smaller scales of other minerals (o oi mm. in diameter and less than this). Some of these are recognizable as pale yellowish and white mica; others seem to be chlorite, the remainder is perhaps kaolin, but, owing to the minute size of the flakes, they yield very indistinct reactions to polarized light. They are also often stained with iron oxide and organic substances, and in consequence their true nature is almost impossible to determine. It is certain, however, that the finer-grained rocks are richest in alumina, and in combined water; hence the inference is clear that kaolin or some other hydrous aluminium silicate is the dominating constituent. These results are confirmed by the mechanical analysis of clays. This process consists in finely pulverizing the soil or rock, and levigating it in vessels of water. A series of powders is obtained progressively finer according to the time required to settle to the bottom of the vessel. The clay is held to include those particles which have less than o o05 mm. diameter, and contains a higher percentage of alumina than any of the other ingredients.

As might be inferred from the differences they exhibit in other respects, clay rocks vary greatly in their chemical composition. Some of them contain much iron (yellow, blue and red clays); others contain abundant calcium carbonate (calcareous clays and marls). Pure clays, however, may be found almost quite free from these substances. Their silica ranges from about 60 to 45%, varying in accordance with the amount of quartz and alkali-felspar present. It is almost always more than would be the case if the rock consisted of kaolin mixed with muscovite.

Alumina is high in the finer clays (18 to 30%), and they are the most aluminous of all sediments, except bauxite. Magnesia is never absent, though its amount may be less than i %; it is usually contained in minerals of the chlorite group, but partly also in dolomite. The alkalis are very interesting; often they form 5 or io % of the whole rock; they indicate abundance of white micas or of undecomposed particles of felspar. Some clays, however, such as fireclays, contain very little potash or soda, while they are rich in alumina; and it is a fair inference that hydrated aluminous silicates, such as kaolin, are well represented in these rocks. There are, in fact, a few clays which contain about 45% of alumina, that is to say, more than in pure kaolin. It is probable that these are related to bauxite and certain kinds of laterite.

A few of the most important clay rocks, such as china-clay, brick-clay, red-clay and shale, may be briefly described here. China-clay is white, friable and earthy. It occurs in regions of granite, porphyry and syenite, and usually occupies funnelshaped cavities of no great superficial area, but of considerable depth. It consists of very fine scaly kaolin, larger, shining plates of white mica, grains of quartz and particles of semi-decomposed felspar, tourmaline, zircon and other minerals, which originally formed part of the granite. These clays are produced by the decomposition of the granite by acid vapours, which are discharged after the igneous rock has solidified ("fumarole or pneumatolytic action"). Fluorine and its compounds are often supposed to have been among the agencies which produce this change, but more probably carbonic acid played the principal role. The felspar decomposes into kaolin and quartz; its alkalis are for the most part set free and removed in solution, but are partly retained in the white mica which is constantly found in crude china-clays. Semi-decomposed varieties of the granite are known as china-stone. The kaolin may be washed away from its original site, and deposited in hollows or lakes to form beds of white clay, such as pipe-clay; in this case it is always more or less impure. Yellow and pinkish varieties of china-clay and pipe-clay contain a small quantity of oxide of iron. The best known localities for china-clay are Cornwall, Limoges (France), Saxony, Bohemia and China; it is found also in Pennsylvania, N. Carolina and elsewhere in the United States.

Fire-clays include all those varieties of clay which are very refractory to heat. They must contain little alkalis, lime, magnesia and iron, but some of them are comparatively rich in silica. Many of the clays which pass under this designation belong to the Carboniferous period, and are found underlying seams of coal. Either by rapid growth of vegetation, or by subsequent percolation of organic solutions, most of the alkalis and the lime have been carried away.

Any argillaceous material, which can be used for the manufacture of bricks, may be called a brick-clay. In England, Kimmeridge Clay, Lias clays, London Clay and pulverized shale and slate are all employed for this purpose. Each variety needs special treatment according to its properties. The true brick-clays, however, are superficial deposits of Pleistocene or Quaternary age, and occur in hollows, filled-up lakes and deserted stream channels. Many of them are derived from the glacial boulder-clays, or from the washing away of the finer materials contained in older clay formations. They are always very impure.

The red-clay is an abysmal formation, occurring in the sea bottom in the deepest part of the oceans. It is estimated to cover over fifty millions of square miles, and is probably the most extensive deposit which is in course of accumulation at the present day. In addition to the reddish or brownish argillaceous matrix it contains fresh or decomposed crystals of volcanic minerals, such as felspar, augite, hornblende, olivine and pumiceous or palagonitic rocks. These must either have been ejected by submarine volcanoes or drifted by the wind from active vents, as the fine ash discharged by Krakatoa was wafted over the whole globe. Larger rounded lumps of pumice, found in the clay, have probably floated to their present situations, and sank when decomposed, all their cavities becoming filled with sea water. Crystals of zeolites (phillipsite) form in the red-clay as radiate, nodular groups. Lumps of manganese oxide, with a black, shining outer surface, are also characteristic of this deposit, and frequently encrust pieces of pumice or animal remains. The only fossils of the clay are radiolaria, sharks' teeth and the ear-bones of whales, precisely those parts of the skeleton of marine creatures which are hardest and can longest survive exposure to sea-water. Their comparative abundance shows how slowly the clay gathers. Small rounded spherules of iron, believed by some to be meteoric dust, have also been obtained in some numbers. Among the rocks of the continents nothing exactly the same as this remarkable deposit is known to occur, though fine dark clays, with manganese nodules, are found in many localities, accompanied by other rocks which indicate deep-water conditions of deposit.

Another type of red-clay is found in caves, and is known as cave-earth or red-earth (terra rossa). It is fine, tenacious and bright red, and represents the insoluble and thoroughly weathered impurities which are left behind when the calcareous matter is removed in solution by carbonated waters. Similar residual clays sometimes occur on the surface of areas of limestone in hollows and fissures formed by weathering.

Boulder-clay is a coarse unstratified deposit of fine clay, with more or less sand, and boulders of various sizes, the latter usually marked with glacial striations.

Some clay rocks which have been laid down by water are very uniform through their whole thickness, and are called mud-stones. Others split readily into fine leaflets or laminae parallel to their bedding, and this structure is accentuated by the presence of films of other materials, such as sand or vegetable debris. Laminated clays of this sort are generally known as shales; they occur in many formations but are very common in the Carboniferous. Some of them contain much organic debris, and when distilled yield paraffin oil, wax, compounds of ammonia, &c. In these oil-shales there are clear, globular, yellow bodies which seem to be resinous. It has been suggested that the admixture of large quantities of decomposed freshwater algae among the original mud is the origin of the paraffins. In New South Wales, Scotland and several parts of America such oil-shales are worked on a commercial scale. Many shales contain great numbers of ovoid or rounded septarian nodules of clay ironstone. Others are rich in pyrites, which, on oxidation, produces sulphuric acid; this attacks the aluminous silicates of the clay and forms aluminium sulphate (alum shales). The lias shales of Whitby contain blocks of semi-mineralized wood, or jet, which is black with a resinous lustre, and a fibrous structure. The laminated structure of shales, though partly due to successive very thin sheets of deposit, is certainly dependent also on the vertical pressure exerted by masses of superincumbent rock; it indicates a transition to the fissile character of clay slates. (J. S. F.)

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From BibleWiki

This word is used of sediment found in pits or in streets (Isa 57:20; Jer 38:60), of dust mixed with spittle (Jn 9:6), and of potter's clay (Isa 41:25; Nah 3:14; Jer 18:1-6; Rom 9:21). Clay was used for sealing (Job 38:14; Jer 32:14). Our Lord's tomb may have been thus sealed (Mt 27:66). The practice of sealing doors with clay is still common in the East. Clay was also in primitive times used for mortar (Gen 11:3). The "clay ground" in which the large vessels of the temple were cast (1 Kg 7:46; 2Chr 4:17) was a compact loam fitted for the purpose. The expression literally rendered is, "in the thickness of the ground,", meaning, "in stiff ground" or in clay.

This entry includes text from Easton's Bible Dictionary, 1897.

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Simple English

Clay is a type of soil that is sticky to touch. It is often used in the making of gardening pots. Depending on the type, it sometimes has a gritty texture, and is sometimes very smelly. Clays are also complex materials containing many different types of atoms, however, all clays contain large numbers of silicon (Si) and oxygen (O) atoms, joined together to form silicate. They form when rocks are broken down or weathered. To make pottery, clay is molded in shape, dried, and then fired in a furnace or kiln to make it hard.

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