The term smoked glass refers to two different types of glass. It can be either of the following:
(1) Smoked glass is a flat sheet of glass held in the smoke of a candle flame (or other inefficiently burning hydrocarbon) such that one surface of the sheet of glass is covered in a layer of smoke residue. The glass is then used as a medium for recording pen traces in scientific instruments. The advantages of using the glass are that the recording medium is easily renewable (just re-smoke the glass), and that the trace obtained can easily be magnified by projection onto a suitable surface. A variation on this scheme is the use of smoked paper in early seismographs.
(2) Smoked glass is glass manufactured with darkening materials incorporated, such that light passing through the glass is decreased in brightness. It can be used aesthetically, for example, in the manufacture of coffee tables with smoked glass tops. It can also be used in scientific instruments as a filter, as in the use of smoked glass in cross-staves and sextants, allowing the operator to make sun sightings without damaging their eyesight.