Smoked glass
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.