Smoked glass
Smoked glass is 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 used as a medium for recording pen traces in scientific instruments, and is also used to track pheromone deposition in ants The advantages of using the smoked glass are that the recording medium is easily renewable (the glass can be easily re-smoked), and that the trace obtained can easily be projected and magnified.