It is common for draftsmen to sprinkle SCUMEX (powdered rubber
eraser) on tracings prior to doing any drawing on it. This reduces
smearing of the pencil marks and such and results in a cleaner
tracing.

At a former employers we had replaced the Scumex at one draftsmans
desk with dried parmesian cheese. It looked about the same. It was
extremely interesting watching him draw for a while and then begin
to smell the paper. Took the poor dude almost 10 minutes to guess
that he had been gigged!