§ 46 Making Ground Glass.
This is easily done by rubbing the surface of polished glass with a bit of cast iron and washed “flour of emery.” Of course, if the fineness of grain of the surface is of importance, appropriate sizes of emery must be employed. The iron may be replaced by a bit of glass cut with transverse grooves to allow the emery to distribute itself, or even by a bit of glass without such grooves, provided it does not measure more than one or two inches each way. If great speed is an object rather than the fineness of the surface, use a bit of lead and coarse emery, say any that will pass a sieve with fifty threads to the inch.
It may perhaps be mentioned here that it is a pity to throw away emery which has been used between glass and glass. In the chapter dealing with fine optical work the use of emery of various grades of fineness will be treated, and the finer grades can only be obtained (to my knowledge) from emery which has been crushed in the process of glass or metal grinding, especially the former. A large jam-pot covered with a cardboard lid does well as a receptacle of washings.