Put the shot and emery in the box, fasten the lid on tight and then shake it hard up and down so that the emery and shot will strike the surface of the glass with as much force as possible. Keep this up for 15 minutes or half an hour when the glass will be etched deep enough.
When you open the box you will find that the particles of emery have been embedded in the lead shot and each of the latter has become a cutting tool. This process of etching can be used for metals as well as for glass.
How to Make Ground Glass.
—To make ground glass go about it as above described but in this case no stencil is needed.
The Acid Process.
—Hydrofluoric acid is made by treating fluor-spar[102] with sulphuric acid. The acid which is thus formed acts on glass by eating into it and for this reason it must be kept in either rubber, lead or platinum bottles upon which it has no effect. In etching large surfaces the acid is not put on the glass directly; because it eats so smoothly the effect is not striking enough; instead the following process is used which leaves a rough surface more nearly like that of the sand blast.
[102] Fluor-spar is calcium fluoride; you can buy it of Eimer and Amend, 4th Ave. and 18th St., New York, or of the L. E. Knott Apparatus Co., Boston, Mass.
Make a lead dish the size of the glass you want to etch and with the sides an inch high. Put about an ounce of powdered fluor-spar into the dish and pour enough concentrated sulphuric acid on it to make a thick paste.
Coat the surface of the glass with paraffin, or beeswax and rosin, and then with a steel scriber, or other pointed instrument scratch on your name or the design you intend to etch, clear through to the glass. Lay the glass with the waxed side down on the dish containing the fluor-spar mixture, as shown at A in [Fig. 95], and let it stand over night. The vapor formed by generating hydrofluoric acid in this way attacks the silica, that is the sand, of the glass with which it has a great tendency to unite, and thus the glass disintegrates or is eaten away.
