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,97 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.

97 Fluorspar 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 flour-spar into the dish and pour enough concentrated sulphuric acid on it to make a thick paste.

Fig. 95. Etching Glass With Acid

A. Etching a sheet of glass with fluor-spar.

B. Etching a thermometer tube with hydrofluoric acid.

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.

The next morning when you take off the glass, scrape off the wax and you will find the name or design etched on it.

To etch the graduations on thermometers, burettes, etc., coat them with wax and scratch the lines and figures on them just as described above - but in this case you can put the hydrofluoric acid on direct as shown at B, using a splinter of wood for the purpose.