Fancy work, ornamental figures, lettering and monograms, are most easily and neatly cut into glass by the sand blast process, a simple apparatus for which will be found described in the Young Scientist. Lines and figures on tubes, jars, etc., may be deeply etched by smearing the surface of the glass with beeswax, drawing the lines with a steel point, and exposing the glass to the fumes of hydrofluoric acid. This acid is obtained by putting powdered fluorspar into a tray made of sheet lead and pouring sulphuric acid on it, after which the tray is slightly warmed.
The proportions will, of course, vary with the purity of the materials used, fluorspar (except when in crystals) being generally mixed with a large quantity of other matter, but this point need not affect the success of the operation. Enough acid to make a thin paste with the powdered spar will be about right. Where a lead tray is not at hand, the powdered spar may be poured on the glass, and the acid poured on it and left for some time. As a general ride, the marks are opaque, but sometimes they are transparent. In this case, cut them deeply and fill up with black varnish, if they are required to be very plain, as in the case of graduated vessels.
Liquid hydrofluoric acid has been recommended for etching, but as it leaves the surface on which it acts transparent, it is not suitable.
The agent which corrodes the glass is a gas which does not remain in the mixture of fluorspar and sulphuric acid, but passes off in the vapor. To mix fluorspar and sulphuric acid and keep it in leaden bottles under the idea that the mixture is hydrofluoric acid, is a gross mistake. Such an idea could enter into the head of none but the compiler of a cyclopaedia of recipes.