(1) Glass is etched by hydrofluoric acid, or by hydrofluoric acid gas. The gaseous acid has the property of producing a surface which resembles ground glass in its appearance; the liquid acid produces clear etching. Etching glass, therefore, consists of 2 distinct branches - 1st, the production of a dull image on a clear surface (when the gas is used); 2nd, the production of a clear image on a surface previously ground or dulled by means of the liquid acid. 1st, the glass plate to be etched is cleaned and gently warmed until hot enough to melt wax. The surface is then covered with an equable layer of white wax, by rubbing the wax over it. When cold, the design is cut out of the wax with a graver. A shallow leaden trough, about the size of the plate (but a trifle smaller) is obtained, into which is placed a small quantity of finely - powdered fluorspar. This must be weighed and then gently sifted over the bottom of the trough. To every 2 parts by weight of fluorspar add 3 of good oil of vitriol. Stir quickly with a wooden stick, and place on the hob or other warm place. Vapour will soon rise. Now remove the trough and cover over with the waxed and graved plate, wax side downwards. In a very short time the acid will have etched the bare portions of the glass.

When sufficiently etched, remove the wax by melting. Conduct these operations under a chimney, to carry off the vapours, as they are very injurious. To prepare the liquid acid for clear etching, place 2 parts fluorspar and 3 of sulphuric acid in a leaden retort, the tube of which must dip into a leaden bottle half - filled with water. Apply heat to the retort as long as the water will absorb the fames generated. If a ground glass be prepared with wax, as above, and a ledge of wax or putty be made round it, on pouring the liquid acid on the plate, clear lines on the dull ground will result; or a "flashed" coloured glass may, by the same means, bo made to present a colourless picture on a coloured ground. The sheets of clear glass may themselves be dulled by exposing them, without previously waxing, to the fumes of the acid gas. N.B. - Hydrofluoric acid produces most painful and malignant ulcers.

(2) The etching of photographs on glass is a process little practised by photographers, and yet some pretty applications may be made by ornamenting globes, vases, and drinking - glasses in this manner. It is true some glass is better adapted for the purpose than others, and that which contains a good deal of lead is said to be the most suitable. Any photographic film is sufficiently thick for protecting the glass where the etching liquid is not to act, and designs or pictures may therefore easily be formed capable of resisting the action of hydrofluoric acid, which dissolves the rest of the surface. Glass which is faced with a surface of colour or opal may be worked with great ease, and the acid coming in contact with the unprotected surface dissolves away the facing, and leaves either a design in transparent glass, or one in colour, according as a negative or positive has been made use of to produce the mask upon the glass. A dilute acid is employed for etching in preference to a strong one, and the.time that is required to dissolve away the surface depends in great measure upon the character of the glass itself. A carbon film will suit very well as a mask, and in this case it is well to apply the moist solarized tissue to the glass surface, and develop the image upon the glass.

The image, under these circumstances, holds very tenaciously to the glass surface (which has not been waxed, it must be remembered), and, when dry, the glass object is transferred to the etching - bath, where, if it happens to be a lamp globe, it is kept slowly revolving. The etching - bath is made of guttapercha, or wood protected with sheet - lead, and the object remains herein until the etching has proceeded far enough. When this is the case, the glass is withdrawn, washed, and then the carbon or other images removed in any ready manner that may suggest itself. In the case of designs or lithographs which it is desired to etch upon glass, a special ink is employed, which is applied to the surfaces to be protected from the action of the acid. This ink is a thick solution of asphalte in turpentine, thickened by means of beeswax and rosin. In this case, as also in the case of photographs to be reproduced in ground glass or colours, it is necessary that the glass surface to be etched should, in the first place, be of ground glass or faced with coloured glass, blue, red, green, violet, etc, according to desire.

The lithographic ink or photographic film forming the design then protects the coloured or ground glass, and the rest of the surface being washed away until the transparent glass underneath comes into view, there results a coloured or ground - glass design upon transparent glass.