(a) The ordinary embossing one sees on shop-doors, etc, is almost always done on plate-glass. A drawing on paper is first made; then by placing a piece of "transfer paper" (this can be bought at any artists' colour shop) under it and going over the design again, we obtain a reversed drawing on the other side of the paper. This is placed under the glass to work by. Those parts which are not to be acted on by the acid (hydrofluoric) are "protected" by a coating of Brunswick black, using a camel-hair brush. In a few hours or the next day a ridge of tallow is made around the design about 3/4 in. high; the glass is placed perfectly level, and the acid is poured on to the depth of 1/8 to 1/4 in. When etched deep enough (a trial is made on a small piece of glass previously, as the time varies with the strength of the acid), in about ten minutes, pour off at one corner, wash with water, remove the black with turpentine, and clean well. The parts not acted on by the acid can now be ground with a small square of plate-glass and emery (medium). If the acid is too strong, it will not give sufficient time to pour on steadily. If it is too weak, it will require more than ten minutes. Good acid direct from the makers will take one-half water at least.
The acid and water may be mixed in an ordinary clay pipkin, with handle such as used by gilders; but first melt some beeswax in it, and turn it about so as to give it a perfect coating, or the acid will eat its way through in a very short time.
Some very pretty effects are produced by what is known as "white" or "frosting" acid, used in conjunction with the ordinary hydrofluoric acid. For instance, we get out a design of a stork standing in the water among some rushes and water-lilies. The sky, water-lilies, and rushes may be frosted with the white acid, likewise some short horizontal lines on the water. The stork and the water may be done with the ordinary hydrofluoric, and will be semi-transparent. The outlines of all the objects must have a burnished line around them - that is, the feathers of the stork, the petals of the flowers, etc. The burnished line is clear- glass that has been protected by the Brunswick black.
The method is to protect all the burnished outlines and all the parts intended to be frosted, then treat with hydrofluoric acid in the way previously described, then clean all off nicely, and protect all the outlines again and all the parts acted on by the acid, and then pour on the white acid; the white acid must not be diluted.
White acid is prepared by adding ammonia to strong hydrofluoric acid, together with a preparation of barium. The reason amateurs do not succeed in making it is, they cannot, as a rule, procure the fluoric acid strong enough.
(b) Cosmos recommends the following for marking designs or inscriptions on glass bottles, etc.: - Dissolve about .72 oz. fluoride of soda with .14 02. sulphate of potash in 1/2 pint of water. Make another solution of .28 oz. chloride of zinc and 1.30 oz. hydrochloric acid in an equal quantity of water. Mix the solutions, and apply to the glass vessel with a pin or brush. At the end of half an hour the design should be sufficiently etched.
(c) Another process, devised by Meth and Kreitner, of Berlin, is given in Invention as follows: - A mixture consisting of ammonium fluoride, common salt, and carbonate of soda is prepared, and then placed in a guttapercha bottle containing fuming hydrofluoric acid and concentrated sulphuric acid. In a separate vessel which is made of lead, potassium fluoride is mixed with hydrochloric acid, and a little of this solution is added to the former, along with a small quantity of sodium silicate and ammonia. 'some of the solution is dropped upon a rubber pad, and by means of a suitable rubber stamp, bearing the design which is to be reproduced, is transferred to the glass vessel that is to be etched. (See also iii. 233, v. 5, 317.)