:\lessrs Brown and AIawe of Derby, and in the Strand, London, have lately introduced etching upon the very elegant articles which they form of various coloured marbles and gypsum. Vases and other antique orna ments are beautifully enriched with figures etched upon them by acids. If sulphuric acid of tolerable strength be laid upon polished gypsum, it gives to the part a dead white, which forms a good contrast with the po lished part. For marbles or any carbonate of lime, the uturiatic acid will answer best. The etching has an excellent effect upon black marble. It does not merely deaden the polish, but forms a greyish ground, which is an agreeable contrast to the black polish.
Although glass plates do not answer for prints, etch ing has been employed to otnament glass with good effect. The corrosion of glass by the fluoric acid gives it the appearance of ground glass, so that when the fi gures formed upon glass are defended from the acid by any substance, the ground becomes opaque, forming a good contrast to the transparent part. The latter is ge rally painted representing figures or flowers, in the man ner of painting transparencies. The common etching ground is generally used to cover the glass, in the same manner as the copperplates are first coated, taking the same pains to spread it uniformly. When cold, the figures, but more commonly the ground, require to be formed partly by needles of different sizes; and a flat pointed tool when greater surfaces are to be removed. As soon as those parts intended to be corroded are cut out, they are surrounded with soft wax, as directed to be placed round the margin of the coppe•-plate, to form a recess for placing the acid. The etching is performed by three processes: I. By the liquid acid; 2. By pound ed fluor spar; and 3. By the acid gas, or what .is more properly called the superfluat of silex. The liquid acid is obtained by distilling the gas from fluat of lime and sulphuric acid, from a leaden retort into a receiver of the same metal, surrounded with ice, and containing a little water. The liquid obtained is to be employed on the glass in the same manner as the dilute nitric acid in etching copper plates. The second method is the most simple, and is on that account generally practised. It consists in first reducing the fluat of lime, commonly called fluor spar, and Blue John, to powder. When the sides of the outlined plate are surrounded with wax to about three-fourths of an inch high, the powdered fluor spar is to be spread upon the plate in a uniform stratum about 3-16ths of an inch thick. The remainder of the cavity must now be filled with diluted sulphuric acid, the water being to the acid as about three to one by weight. It must be placed in a warm situation, but not so hot as to melt the wax or varnish. The sulphuric
acid soon begins to liberate the fluoric acid, which in its nascent state corrodes the glass. The gas which es capes carries the silex of the glass along with it in the form of supeilluat of silex. The fluat of lime used for this purpose should be very pure, that is, free front silex. If the latter be present, the acid will be apt to escape with it, instead of getting the silex from the glass, by which it becomes corroded. The same evil occurs when the bodies to be corroded are exposed to the gas in which silex is present, because it is already saturated with what it should take from the glass. The fluoric acid free from silex puts on the elastic form at a very low temperature, at less than GO°. Hence glass may be very advantageously corroded by being immerg ed in this vapour. As it combines with the silex of the glass, it becomes a more elastic fluid, called the super. fluat of silex. In etching round vessels of glass, it is not so convenient to use the wall of wax to contain the liquid ; and hence it will be better to etch bodies of such form with vapour, observing not to apply it so hot as to melt the wax.
When the plate has been exposed to the action of pulp composed of pounded fluor spar and sulphuric acid, till the finest lines are corroded sufficiently, which can only be ascertained by trial and observation, the acid mass is to be removed, and the plate washed with clean water, and dried with a low heat. The parts corroded sufficiently are to be stopped out with turpentine varnish used in etching copper. The rest of the process will be precisely the same as that of the copper-plates, ap plying every time the same quantity of pounded spar and acid. The pulp which has been used once, may sometimes be used again, since it will be capable of corroding so long as any effervescence appears.
This art of etching upon steel, has been practised at Sheffield and Birmingham for ornamenting polished steel. The corroded part becomes a dead white, while the part unaffected remains polished. Those parts in tended to be preserved from the acid, are drawn with turpentine varnish, while that which is to form the white ground is left bare. The space where the acid is to be exposed is first surrounded by a wall, of a mixture of bees wax and pitch, with the addition of a little tallow. The acid employed is the nitric, diluted with 3 or 4 parts of water, more or less, according to the strength of the acid. The proportions will be those which produce the whitest ground. This will be easily formed, by try ing different strengths of acid, with a pair of polished steels kept for the purpose. This beautiful art is much assisted by bluing and gilding. (e. s.)