ETCH′ING

A species of engraving, in which the design is formed on the plate by the action of an acid, or some other fluid, instead of being cut out by the graver. Proc. In the ORDINARY PROCESS OF ETCHING the plate is covered with ‘etching ground’ (an acid-resisting varnish), and the design is scratched on the metal through the ground, by means of a pointed tool of steel called the ‘etching needle’ or ‘point.’ A border of wax is then placed round the plate, and the ‘biting’ fluid poured on, and allowed to remain till the ‘lights’ or finest portions of the design are sufficiently ‘bitten in,’ The etching fluid is then poured off, the plate washed, and the light parts ‘stopped out’ with Brunswick black or other varnish; the solvent is again poured on, and allowed to remain until the finest portion of the exposed lines are sufficiently deep, when the acid is again poured off, and the whole proce is repeated till the very darkest lines or shadows are sufficiently ‘bitten in,’ The plate is then cleaned, and is ready to be printed from. Occasionally the etched design receives a few finishing touches with the ‘graver.’ There are several varieties of etching, of which the following are the principal:— Etching with a soft ground , when a coating of lard or tallow is employed, and the design is drawn on a piece of paper, laid evenly on the ground, by which means the fatty matter adheres to the paper, on the parts pre ed on by the point or pencil, and the copper beneath becomes exposed, and is then acted on by the acid. The effect resembles that of chalk or pencil drawings.— Stippling , or executing the design in dots instead of lines.— Aquatinta or AQUATINT , a mode of etching on copper for producing an effect resembling a drawing in Indian ink. It is performed by sifting powdered asphaltum or lac resin on the plate, previously slightly greased, and, after shaking off the loose powder, gently heating it over a chafing dish; on cooling, the lights are covered with turpentine varnish coloured with lampblack, by means of a hair pencil, and a rim of wax being placed round the plate, a mixture of ‘aquafortis’ and water is poured on it, and allowed to remain for 5 or 6 minutes, when it is poured off, the plate dried, and recourse had to the pencil as before. The proce of ‘stopping’ and ‘etching’ is repeated again and again, until the darkest shades are produced. Sometimes, instead of using asphaltum, an alcoholic solution of shell-lac or gum mastic is poured over the plate, placed in a slanting direction; this varnish forms a film, which, on drying, leaves innumerable cracks or minute fi ures through which the acid acts on the plate. The finene or coarsene of the grain depends entirely upon the condition of the powdered asphaltum, or on the quantity of matter di olved in the spirit employed to form the ground. The fluids employed for ‘biting in’ the designs vary considerably, almost every artist having his own receipt. Aquafortis, more or le diluted, is, however, generally employed for COPPER , and this, with the addition of pyroligneous acid, for etching on STEEL ; but any fluid that rapidly di olves the metal may be used for the purpose. The ‘etching ground’ may be formed of any substance capable of resisting the action of the etching fluid, and which is, at the same time, sufficiently soft to allow of the free use of the needle or point, and sufficiently solid to prevent an injury to the design during the ‘scratching in,’ In ETCHING ON GLASS , the ground is laid on, and the design ‘scratched in’ in the usual way, when liquid hydrofluoric acid is applied, or the gla is exposed to the action of hydrofluoric acid gas. The former renders the surface of the etching transparent, the latter opaque. A simple modification of the proce is to wet the design with sulphuric acid, and then to sprinkle on some finely pulverised fluor spar (fluoride of calcium), by which means hydrofluoric acid is set free and attacks the gla . This method may be very easily applied to the graduation of gla ve els, thermometer tubes, . Etching on Gla by Electricity. Planté (‘Ann. Chem. Phys.’ [5], xiii, 143-144). The author had previously drawn attention to the fact that when an electric current is pa ed through saline solutions in gla ve els, platinum wire serving as electrodes, the gla is immediately attacked, and he therefore proposes the following method for etching on gla : The surface of the gla to be engraved is coated with a concentrated solution of pota ium nitrate, and beneath the layer of liquid a platinum wire, connected with one of the poles of a battery, is stretched acro the plate. With the other pole is connected another platinum wire, the whole of which, except the point, is insulated; with this the designs are drawn on the gla , which is engraved wherever the wire comes in contact with it, flashes of light being emitted at the same time. The depth of engraving depends on the rate at which the platinum wire moves; the slower the rate the deeper the line. A rapid method of etching on iron or steel , capable of very general application, is as follows:—“The metal is warmed until it is capable of melting a piece of beeswax, or ‘etching ground,’ which is then carefully rubbed over it, so as to form a thin and even coating; when cold, the design is ‘scratched in’ in the common way; a little powdered iodine is then sprinkled on the exposed parts, and at the same time a few drops of water are added, and the two worked into a liquid paste with a camel-hair pencil. The paste is then moved about over the intended etching, for a period varying from one to five minutes, according to the depth of the lines required to be produced. Afterwards the whole is removed, and reapplied, ., as with the usual etching fluids. The same etching-paste, by being kept for a few days, again acquires the property of di olving iron, and may be used again and again; but independently of this, the iodide of iron formed during the proce , if rapidly evaporated to dryne in a clean iron ve el by a moderate heat, and placed in stoppered bottles, will sell for more than the original cost of the iodine. To travellers and amateurs who amuse themselves with the delightful art of etching, iodine, from its portability and convenience, will, doubtle , prove invaluable. We have adopted it with considerable succe , and have found it especially useful in marking surgical instruments, razors, and other edge tools. We published this method many years ago. Several parties have since availed themselves of our suggestions and formulæ, but without the slightest acknowledgment of the source from which they obtained them.” (A. J. Cooley.)