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 pressed 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 process 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 fissures through which the acid acts on the plate. The fineness or coarseness of the grain depends entirely upon the condition of the powdered asphaltum, or on the quantity of matter dissolved 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 less diluted, is, however, generally employed for COPPER, and this, with the addition of pyroligneous acid, for etching on STEEL; but any fluid that rapidly dissolves 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 glass 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 process 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 glass. This method may be very easily applied to the graduation of glass vessels, thermometer tubes, &c.
Etching on Glass 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 passed through saline solutions in glass vessels, platinum wire serving as electrodes, the glass is immediately attacked, and he therefore proposes the following method for etching on glass:
The surface of the glass to be engraved is coated with a concentrated solution of potassium nitrate, and beneath the layer of liquid a platinum wire, connected with one of the poles of a battery, is stretched across 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 glass, 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, &c., as with the usual etching fluids. The same etching-paste, by being kept for a few days, again acquires the property of dissolving iron, and may be used again and again; but independently of this, the iodide of iron formed during the process, if rapidly evaporated to dryness in a clean iron vessel 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, doubtless, prove invaluable. We have adopted it with considerable success, 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.)
Etching, Electro-. This mode of etching, which is in many respects superior to the ordinary mode, is based upon the destructive action of certain ‘anions’ during ‘electrolysis.’[281] If two plates of copper be connected with the opposite ends of a voltaic battery, and placed in a vessel containing very dilute sulphuric acid, the plate connected with the copper of the battery will be attacked by the anion oxygen which is released during the decomposition of the acid. This destructive action can be localised at pleasure by covering certain parts of the plate with a protecting stratum of varnish, ordinary ‘etching ground’ for instance. In the practice of electro-etching, the drawing is ‘scratched in’ in the usual way through an ordinary ground; a stout wire is then soldered to the plate, and this, as well as the back of the plate, is coated with sealing-wax varnish. Thus prepared, the plate is placed in a suitable ‘decomposition cell’ opposite a plate of somewhat similar size, and the two are connected respectively with the copper and zinc of a ‘Daniell’s cell,’ or the silver and zinc of a ‘Smee’s cell.’[282] After about ten minutes the plate is removed, washed, and dried; and when the ‘fine work’ has been stopped out with Brunswick black, it is returned for another space of ten minutes. By alternately exposing the plate to the action of the decomposing fluid, and ‘stopping out’ parts of the work, the required gradation in tints is obtained. The exact duration of the various exposures, as well as their number, must, of course, be regulated by circumstances. See Etching Fluids (below).
[281] See Electrolysis and Electrotype, pages 428 and 429.