A third method of engraving consists in corroding the various lines by means of aquafortis, the remaining parts of the plate being defended from the action of the acid by being covered with a thin stratum of a composition which resists its effects. This method is termed etching, and is employed both for preparing the outline in other styles of engraving, and also for filling in and completing a picture ; and the prints so produced resemble pen and ink draw ings. This style is distinguished for the inimitable spirit and freedom of which it admits. The first stage of the process is the preparation of the plate, by covering it with a thin even film of the composition or ground, as it is termed, which is to protect it from the acid. This ground is a combination of asphaltum, gum mastich, and virgin wax, melted over a fire in an iron pot. A piece of this ground is tied on a piece of lustring for use, and another piece of silk is made into a dabber by tying a quantity of cotton wool in it. The copper-plate, secured at one corner by a hand vice, is to be held over a charcoal fire, and the silk containing the ground rubbed over until every part is covered by the melted composition ; and before it cools, the silk dabber is applied in all directions, until the surface of the plate is thinly and equally varnished. When this is completed, several lengths of wax taper twisted together are to be lighted, and the plate being held in the left hand by the vice, the right holding the wax taper, is to be waved gently to and fro under the ground, carefully avoiding touching it with the wick, yet causing the flame to spread smoothly over the surface, which will render it perfectly black, smooth, and shining in a short time. The next object is to transfer the design to the ground, which may be done either by making a tracing with a black lead pencil, or with vermilion, upon thin paper, and applying it carefully to the plate, pass the plate through a rolling-press ; or the back of the design may be rubbed with red chalk and fas tened to the plate at the corners, and the outline then gone carefully over with a blunt tracer. The outline thus prepared is then gone over with an etching needle, which cuts through the ground ; but particular care must be taken that the point of the needle, though taper, be rounded, so as to avoid the possibility of its tearing the surface of the copper, which would prevent the progress of the point, and ruin the plate when bitten. The different shades and tints are then worked up by the needle, the arrangement of the lines of which they are com posed depending entirely upon the taste and judgment of the artist. When the etching is completed, the edges of the plate are surrounded by a high border of wax, so well secured that water will not penetrate between it and the plate. The best spirits of nitre must then be diluted with water, in the pro portion of one part of the former to four of the latter, and poured upon the plate. In a short time numerous small air bubbles collect over every line traced by the needle through the ground ; these bubbles are caused by the action of the acid upon the copper, and must be removed with a feather. When it is judged that the lighter tints are sufficiently corroded, the acid is poured off the plate, which is then washed, suffered to dry, and the light parts stopped out, as it is termed, that is, covered with a composition of turpentine, varnish, and lamp black, diluted so as to be used freely with a camel's-hair pencil; this prevents the aquafortis from touching these parts again. After this the acid is again applied until the next depth of tint is obtained, which parts are in their turn stopped out; and thus the process continues, alternately biting in and stopping out, until every gradation of tint is obtained. The ground is then removed by covering the-plate with olive oil, heating it, and then wiping it with a piece. of old linen dipped in spirit of turpentine, which effectually removes all remaining dirt. If upon proving the plate, any part should be found not sufficiently cor roded, it must be rebitten, which is effected by applying the ground so care fully as not to fill in the lines, but merely to protect the surface of the plate; and then raising a border of wax round the parts to be rebitten, apply the acid as before. When the operations of etching and rebiting are entirely finished, nothing remains to be done but to examine the plate attentively, and improve it with the graver and dry point.
The last style of copper-plate engraving which we shall notice is the aqua tinta. The prints from an aqua-tinted plate greatly resemble a neatly-tinted Indian ink drawing. This effect is produced by covering the plate with a thin coating of various substances finely granulated, which defend it from the acid where they cover it, whilst the interstices amongst the particles or grains are corroded. The first step is to lay an etching ground, and to bite in lightly the outline, after which the plate is cleaned and polished with whiting, previous to laying the grain. The best mode of laying the grain is as follows: common resin, gum mastich, or Burgundy pitch, is dissolved in highly rectified spirits of wine of the best quality ; each of these substances produce a different description of grain, of which that from resin is the coarsest; but they may be in such proportions as the artist prefers, who, to satisfy himself on this point, should try the grain of each proportion on useless slips of copper. Having obtained a solution to his mind, it must remain undisturbed until every impure particle has subsided, when it is poured upon the plate, which is held slightly slanting, until the most fluid parts run off, after which it is laid to dry, in the progress of which the resin granulates, and adheres firmly to the surface. The grain being thus laid, the various tints are obtained by biting in and stopping out, as in etching.
Another style of engraving has been introduced within these few years, called machine engraving or ruling, in which the lines are ruled with a diamond point on an etching ground, by means of a machine invented for the purpose by the celebrated Lowrie. This machine is capable of producing lines straight or
waved, and either parallel or converging to any given point, as also parabolas, hyperbolas, and most other geometrical curves and circles varying from a point to a five-feet radius. The lines thus ruled are afterwards bitten in, in the usual manner, as in ordinary etching. This style is now universally employed for architectural and mechanical subjects, as also for putting in the buildings and skies in the works of line engravers.
Steel Engraving, as we have already said, is extensively employed in the illustration of works of which very large editions are printed, on account of the durability of the steel plates, which is so great, that artists' proofs have sometimes been taken after 20,000 impressions have been thrown off, whereas a copper-plate will generally require touching after 1500 or 2000 impressions. But before steel can be cut by the graver it requires to be softened, by depriving it of a portion of its carbon, which is effected by imbedding the steel to the depth of half an inch on all sides in a bed of pure iron filings, contained in a cast iron box with a well closed lid. In this box the steel is to be exposed for four hours to a white heat, after which it is to be suffered to cool very slowly, which is best effected by shutting off all access of air to the furnace, and covering the box with a layer of fine cinders to the depth of six or seven inches. After the steel plate has been engraved, (which engraving may be in any of the styles practised on copper,) it requires to be hardened or reconverted into steel, which is effected by the following process : a suitable quantity of leather is reduced to charcoal, by exposing it to a red heat in an iron retort for a sufficient length of time ; a cast iron box, whose cavity is about an inch greater than the thickness of the steel plate, is then to be filled with this charcoal reduced to a fine powder, and being covered with a well-fitted lid, it is to be exposed to a heat somewhat above a red heat, until all the volatile or evaporable parts are driven off from the charcoal. The lid is then removed, and the plate immersed in the charcoal, as nearly as possible in the middle, so as to surround it on all sides with a stratum of uniform thickness. The lid being replaced, the box must remain in the degree of heat before described, from three to five hours, according to the thickness of the plate. After remaining in the fire the requisite time, the plate is taken from the box and plunged immediately into cold water, from which it must be withdrawn before the hissing noise has ceased ; but the precise point for this cannot be explained in words, and can only be learned by actual observation. The plate is then to be immediately laid over a fire, until its temperature is raised to that degree that smoke would arise upon rubbing the surface with tallow, when it must be again plunged into water, where it remains until the hissing sound becomes somewhat weaker than before. The process of heating and cooling is then to be twice repeated, after which, the surface of the plate is to be cleaned, and the temper finally reduced by heating it over a fire, until it acquires such a shade of colour as denotes that the steel is of the fit quality for the required purpose. For the process above described for softening and then rehardening steel plates, we are indebted to Mr. Perkins, it constituting an important branch of an art invented by that gentleman, and to which he has given the name of Siderographia. By means of this truly wonderful invention, not only are en graved steel plates obtained, whose durability is unknown, (since Mr. Perkins states that he has taken 500,000 impressions from one plate,) but a plate being engraved, other plates may be produced from it, which shall be fac similes of the original. The method by which this astonishing effect is obtained is as follows : A steel plate is engraved or etched in the usual way ; it is then har dened. A cylinder of very soft steel, of from two to three inches diameter, is then made ;to roll backwards and forwards on the surface of the steel plate, until the whole of the impression from the engraving is seen on the cylinder in relievo ; after this cylinder has been hardened, it is made to roll backwards and forwards on a copper or soft steel plate, and a perfect fac simile of the original is produced of equal sharpness. But not only is perfect identity thus obtained, but the two styles of work, viz. copper-plate printing and letter-press may be beautifully combined, by means of the process of transferring and retransferring. This invention is admirably adapted to the prevention of forging bank notes, as by this means several first rate artists may be employed at one time to produce portions of a plate in detached parts, which may afterwards be combined and arranged in any order to produce a single plate, and before the note could be imitated by forgers it may be called in and a new note issued, consisting of the same parts differently arranged; the principle, also, offers various other modes of defeating attempts at imitation. the invention has accordingly been very extensively patronized by the banks in America from its outset ; and at the present time the notes of most of the provincial banks in this country are pro duced by this process, some of them presenting the most beautiful specimens of the art of engraving ever witnessed. The bank of England, however, for reasons which are variously stated, has declined availing itself of the advantages which the plan'holds out, and after going to an expense of more than 30,0001. in endeavouring to improve the quality of its notes, and to render them more difficult of imitation, still continues to issue the same wretched description of notes as have been so extensively and successfully counterfeited for many years past.