Engraving of Mezzotintos differs en tirely from the manner above described ; this method of producing prints, which resemble drawings in Indian ink, is sad by Evely n, in his history of clialcography, to have been discovered by Prince Ru pert, and was some years pm, a very fa vourite way of engraving portraits and historical subjects ; of the former, the large heads by Fry are of superior ex cellence.
The tools required for this easy and rapid mode of proceeding are, the ground ing-tool, the scraper, and the burnisher ; the copperplate should be prepared as if intended for the graver, and laid flat upon a table, with a piece of flannel spread under it, to prevent the plate from slip ping ; the grounding-tool is then held perpendicularly on it, and rocked with moderate pressure backwards and for wards, till the teeth of the tool have equally and regularly marked the copper from side to side; the operation is after wards repeated from end to end, and from each corner to the opposite ; but it is ne cessary to observe, that the tool must never be permitted to cut twice in the same place ; by this means the surface is converted into a rough chaos of intersec tions, which, if covered with ink and printed, would present a perfectly black impression upon the paper.
To transfer the design to be scraped, it is usual to rub the rough side of the plate with a rag dipped into 'the scra pings of black chalk, or to smoke it with burning wax taper, as in the process for etching ; the back 'of the design is then covered with a mixture of powdered red chalk and flake white, and laid on the plate through which it is traced ; parti cles of red, in the form of the outlines, are thus conveyed to the black chalk on the plate, which are to be secured there by the marks of a blunted point ; the process must then be carried on with the scraper, by restoring the plate in the perfectly light parts of the intended print to a smooth surface, from which the gra dations arc preserved by scraping off more or less of the rough ground ; but the burnisher is necessary to polish the extreme edges of drapery, &c., where the free touch of the brush in painting repre sents a brilliant spot offight. The deep est shades are sometimes etched and cor roded by aqua fortis, and so blended with the mezzotint° ground added afterwards, that there is nothing offensive to the eye in the combination.
Many proofs are required to ascertain whether the scraping approaches the de sired effect, which is done by touching the deficient parts with white or black chalk, on one of the proofs from the original drawing, and then endeavouring to make the plate similar by further scraping, or relaying the ground with a small tool made for this particular pur pose, where too much of the roughness has been effaced.
Engraving on Steel is confined to the cutting of punches, for the conveyance of any form a certain depth into that or any other metal, seals, and dyes, for im pressing the designs of coins, medals, &c. on gold, silver, or copper, &c. The punches are engraved from models in wax made in relievo, and, when com pleted, are tempered to that degree of solidity, which will bear the violent blows, without blunting the finest parts or break ing them, necessary to produce the ma trix in the steel intended for striking of medals or coins, which must be heated to prevent such a disaster, and tempered again, for a similar reason to the preced after it is finished.
There are several tools used in finish ing of dyes, which are, gravers, chissels, and flatters; and many little punches for making ornamental borders and mould ings to coins and medals ; the latter are always in greater relief than the former, and consequently more difficult to exe cute in perfection.
Engraving on precious Stones is accom plished with the diamond or emery. The diamond possesses the peculiar property of resisting every body in nature, and, though the hardest of all stones, it may be cut by a part of itself, and polished by its own particles. In order to render this splendid substance fit to perform the operations of the tool, two rough dia monds are cemented fast to the ends of the same number of sticks, and rubbed together till the form is obtained for which they are intended ; the powder thus produced is preserved, and used for polishing them in a kind of mill fur nished with a wheel of iron ; the diamond is then secured in a brazen dish, and the dust mixed with olive oil applied, the wheel is set in motion, and the friction occasions the polished surface so neces sary to give their lustre due effect. Other stones, as rubies, topazes, and sapphires, are cut into various angles on a wheel of copper, and the material for polishing those is tripoli diluted with water.