A leaden wheel, covered with emery mixed with water, is preferred for the cutting of emeralds, amethysts, hya cinths, agates, granites, &c. &c. and they are polished on a pewter wheel with tripoli ; opal, lapis lazuli, &c. are polished on a wheel made of wood.
Contrary to the method used by per sons who turn metals, in which the sub stance to be wrought is fixed in the lathe, turned by it, and the tool held to the sub stance, the engraver of the crystal, lapis lazuli, &c. fixes his tools in the lathe, and holds the precious stone to them, thus forming vases, or any other shape, by interposing diamond dust mixed with oil, or emery and water, between the tool and the substance, as often as it is dis persed by the rotary motion of the for mer.
The engraving of armorial bearings, single figures, devices, Ire. on any of the above stones, after they are polished, is perfbrmed through the means of a small iron wheel, the ends of the axis of which are received within two pieces of iron, in a perpendicular position, that may be closed, or otherwise, as the ope ration requires ; the tools are fixed to one end of the axis, and screwed firm ; the stone to be engraved is then held to the tool, the wheel set in motion by the foot, and the figure gradually formed. The materials of which the tools are made is generally iron, and sometimes brass; they are flat, like chissels, gouges, ferules, and others have circular heads. After the work is finished, the polishing is done with hair brushes, fixed on wheels and tripoli.
Engraving in wood, has been practised for several centuries, and originally with tolerable success ; it languished for great part of the 18th century, but revived to wards the close, and is still practised in a manner which reflects credit on the in genuity of the age. Bewick will long be remembered by his works in this style of engraving, and his imitators have been numerous and successful. As it is entirely different from engrav ing on copper, the artist already ac quainted with that mode would find himself at a loss how to proceed on wood, as the lines, instead of being cut into the substance, are raised, like the letters of printing types, and print ed in the same manner.
The wood used for this purpose is box, which is preferred for the hardness and closeness of its texture ; the surface must be planed smooth, and the design drawn on it with a black lead pencil ; the ver is then used, the finer excavations from which are intended for white in terstices between the black lines pro duced by leaving the box untouched, and the greatest lights are made by cut ting away the wood entirely, of the in tended form, length and breadth ; but the deepest shades require no engraving.
Much of the beauty of this kind of en graving depends upon the printing, nor is it every artist who can excel in it as expedition and freedom are not to be attained : in short, the best wooden cuts are evidently the products rather of per se; erance and ingenuity than easy fidence in ability, observable in every line of fine etchings. There are some who succeed to admiration in represent ing foliage and plants, but unfortunately a few months practice will enable a pu pil to etch them on copper with grew er truth : drapery and architecture may be well done in wood, but the faces and limbs of figures never look well.
Such are the different descriptions of engraving which do not require the aid of aqua fortis ; of those wade by the in tervention of that liquid, the principal is Etching. He that would excel in this branch of the arts must be thoroughly acquainted with drawing; otherwise his works will appear tasteless indeed. The ground used in etching is a combina tion of asphaltum, gum mastic, and vir gin wax, mixed in such proportions as will prevent the asphaltum from •break ing the composition, when under the aqua fortis, or the wax from making it so soft as to close the lines when cut through it by the needle. As every thing depends upon the stability of the ground, it should be purchased of those persons who are most celebrated for making it ; or if the person wishing to use it prefers doing it himself, let him remember, that he must keep every par ticle of grease or oil far from him and his materials, and that, without the great est care, the inflammability of the as phaltum will ruin his operations in melt ing them. The proportions of the ingre dients should be obtained by experi ment.