The substances on which bronzing is employed are either metals, wood, ivory, clay, or plaster ; but more general preference is given to wood or plaster. The colours are of various shades and intensity ; their composition and application being in a great mea sure arbitrary, according to the will of the artist. This art is nothing but a species of painting, far• from the most delicate kind ; and, when applied to plaster figures, may be done either with or without cement, the latter rendering it more durable. One principal ingredient in bronzing is gold-powder, for the prepa ration of which the following receipt is given. A quantity of leaf-gold is ground with virgin honey a stone, until the texture of the leaves be complete ly broken, and their parts divided to the most mi nute degree. The mixture of gold and honey is then removed from the stone, and put into a basin of water, whereby the honey may be melted, and the gold freed from it ; and the basin is allowed to stand at rest until the gold subsides. When it does so, the water is poured off, and fresh quantities are added, until the honey be entirely washed away ; after which, the gold is put in paper, and dried for use. This is the true geld-powder ; besides which, there is another, called German gold, in common use ; and also a third, called aunty, snosaicum or musicum, greatly employed'in bronzing, and is thus prepared. A pound of tin, seven ounces of flour of sulphur, half a pound of purified quicksilver, and the same quantity of sal ammoniac, are taken as the ne cessary ingredients. The tin being melted in a cru cible, the quicksilver is added to it ; and, when this mixture is cold, it is reduced to posider, and ground with the sal ammoniac and sulphur until the whole be thoroughly mixed. They arc then to be cal cined in a mattress, and the sublimation of the other ingredients leaves the tin converted into the a'urion mosaicurn, which is found at the bottom of the glass like a mass of bright flaky geld-powder. Should any black or discoloured particles appear, they must be removed. The sal ammoniac used here must be very white and clean, and the mercury quite pure, and unadulterated with lead. These colours are commonly employed in bronzing ; but when a shade more of a red, resembling copper, is required, it can easily be obtained by grinding a very small quantity of red lead along with them. Copper powder may be procured by dissolving filings or slips of that me tal with nitrous acid in a receiver. When the acid is saturated, the slips are to be removed; or, if filings be employed, the solution is to be poured off from what remains undissolved. Small iron bars are then put in, which will precipitate the copper from the saturated acid, in a powder of the peculiar appear ance and colour of copper ; and the liquid being poured from the powder, this is to be washed clean off the crystals by repeated levigations. In addition to these compounds, we may name gold size, which is of particular use in bronzing, and several other branches of the arts. This is prepared from a pound of linseed-oil, with four ounces of gum animi. The latter is gradually supplied in powder to the oil, while boiling, and it is necessary that it should be stirred with every successive dose, until the whole be dissolved and incorporated w,it• the oil. The mixture is still allowed to continue boiling, until a small quantity, when taken out, appears of a thicker consistence than tar, and the whole being then strain ed through a coarse cloth, is put aside. When used, it must be ground with as much vermillion as will render it opaque, and, at the same time, diluted with such a quantity of oil of turpentine as will bring it to a proper consistence for working freely with the pencil.
In regard to the operation of bronzing itself, if a cement is to be used, the powders now described may be mixed with strong gum water or isinglass, and laid on the subject with a brush or pencil ; in doing which, some artists recommend beginning at the bottom, and proceeding upwards. By a different
process, gold size, prepared with a due proportion of turpentine, may be taken, and the subject covered with it ; then being allowed to dry very nearly, but still preserving a certain clamminess, a piece of soft leather wrapped round the finger is dipped in the powder, and rubbed over the work ; or, what is judged preferable, it may be spread with a soft camelhair penCil. The whole, now covered, must be left. to dry, and the loose powder then cleared away by a hair pencil also. Here the principal nicety consists in ascertaining the proper period of dryness for applying the powder, as much of the effect depends on it. But this method of bronzing is esteemed better, because the gold size binds the powders to the ground, without any hazard of their scaling or rubbing off, which sometimes happens when gm or isinglass are employed. The precise tint of bronzing to regulated by taste ; and, indeed, a very perceptible difference appears both in an cient and modern statues, resulting either from age or the metallic proportions.
Bronzing on wood may be effected by a para. cular process, somewhat varying from the general rules. Prussian blue, patent yellow, raw umber, lamp-black, and pipe-clay, are ground separately. with water, on a 'stone. and as much of them as will make acolour put into a small vessel three-fourths fir of size, not quite so strong as what is called Clean Size in gilding. This mix ture is found to succeed best on using about half as much more pipe-clay as of the rest; but this depends on taste and fancy in preferring a peculiar tint. The wood being previously cleaned and smooth ed, and coated with a mixture of clean size and lamp-black, receives a new coating with the preced ing ingredients, twice successively, having allowed the first to dry ; afterwards the bronze-powder is to be laid on with a pencil, and the whole burnished or cleaned anew, observing to repair the parts which may be injured by this operation. Next, the •work must be coated over with a thin lather of Castile soap, which will take off the glare of the burnishing, and afterwards carefully rubbed with a woollen cloth. The gangrenous appearance of the cavities is effect ed by slightly wetting them with a camel-hair pencil dipped in the lather, and then The them with a little dust of verditer gum. The superfluous pow der may be rubbed off when dry.
In bronzing iron, the subject should be heated to a greater degree than the hand can bear, and Ger man gold, mixed with a small quantity of spirit of wine varnish, spread over it with • pencil. Should the iron he already polished, it is necessary to beat it well and moisten it with a linen rag wet in vinegar, On purpose to obscure the glare, that the bronze powder may be sufficiently incorporated with the surface. There are other methods of accomplishiog the same object, as by employing mine coloured mordant„.when the iron is not to be exposed to heat, and spreading the bronze over the mordant, when half dry, with a pencil.
Bronze is injured by humidity ; and it is said not to preserve its proper quality beyond ten years; bet it may be renewed, in which case the subject must be completely cleaned.
There is an analogous method of silvering casts of plaster of Paris, and other substances. which is also called Bronzing, and conducted after the manner above described, but it is not in general repute.
Conjectures have been entertained, that artists originally resorted to bronzing solely for the purpose of correcting the glare of colours; but this is ex. ceedingly improbable, and it is certainly unnecestra,‘ ry to seek farther than the inducement of easily imitating metallic figures esteemed by the curious. This art has, of late years, come into veep Repeal me, and bas received many improvements. (s.)