Imágenes de páginas
[ocr errors][merged small][ocr errors]
[merged small][ocr errors][ocr errors][ocr errors]



GILDING, art of. The art of gilding, or of laying a thin superficial coating of metal on wood, metal, and other substances, has been long practised and highly esteemed, both for its utility, and the splendid effect which it produces. Gold, from the extreme beauty of its colour, and from the length of time during which it may be exposed to the action of the air without tarnishing, is perhaps the most valuable of all substances for the purpose of decoration; but on account of its dearness and weight, it can very seldom be employed in substance, and its ornamental use would be limited, indeed, if it were not at the same time the most extensible of all substances; so that a given weight of gold, notwithstanding its high specific gravity, may, by beating, be made to cover a larger surface than an equal quantity of any other body. Among the ancients, the Romans, and, among the moderns, the French have been remarkable for their large and profuse consumption of gold ; not only the temples, theatres, and other public building, being adorned with gilding, but even the private houses of the wealthier classes.

The materials for gilding, or rather the different states in which gold is used for the purpose, are the following: leafgold of different thicknesses, and formed either of the pure metal, or of an alloy of this with silver, amalgam of gold, and gold-powder. The leaf-gold is procured by the gilder from the gold-beater, for an account of which we shall refer the

reader to the article Goln; but the other two substances being prepared by the gilder himself, may be with propriety described here. The amalgam of gold is made, by heating in a crucible some pure quicksilver; and when it is nearly boiling, adding to it about a sixth of its weight of fine gold in thin plates, heated red hot; the mixture, after being kept hot for a few minutes, becomes of a perfectly homogeneous consistence, and may then be allowed to cool : when cold, it is to be put in a piece of soft leather, and, by gradual pressure, the fluid part of the amalgam, consisting almost wholly of mercury, may be forced through the ores of the leather, while the gold comined with about twice its weight of mercury will remain behind, forming a yellowish silvery mass of about the consistency of soft butter. This, after being bruised in a mortar, or shaken in a strong phial, with repeated portions of salt and water, till the water ceases to be fouled by it, is fit for use, and may be kept for any length of time without injury in a corked phial. It is of essential importance that the materials of this amalgam, and especially the mercury, should be erfectly pure, as the least portion of ead or bismuth would very materially injure the beauty of the gilding, by deteriorating the colour of the gold, and filling it with black specks ; on this account no mercury ought to be employed, that has not been procured by distillation from the red precipitate (nitrous red oxide of mercury) either alone or mixed with a little charcoal powder. Gold is prepared in three different ways. The first and simplest is, to put into a glass or earthen mortar some goldleaf, with a little honey or thick gum-water, and grind the mixture for a considerable time, till the gold is reduced to extremely minute fragments; when this is done, a little warm water will wash out the honey or gum, leaving the gold behind in a flaky pulverulent state. A less tedious and more effectual way of comminuting the gold is, to dissolve it in nitro-muriate acid, and then precipitate it with a piece of copper: the precipitate, after being digested in distilled vinegar, and then washed in water and dried, is in the form of a very fine powder, and both works better, and is easier to burnish, than the ground leaf-gold. The finest ground gold is however produced by heating very gradually the gold amalgam in an open earthen vessel, and continuing the fire till the whole of the mercury

is evaporated, taking care that the amalgam shall be constantly stirred with a piece of glass, rod, or tobacco-pipe, in order to prevent the particles of gold from adhering as the mercury flies off. When the mercury is completely evaporated,the residual gold being then ground in a Wedgewood-ware mortar, with a little water, and afterwards dried, it is fit for use. Gilding is performed either with or without heat. By the first of these methods those substances are gilt, which are not liable to alteration by exposure to a moderate heat, such as metals, and sometimes glass and porcelain : the second method is practised with those substances, such as wood, paper, lead, silk, lacquered and japanned ware, &c. which would be injured, and even destroyed, at the temperature requisite for gilding the former. The last of these methods, being the simplest, shall be first described, and we shall begin with the art of gilding on wood. There are two methods for gilding on wood, namely, oil gilding and É. gilding. Oil gilding is thus performed : the wood must first be covered, or primed, with two or three coatings of boiled linseed oil and white-lead, in order to fill up the pores, and to conceal the irregularities of the surface occasioned by the veins in the wood. When the priming is quite dry, a thin coat of gold-size must be laid on. This is prepared by grinding together some strongly calcined red ochre, with the thickest drying oil that can be procured, and the older the better : that it may work freely, it is to be mixed, previously to being used, with a little oil of turpentine, till it is brought to a proper consistence. If the gold-size is good, it will be sufficiently dry in twelve hours, more or less, to allow the artist to proceed to the last part of the process, which is the application of the gold. For this purpose a leaf of gold is spread on the cushion (formed by a few folds of flannel secured on a piece of wood, about eight inches square, by a tight covering of leather), and is cut into strips of a proper size by a blunt palletknife : of strip being then taken up on the point of a fine brush is applied to the part intended to be gilded, and is then gently pressed down by a ball of soft cotton ; the gold immediately adheres to the sticky surface of the size, and after a few minutes the dexterous application of a o camel's hair brush sweeps away the loose particles of the

gold leaf without disturbing the rest. In a day or two the size will be completely dried, and the operation is finished. The advantages of this method of gilding are, that it is very simple, very durable, not readily injured by changes of weather, even when exposed to the open air, and when soiled it may be cleaned by a little warm water and a soft brush; its disadvantage is, that it cannot be burnished, and therefore wants the high lustre produced by the next method. Its chief employment is in out-door work. Burnished gilding, or gilding in distemper, is thus performed. The surface to be gilt must be carefully covered with strong size, made by boiling down two pieces of white leather, or clippings of parchment, till they are reduced to a stiffjelly; this coating being dried, eight or ten more must be applied, consisting of the same size, mixed with fine Paris plaster or washed chalk: when a sufficient number of layers have been put on, varying according to the nature of the work, and the whole is become quite dry, a moderately thick layer must be applied, composed of size and bole, or yellow ochre : while this last is yet moist, the gold leaf is to be put on in the usual manner; it will immediately adhere on being pressed by the cotton ball, and before the size is become perfectly dry, those parts which are intended to be the most brilfiant are to be carefully burnished with agate or dog's tooth. In order to save the labour of burnishing, it is a common, but bad practice, slightly to burnish the brilliant parts, and to deaden the rest by drawing a brush over them dipped in size: the required contrast between the polished and the unpolished gold is indeed thus obtained; but the general ef. fect is much inferior to that produced in the regular way, and the smallest drop of water falling on the sized part occasions a stain. This kind of gilding can only be applied on in-door work, as rain, and even a considerable degree of dampness, will occasion the gold to peel off. When dirty, it may be cleaned with a soft brush, and hot spirit of wine, or oil of turpentine. It is chiefly used on picture frames, mouldings, and stucco. Letters written on vellum or paper are gilded in three ways: in the first, a little size is mixed with the ink, and the letters are written as usual; when they are dry, a slight degree of stickiness is produced by breathing on them, upon which the gold leaf is immediately applied, and by

a little pressure may be made to adhere with sufficient firmness. In a second method, soule white-lead or chalk is ground up with strong size, and the letters are made with this by means of a brush: when the mixture is almost dry, the gold-leaf may be laid on, and afterwards burnished. The last method is, to mix up some gold powder with size, and make the letters of this by means of a brush. The edges of the leaves of books are gilded, while in the binder's press, by first applying a composition formed of four parts of Armenian bole and one of sugar candy, ground together to a proper consistence, and laying it on with a brush with the white of egg : this coating, when nearly dry, is smoothed by the burnisher; it is then slightly moistened with clean water, and the goldleaf applied, and afterwards burnished. In order to impress the gilt figures on the leather covers of books, the leather is first dusted over with very fine rezin, or mastich, then the iron tool by which the figure is made is moderately heated. and pressed down on a piece of leafgold, which slightly adheres to it; being then immediately applied to the surface of the leather with a certain force, the tool at the same time makes an impression, and melts the mastich which lies between the heated iron and the leather; in consequence of this the gold with which the face of the tool is covered is made to adhere to the leather, so that on removing the tool a gilded impression of it remains behind. Drinking glasses, and other utensils of this material, are sometimes, especially in Germany, gilt on their edges; this is done in two ways, either by a simple adhesive varnish, or by means of fire. The varnish is prepared, by dissolving in drying linseed oil a quantity of gum amine, or still better of clear amber, equal in weight to the linseed oil : a very dryin and adhesive varnish is thus prepared, which being diluted with a proper quantity of oil of turpentine, is to be applied as thin as possible to those parts of the glass which are intended to be gilded ; when this is dry, which will be about a day, the glass is to be placed by the fire side, or in a stove, till it is so warm as almost to burn the fingers when handled; at this temperature the varnish will become glutinous, and a piece of gold leaf applied in the usual way will immediately adhere; when the gilding is thus put on, and before it is grown quite cold, it may be burnished, taking care only to interpose a piece of verythin paper between the gold and burnisher. If the varnish is very good, this is the best method of gilding glass, as the gold is thus fixed on more evenly than in any other way: it often happens, however, when the varnish is but indifferent, that by repeated washing the gold soon wears off: on this account, the practice of burning it in is sometimes had recourse to. For this purpose some gold powder is tempered with borax, and in this state applied to the clean surface of the glass, with a clean camel's hair pencil: when quite dry, the glass is put in a stove heated to about the temperature of an annealing oven, the gum burns off, and the borax by vitrifying cements the gold with great firmness to the glass; after which it may be burnished. The gilding upon porcelain is in like manner fixed by fire and borax; and this kind of ware being neither transparent nor liable to soften, and thus injure its form in a low red heat, is free from the risk and injury, which the finer and more fusible kinds of glass are apt to sustain from such treatment. All the methods of gilding hitherto described resemble each other, by being accomplished by means of some adhesive mediurn; this,however, is not the case with gilding upon metals; the gold is brought into immediate contact with the other me. tal, and they both remain firmly united merely by the attraction of adhesion subsisting between them. The simplest of all the kinds of gilding on metal, and which strikingly demonstrates the power of the affinity of adhesion, is one which is sometimes practised on plane surfaces of copper and iron, with considerable success. The metal, being previously polished, is heated to about the temperature of melted lead, and covered with a double layer of gold leaf: by the cautious application of a blood stone burnisher, applied gently at first, and increasing the force of the pressure by degrees, the surface of the gold and copper are brought to touch each other in almost every point, and then adhere with a force proportionate to the completeness of the contact. The first jayer being thus burnished down, a second is made to adhere in the same manner, and sometimes a third, if the gilding is intended to be very solid. The objection to this kind of gilding is its tediousness, and the almost impossibility of using a sufficient pressure without injuring the evenness of the gilded surface : where

these objections do not apply, there cannot be a more effectual mode of gilding, as is evident from the manufacture of gilt silver and copper wire. The bar, before it is committed to the wire-drawer, is plated with gold, by having several plates of gold successively burnished down upon it,and being thensubjected to the stronger compression which takes place in wiredrawing, the gold and the other metal become so perfectly united as to form in a manner but one substance. The most usual method of covering the face of a metal with gold is by means of an amalgam, or, as it is technically called, water-gilding. If the metal to be gilt is silver, the best method of proceeding is, first to soak it in warm dilute muriatic acid, that the surface may be rendered perfectly clean ; it must then be washed in clean water, changed two or three times, to get rid of the whole of the acid : being afterwards dried, and made moderately warm, a little gold amalgam, also warm, is to be carefully and evenly spread upon the silver, to which it will immediately adhere: when this is completed, the piece is placed upon a convenient support over a charcoal fire, and while the mercury is evaporating, if any specks or places appear, which have escaped the amalgam, a small piece is to be laid on and spread with a brush, to supply the deficiency, without removing the article from the fire. After a time, the whole of the mercury will be driven off, and the piece, after cooling, being accurately examined, will be found to be entirely covered with a thin coating of pale dull gold. The small roughnesses, and loosely-adhering particles, are now to be removed with a scratch-brush, which is made of some extraordinary fine brass wire, bound together into a tuft; by it the surface is rendered perfectly smooth and bright; but it still remains of a pale yellowish colour: this defect is next removed by warming the piece, and smearing it over with gilders' wax, a composition of bees' wax, red ochre, verdigris, and green vitriol or alum. The wax being burnt over a charcoal fire, and the piece quenched in urine, the colour of the gilding will be found to be much heightened; if it is not sufficiently so, the application of a succeeding one will complete the desired ef. fect, after which the work may be burnished or not, according to the taste of the artist. Instead of the common gilders’ wax, a mixture of equal parts of nitre, sal-ammoniac, green vitrol, and

« AnteriorContinuar »