GILDING, the art of spreading or covering over a thing with gold, either in leaf or in liquid. The art of gilding was not unknown amongst the ancients, though it never arrived among them at the perfection to which the moderns have carried it. Pliny assures us that the first gilding seen at Rome was after the destruction of Carthage, under the censorship of Lucius Mummius, when they began to gild the ceilings of their temples and palaces; the Capitol being the first place on which this enrichment was bestowed. But he adds, that luxury advanced on them so rapidly, that in a little time you might see all, even private and poor persons, gild the walls, vaults, and other paru of their dwellings.

We need not doubt that they had the same method as ourselves, of beating gold, and reducing it into leaves; though it should seem that they did not carry it to the same extent, if it be true which Pliny relates, that they only made 750 leaves of four fingers square out of an ounce. Indeed he adds that they could make more; that the thickest were called bractea Pranestina, by reason of a statue of fthe goddess Fortune at Praeneste gilt with such leaves; and that the thinner sort was called bractea questoria.
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The modem gilders also make use of gold leaves of different thickness; but there are some so fine that a thousand do not weigh above four or five drachms. The thickest are used for gilding on iron and other metals, and the thinnest on wood. But we have another advantage over the ancients in the manner of using or applying the gold. The secret of painting in oil, discovered of late ages, furnishes us with means of gilding works capable of enduring all the injuries of time and weather, which to the ancients was impracticable. They had no way to lay the gold on bodies which would not endure the fire, but with the albumen of eggs or sise, neither of which will endure the water; so that they could only gild such places as were sheltered from moisture.

The Greeks called the composition on which they applied their gilding on wood laicopiueum or leucophorum ; which is described as a sort of glutinous compound earth, serving in all probability to make the gold stick and bear polishing. But antiquaries and naturalists are not agreed as to the particulars of this earth, its colour, and ingredients.

The lustre and beauty of gold have occasioned several inquiries and discoveries concerning the different methods of applying it to different substances. Hence the art of gilding is very extensive, and contains many particular operations and various management.

A colour of gold is given by painting and by varnishes, without employing gold; but this is a false kind of gilding. Thus a very fine golden colour is given to brass and to silver, by applying to these metals a gold-coloured varnish, which, being transparent, shows all the brilliancy of the metals underneath. Many ornaments of brass were varnished in this manner, which is called gold laquering, to distinguish them from those which are really gilt. Silver leaves thus varnished are put upon leather, which is then called gilt leather. (See Laquer.)

There are two methods of gilding on wood, which are severally termed size and oil gilding. The leaves of several metals are used for this purpose, viz. gold, silver, mosaic gold, Egyptian gold, Peruvian gold, and Dutch metal. Gold and silver alone are used when the work is to be burnished; the others only for the coarser sorts of oil-gilding, and that rarely, owing to their liability to blacken with the atmosphere, from being merely a composition of the baser metals.

The practical operations are nearly the same with respect to all the different metals; but as various size» are required to prepare the work, we shall describe the manner of procuring them before we describe how they are employed: Isŕ, Strong size; 2d , clear size; 3d , burnish gold size, and oil gold size.

1st, Strong size is prepared from the parings of animal hides: the best, and that in most common use, is known at the tan-works by the name of bate shavings. To one pound of these shavings add a gallon of water, and boil them together for two or three hours, so that when cold they may become a very stiff jelly. After it has been strained it is fit for use. 2d , To prepare clear size, take of vellum two ounces, add one pint of water, and boil them for one hour, or until, when cold, they become a stiffish jelly, about half the strength of the above. 3d , To make gold size for burnishing, reduce one pound of fine pipe-clay to powder, take one quarter of an ounce of black lead, and half an ounce of French bole, and grind the two together on a painter’s stone with water; mix in the clay, and add more water, until it attains the consistency of a thick paste; put in a quarter of an ounce of olive oil; grind the whole, and it is then ready for use.

The tools to be used are the tip, the cushion, and knife. The tip is a broad thin brush, made of the long hairs of the squirrel’s tail glued between two cards. The cushion is a board covered with calf leather, the rough side up, and a fold of cotton between.

Warm the strong size, and mix with it the finest washed whitening until it is of the consistency of very thick cream; lay it on the work to be gilded with a sash tool; allow it to dry, and repeat the coats five or six times, allowing it to dry each time. If plain or moulded, take pumice-stone and smooth the work; allow it to dry, and smooth again with fine sand-paper; then take a portion of the gold size (No. 3), and mix it with a part of the clear size (No. 2), in a warm state, until it has been reduced to the consistency of cream; give the work four coats of this, allowing it to dry each time; rub it down with flour-emery paper, and coat again the parts to be burnished: the mat or flat part must be coated with the clear size (No. 2) reduced with equal part of water. Take the gold leaf and spread it on the cushion, cut it with the knife to the size required, take the tip, grease it slightly, to make the gold lift; and having drawn a wetted hair pencil over the part meant to be covered, lift the leaf with the tip and lay it on, repeating this until the whole is covered: allow this to stand for five or six hours, and rub carefully over the parts to be burnished with the burnisher. The burnisher is a piece of smooth pebble fixed into a handle. Any faults in the mat part should be mended with leaf, and coated over with clear size, and then the work is finished.

Oil-gilding. —Oil gold size is made by grinding Oxford ochre with drying oil, adding, when used, a little fat oil (fat oil is procured by spreading a thin film of linseed oil on water, or shaking it in a bottle with oxygen gas).

Work which is not to be exposed to damp or moisture should be prepared in the same manner as for burnishing, and in the place of burnish gold size it should get two coats of strong size (No. 1) a little reduced with water; and when dry-coated with the oil gold size, and allowed to stand till the breath lies on it, then lay the leaf as formerly described, pressing it down with cotton wool; dust it off with a brush, coat it with clear size, and the work is finished.

Out-door work is prepared entirely with oil-paint in place of size, and when dry it is coated with the oil gold size, and gilded in the same manner as described above; but no size is applied on the surface of the gold.

It will readily be perceived that burnish and mat gilding is a more laborious process than gilding in oil, consequently more expensive at the first. It has, however, the advantage over the oil, for inside work, of keeping longer clean, owing to the smoothness of its surface; for the best executed work in oil has a roughness which allows the smoke and dust more readily to adhere. On the other hand, when the work is to be exposed to the weather, oil gilding has a power of resisting its action scarcely to be conceived.

The method of applying gold upon metals is entirely different, and it is done in various ways. One of these is by previously forming the gold into a paste or amalgam with mercury. In order to obtain a small amalgam of gold and mercury, the gold must first be reduced into thin plates or grains, which are heated red hot, and thrown into mercury previously heated, till it begins to smoke. Upon stirring the mercury with an iron rod, the gold totally disappears. The proportion of mercury to gold is generally as six or eight to one. With this amalgam the surface of the metal to be gilded is to be covered; then a sufficient heat is to be applied to evaporate the mercury; and, lastly, the gold is to be burnished with a bloodstone.

This method of gilding by amalgamation is chiefly used for gilding copper, or an alloy of copper, with a small portion of zinc, which more readily receives the amalgam; and it is also preferable for its colour, which more resembles that of gold than the colour of copper. When the metal to be gilt is wrought or chased, it ought to be pre·
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viously covered with quicksilver before the amalgam is applied, that this may be easier spread; but when the surface of the metal is plain, the amalgam may be applied directly to it. It is to be observed that the use of the blood-stone is now generally abandoned, and a steel burnisher substituted for it. The steel burnisher is case-hardened and then polished; the clearer the polish is, the brighter will the metal be burnished. The quicksilver or amalgam is made to adhere to the metal by means of a little aquafortis, which is rubbed on the metallic surface at the same time, and by which this surface is cleansed from any rust or tarnish that might prevent the union or adhesion of the metals. But the use of the nitrous acid in this operation is not, as is generally supposed, confined merely to the cleansing the surface of the metal to be gilt, from any rust or tarnish which it may have acquired, but also greatly facilitates the application of the amalgam to the surface of that metal. In the first place it dissolves part of the mercury of the amalgam; and when this solution is applied to the copper, the latter metal, having a stronger affinity for nitrous acid than the mercury has, precipitates the mercury upon its surface, in the same manner as a polished piece of iron precipitates copper upon its surface from a solution of blue vitriol. When the metal to be gilt is thus covered over with a thin precipitated coating of mercury, it readily receives the amalgam. In this solution and precipitation of mercury, the principal use of the nitrous acid in the process of gilding appears to consist. The amalgam being equally spread over the surface of the metal to be gilt, by means of a brush, the mercury is then to be evaporated by a heat just sufficient for that purpose; for if it be too great, part of the gold may also be expelled, and part of it will run together, and leave some of die surface of the metal bare. Whilst the mercury is evaporating, the piece should be from time to time taken from the fire, that it may be examined, that the amalgam may be spread more equally by means of a brush, that any defective parts of it may be again covered, and that the heat may not be too suddenly applied to it; and when the mercury has evaporated, which is known by the surface having entirely become of a dull yellow colour, the metal must then undergo other operations, by which the fine gold colour is given to it. First, the gilded piece of metal is rubbed with a scratch brush, composed of brass wire, until its surface be made smooth; then it is covered over with a composition called gilding wax, and again exposed to the fire until the wax be burnt off. This wax is composed of bees-wax, mixed with some of the following substances, viz. red ochre, verdigris, copper scales, alum, vitriol, borax; but, according to Dr Lewis, the saline substances alone are sufficient, without any wax. By this operation the colour of the gilding is heightened; and the effect seems to be produced by a perfect dissipation of some mercury remaining after the former operation. This dissipation is well effected by this equable application of heat. The gilt surface is then covered over with a saline composition, consisting of nitre, alum, or other vitriolic salt, ground together, and mixed up into a paste with water or urine. The piece of metal thus covered is exposed to a certain degree of heat, and then quenched in water. By this method its colour is further improved, and brought nearer to that of gold. This effect seems to be produced by the acid of nitre (which is disengaged by the vitriolic acid of the alum, or other vitriolic salt, during the exposure to heat) acting upon any particles of copper which may happen to lie on the gilded surface. Lastly, some artists think that they give an additional lustre to their gilt work by dipping it in a liquor prepared by boiling some yellow materials, as sulphur, orpiment, or turmeric. The only advantage of this operation is, that a part of the yellow matter, as the sulphur or turmeric, remains in some of the hollows of the carved work, in which the gilding is apt to be more imperfect, and to which it gives a rich and solid appearance.

Iron cannot be gilt by amalgamation, unless, as it is said, it be previously coated over with copper by dipping it in a solution of blue vitriol. At Sheffield and Birmingham the metal is generally first covered with copper, and then gilded. This species of gilding, however, does not present any thing like a good colour, and, besides, it is by no means durable. Iron may also receive a gold coating from a saturated solution of gold in nitro-muriatic acid, or in aqua-regia, mixed with alcohol or spirit of wine, the iron having a greater affinity for the acid, from which it necessarily precipitates the gold. Whether any of these two methods be applicable to use, is uncertain; but the method commonly employed of fixing gold upon iron is that above mentioned, of burnishing gold leaf upon this metal when heated so as to become blue; and the operation will be more perfect if the surface has been previously scratched or graved.

Another method is mentioned by authors, of gilding upon metals, and also upon earthen ware, and upon glass; namely, to fuse gold with regulus of antimony, to pulverize the mass, which is sufficiently brittle to admit that operation, to spread this powder upon the piece to be gilt, and to expose it to such a fire that the regulus may be evaporated, whilst the gold remains fixed. The inconveniences of this method, according to Dr Lewis, are, that the powder does not adhere to the piece, and cannot be equally spread; that part of the gold is dissipated along with the regulus; that glass is fusible with the heat necessary for the evaporation of regulus of antimony; and that copper is liable to be corroded by the regulus, and to have its surface rendered uneven.

On the subject of gilding by amalgamation Dr Lewis makes the following observations. “There are two principal inconveniences in this business; one, that the workmen are exposed to the fumes of the mercury, and generally, sooner or later, have their health greatly impaired by them; the other, the loss of the mercury; for though part of it is said to be detained in cavities made in the chimney for that purpose, yet the greatest part of it is lost. From some trials 1 have made, it appeared that both these inconveniences, particularly the first and most considerable one, might in good measure be avoided, by means of a furnace of a due construction. If the communication of a furnace with its chimney, instead of being over the fire, is made under the grate, the ash-pit door, or other apertures beneath the grate, closed, and the mouth of the furnace left open, the current of air, which otherwise would have entered beneath, enters now at the top, and passing down through the grate to the chimney, carries with it completely both the vapour of the fuel and the fumes of such matters as are placed upon it: the back part of the furnace should be raised a little higher above the fire than the fore part, and an iron plate laid over it, that the air may enter only at the front, where the workman stands, who will be thus effectually secured from the fumes, and from being incommoded with the heat, and at the same time have full liberty of introducing, inspecting, and removing the work. If such a furnace is made of strong forged (not milled) iron plate, it will be sufficiently durable: the upper end of the chimney may reach above a foot and a half higher than the level of the fire: over this is to be placed a larger tube, leaving an interval of an inch or more all round between it and the chimney, and reaching to the height of ten or twelve feet, the higher the better. The external air, passing up between the chimney and the outer pipe, prevents the latter from being much heated, so that the mercurial fumes will condense against its sides into running quicksilver, which, falling down to the bottom, is there catched in a hollow rim, formed by turning inwards a portion of [7:10:522]the lower part, and conveyed, by a pipe at one side, into a proper receiver.^[1. Dr Lewis’s improvements, however, have been found too complicated, and simpler contrivances have been had recourse to. The following is the method lately introduced, and now in use. A sponge saturated with vinegar is fastened underneath the mouth and nostrils, and a vessel filled with cold water is placed at the vent of the furnace. This vessel attracts the principal portion of the fumes of mercury, and the remainder is attracted by the sponge. But still this is not able to attract all the smoke, and the gilders in large manufactories generally die at an early age. ]

“Mr Hellot communicates, in the Memoirs of the French Academy for the year 1745, a method of making raised figures of gold on works of gold or silver, found among the papers of Μ. Dufay, and of which Μ. Dufay himself had seen several trials. Fine gold in powder, such as results from the parting of gold and silver by aquafortis, is directed to be laid in a heap on a levigating stone, a cavity made in the middle of the heap, and half its weight of pure mercury put into the cavity; some of the fetid spirit obtained from garlic root by distillation in a retort, is then to be added, and the whole immediately mingled and ground with a muller till the mixture is reduced into an uniform gray powder. The powder is to be ground with lemon juice to the consistence of paint, and applied on the piece previously well cleaned and rubbed over with the same acid juice; the figures drawn with it may be raised to any degree by repeating the application. The piece is exposed to a gentle fire till the mercury is evaporated so as to leave the gold yellow, which is then to be pressed down, and rubbed with the finger and a little sand, which makes it appear solid and brilliant; after this it may be cut and embellished. , The author observes that, being of a spongy texture, it is more advisable to cut it with a chisel than to raise it with a graver; that it has an imperfection of being always pale; and that it would be a desirable thing to find means of giving it colour, as by this method ornaments might be made of exquisite beauty, and with great facility. As the paleness appears to proceed from a part of the mercury retained by the gold, I apprehend it might be remedied by the prudent application of a little warm aquafortis, which, dissolving the mercury from the exterior part, would give at least a superficial high colour: if the piece is silver, it must be defended from the aquafortis by covering it with wax. Instruments and ornaments of gold, stained by mercury where the gold is connected with substances incapable of bearing fire, may be restored to their colour by the same means.

“The foregoing process is given entirely on the authority of the French writer. 1 have had no experience of it myself, but have seen very elegant figures of gold raised upon silver, on the same principle, by a different procedure. Some cinnabar was ground, not with the distilled spirit, but with the expressed juice of garlic, a fluid remarkably tenacious. This mixture was spread all over the polished silver; and when the first layer had become dry, a second, and after this a third, was applied. Over these were spread as many layers of another mixture, composed chiefly of asphaltum and linseed oil boiled down to a due consistence. The whole being dried with a gentle heat on a kind of wire grate, the figures were traced and cut down to the silver, so as to make its surface rough; the incisions were filled with an amalgam of gold, raised to different heights in different parts according to the nature of the design; after which a gentle fire, at the same time that it evaporated the mercury, destroyed the tenacity of the gummy juice, so that the coating, which served to confine the amalgam, and as a guide in the application of it, was now easily got off. The gold was then pressed down and embellished as in the former method; and had this advantage, that the surface of the silver under it having been made rough, it adhered more firmly, so as not to be in danger of coming off, as Μ. Dufay says the gold applied in this way sometimes did. The artist, however, found the process so troublesome, that though he purchased the receipt for a considerable sum, he has laid the practice aside.”^[2. The process of Μ. Dufay is a very ancient one, and totally abandoned, or else only had recourse to for the purpose of amusement, or to observe chemical action. ]

Finally, some metals, particularly silver, may be gilt in a different manner. Let gold be dissolved in aqua-regia. In this solution pieces of linen are to be dipt, and burnt to black ashes. These ashes being rubbed on the surface of the silver by means of a wet linen rag, apply the particles of gold which they contain, and which by this method adhere very well. The remaining part of the ashes is to be washed off; and the surface of the silver, which in this state does not seem to be gilt, is to be burnished with a blood-stone, till it acquire a fine colour of gold. This method of gilding is very easy, and consumes a very small quantity of gold. Most gilt ornaments upon fans, snuff-boxes, and other toys of much show and little value, are nothing but silver gilt in this manner.

Gold may also be applied to glass, porcelain, and other vitrified matters. As the surface of these matters is very smooth, and consequently is capable of a very perfect contact with gold leaf, it adheres to them with some force, although they are not of a metallic nature. This gilding is so much the more perfect, that the gold is more exactly applied to the surface of the glass. The pieces are then to be exposed to a certain degree of heat, and burnished slightly to give them lustre.

A more substantial gilding is fixed upon glass, enamel, and porcelain, by applying to these substances powder of gold mixed with a solution of gum-arabic, or with some essential oil and a small quantity of borax; after which a sufficient heat is to be applied to soften the glass and the gold, which is then to be burnished. With this mixture any figures whatever may be drawn. The powders for this purpose may be made, first, by grinding gold leaf and honey, which is afterwards to be washed away with water; secondly, by distilling to dryness a solution of gold in aqua-regia; thirdly, by evaporating the mercury from an amalgam of gold, taking care to stir well the mass near the end of the process; fourthly, by precipitating gold from its solution in aqua-regia, by applying to it a solution of green vitriol in water, or some copper, and perhaps some other metallic substances.