the art of spreading or covering a thing over with gold, either in leaf or liquid. The art of gilding was not unknown among the ancients, though it never arrived among them at the perfection to which the moderns have carried it. Pliny affirms 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 hastily, that in a little time you might see all, even private and poor persons, gild the very walls, vaults, &c. of their houses.

We need not doubt but they had the same method with us, of beating gold, and reducing it into leaves; though it should seem they did not carry it to the same height, if it be true which Pliny relates, that they only made 750 leaves of four fingers square out of a whole ounce. Indeed he adds, that they could make more; that the thickest were called bractae Praenestinae, by reason of a statue of the goddess Fortune at Praeneste gilt with such leaves; and that the thinner sort were called bractae quelloriae.

The modern gilders do also make use of gold-leaves of divers thicknesses; 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 that shall endure all the injuries of time and weather, which to the ancients was impracticable. They had no way to lay the gold on bodies that would not endure the fire but with whites of eggs or size, neither of which will endure the water; so that they could only gild such places as were sheltered from the moisture of the weather.

The Greeks called the composition on which they applied their gilding on wood leucophorum 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 the particulars of this earth, its colour, ingredients, &c. the antiquaries and naturalists are not agreed upon.

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 varnishing, 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 upon these metals a gold-coloured varnish, which, being transparent, shows all the brilliancy of the metals beneath. Many ornaments of brass are varnished in this manner, which is called gold lacquering, to distinguish them from those which are really gilt. Silver-leaves thus varnished are put upon leather, which is then called gilt leather. See Lacquer.

Amongst the false gilding may also be reckoned those which are made with thin leaves of copper or brass, called Dutch-leaf. In this manner are made all the kinds of what is called gilt paper.

In the true gilding, gold is applied to the surface of bodies. The gold intended for this purpose ought in general to be beat into thin leaves, or otherwise divided into very fine parts.

As metals cannot adhere well merely by contact to any but to other metallic substances, when gold is to be applied to the surface of some unmetallic body, that surface must be previously covered with some gluey and tenacious substance, by which the gold shall be made to adhere. These substances are in general called sizes. Some of these are made of vegetable and animal glues, and others of oily, gluey, and drying matters. Upon them the leaves of gold are applied, and pressed down with a little cotton or a hare's foot; and when the whole is dry, the work is to be finished and polished with a hard instrument, called a dog's tooth, to give lustre.

When the work is required to be capable of resisting rain or moisture, it ought to be previously covered with a composition of drying oil and yellow ochre ground together; otherwise a water-size may be used, which is prepared by boiling cuttings of parchment or white leather in water, and by mixing with this some chalk or whiting: several layers of this size must be laid upon the wood, and over these a layer of the same size mixed with yellow ochre. Lastly, another mixture, called gold size, is to be applied above these; upon which the gold-leaves are to be fixed. This gold size, the use of which is to make the gold-leaf capable of being burnished, is composed of tobacco-pipe clay, ground with some ruddle or black lead, and tempered with a little tallow or oil of olives. The edges of glasses may be guilt by applying first a very thin coat of varnish, upon which the gold-leaf is to be fixed; and when the varnish is hardened, may be burnished. This varnish is prepared by boiling powdered amber with linseed oil in a brass vessel to which a valve is fitted, and by diluting the above solution with four or five times its quantity of oil of turpentine; and that it may dry sooner, it may be ground with some white lead.

The method of applying gold upon metals is entirely different. The surface of the metal to be gilt is first metals, to be cleaned; and then leaves are to be applied to it, which, by means of rubbing with a polished bloodstone, and a certain degree of heat, are made to adhere perfectly well. In this manner silver-leaf is fixed and burnished upon brass in the making of what is called French plate, and sometimes also gold-leaf is burnished upon copper and upon iron.

Gold is applied to metals in several other manners. 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 is first to be reduced into thin plates or grains, which are heated red-hot, and thrown into mercury previously heated, 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 the gold is lastly to be burnished with a blood-stone.

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 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 chafed, it ought to be previously 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. 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, by which this surface is cleaned from any rust or tarnish which 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 cleanse the surface of the metal to be gilt from any rust or tarnish it may have acquired; but it also greatly facilitates the application of the amalgam to the surface of that metal, probably in the following manner: It first dissolves part of the mercury of the amalgam; and when this solution is applied to the copper, this latter metal having a stronger disposition to unite with the nitrous acid than the mercury has, precipitates the mercury upon its surface, in the same manner as a polished piece of iron precipitates upon its surface copper from a solution of blue vitriol. When the metal to be gilt is thus covered over with a thin precipitated coat 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 the surface of the metal bare; while the mercury is evaporating, the piece is to 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: when the mercury is evaporated, which is known by the surface being 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 (which is a brush composed of braids wire) till its surface is made smooth; then it is covered over with a composition called gilding wax, and is again exposed to the fire till the wax be burnt off. This wax is composed of bees-wax, sometimes mixed with some of the following substances; red ochre, verdigris, copper-scales, alum, vitriols, 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 this effect seems to be produced by a perfect distillation of some mercury remaining after the former operation. This distillation 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 with copper by dipping in a solution of blue vitriol. Iron may also receive a golden coat from a saturated solution of gold in aqua-regia, mixed with spirit of wine, the iron having a greater affinity with the acid, from which it therefore 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; which is, 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 expose it to such a fire that the regulus may be evaporated, while 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 distilled 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 improves on the following remarks. "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 that 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 I 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... stead 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 incommode by 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 10 or 12 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 caught in a hollow rim, formed by turning inwards a portion of the lower part, and conveyed, by a pipe at one side, into a proper receiver.

"Mr Hellot communicated, 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 M. du Fay, and of which M. du Fay himself had seen several trials. Fine gold in powder, such as results from the parting of gold and silver by aqua-fortis, is directed to be laid in a heap on a levitating 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 grey 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 aqua-fortis, 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 aqua-fortis by covering it with wax. Instruments or 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. I 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 garlick, a fluid remarkably tenacious. This mixture was spread all over the polished silver; and when the first layer was 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 M. du Fay says the gold applied in his 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."

Finally, some metals, particularly silver, may be gilt in the following manner:

Let gold be dissolved in aqua-regia. In this solution pieces of linen are to be dipped, 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 leaves, these leaves adhere to them with some force, although they are not of metallic nature. This gilding is so much more perfect, as 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 may be drawn. The powders for this purpose may be made, 1. By grinding gold-leaf with honey, which is afterwards to be washed away with water. 2. By distilling to dryness a solution of gold in aqua-regia. 3. By evaporating the mercury from an amalgam of gold, taking care to stir well the mass near the end of the process. 4. By precipitating gold from its solution in aqua-regia by applying it to a solution of green vitriol in water, or some copper, and perhaps some other metallic substances.