(a) In the centre of a charcoal stove put a crucible holding a given quantity of pure and dry mercury, and when the temperature has reached about 212° F., add J the weight of gold. Stir with an iron rod until the amalgam has acquired the consistency of butter, throw it into cold water, and keep it there for use.
(b) A quantity of mercury is put into a crucible or iron ladle, which is lined with clay, and exposed to heat till it begins to smoke. The gold to be mixed should be previously granulated, and heated red hot, when it should be a lded to the mercury, and stirred about with an iron rod till it is perfectly dissolved. If there should be any superfluous mercury, it may be separated by passing it through clean soft leather; and the remaining amalgam will hare the consistence of butter, and contain about 3 parts of mercury to 1 of gold.
The metal to be gilt is previously well cleaned on its surface, by boiling in a weak picklo of very dilute nitric acid. A quantity of aquafortis is poured into an earthen vessel, and quicksilver is put therein; when a sufficient quantity of mercury is dissolved, the articles to be gilt are put into the solution, and stirred about with a brush till they become white. This is called quicking. During quirking by this mode a noxious vapour continually arises, which proves very injurious to the health of the workmen. To avoid that danger, dissolve the quicksilver in a bottle containing aquafortis, and leave it in the open air during the solution, so that the noxious vapour escapes into the air. Then a little of this solution is poured into a basin, and with a brush dipped therein stroke over the surface of the metal to be gilt, which immediately becomes quicked. The amalgam is now applied by one of the following methods: - (a) By proportioning it to the number of articles to be gilt, and putting them into a vessel together, working them about with a soft brush, till the amalgam is uniformly spread.
(6) By applying a portion of the amalgam upon one part, and spreading it on the surface, if flat, by working it about with a harder brush. The work thus managed is put into a pan, and exposed to a gentle degree of heat; when it becomes hot, it is frequently put into a pan, and worked about with a painters' large brush, to prevent an irregular dissipation of the mercury, till at last the quicksilver is entirely dissipated by the repetition of heat, and the gold is attached to the surface of the metal. This gilt surface is well cleaned by a wire brush, and then artists heighten the colour of the gold by the application of various compositions; this part of the process is called colouring.
(c) Mercury gilding will furnish gold with a bright or a dead lustre, scratch-brushed, ormolued, and with different shades. The amalgam should be about as hard as wax. This amalgam is crystalline,' and a certain crackling sound is heard when the crystals are crushed between the fingers. A stock of amalgam is generally prepared in advance, and is divided into small balls of nearly equal size, the value of which is ascertained from their number, and from the total weight of gold employed. These balls are kept in water, but should not remain too long without being used, as the different parts do not then present the same composition. The amalgam is spread with the finger upon a flat, hard stone, called the gilding stone; and having dipped a scratch-brush Of stout brass wire into a solution of nitrate of binoxide of mercury until it becomes completely white, it is passed over the amalgam, a portion of which is carried away. The object, previously well cleansed, is scratch-brushed in every direction, and the brush must be frequently dipped into the mercurial solution to facilitate the regular and even spreading of the amalgam. This operation requires great care to obtain a uniform coat upon the hollow and raised parts.
When the back part of a piece does not require gilding, the flat outline, and the back edge, should be gilt, so that the naked copper shall cause no injury in the subsequent operations. The article, when uniformly covered with the amalgam, is heated upon a charcoal fire without draught, which rests upon a cast-iron plate. It is advisable to employ a gilding forge, which allows the workman to watch the operation from behind a glass frame, which protects him from the mercurial vapours. The entire attention is now required for watching the process. With the left hand covered with a thick glove of buckskin, turn the piece in every direction upon the fire, and, as the mercury disappears, with the right hand strike the article in every direction with a brush, the handle and the bristles of which must be long, to equalise the gilding, and to push the remaining amalgam upon those parts which appear less charged with it. When all the mercury has volatilised, the gilding has a dull greenish-yellow colour, resembling that of boxwood; examine whether the coat of gold is continuous. Should a few empty places appear, add more amalgam, and heat the whole again. The next operation is scratch-brushing, which furnishes a pale green colour, and requires another heating for arriving at the desired shade.
The reheating should expel any remaining mercury, and produce a fine orange-yellow colour. In case a bright lustre is required, submit the object, with the aid of heat, to the ormolu process. To obtain dead lustre, the object is firmly fixed to an iron rod, by wire of the same metal, and smeared with a hot paste for dead gilding, composed of saltpetre, common salt, and the double sulphate of alumina and potash. The whole is heated upon a brisk char* coal fire, without draught, and moved about until the mixture dries and begins to fuse, when the article is immediately placed in a barrel half filled with water. The covering of salts dissolves, and the dead lustre appears; this operation requires a certain amount of practice. The gilding must be strong to stand the dead lustre process, especially when the first trial is not successful. The red lines left by the iron wire disappear by plunging the object into a not too diluted solution of nitric acid, or pure hydrochloric acid. Mercury gilders do not employ pure gold; what they use is previously alloyed with a certain portion of copper or silver.
With the latter metal the gilding is green.
Equal parts of sal-ammoniac and corrosive sublimate are dissolved in spirit of nitre, and a solution of gold is made with this menstruum. Silver brushed over with it turns black, but on exposure to a red heat it assumes the colour of gold.