GILDING. In the gilding of articles of metal, pure gold is first combined or amalga mated with quicksilver, by boiling the gold in five or six times its weight of quicksilver. The boiling mixture is poured into cold water, by which it loses a great deal of its fluidity ; and it is then squeezed through chamois leather. The amalgam has now the consistence of a stiff clay, has a greasy and gritty feel, and is in the most convenient state for being weighed out into portions requisite for each respective quantity of work. The main object of bring ing the amalgam to this consistence and these proportions is to have it in a form convenient for division and apportionment, as well as for the sake of having a uniform standard by which to ascertain the quantity and value of the gold employed.
On the application, however, of this amal gam to the surfaces of metal, it is found that, as there is no chemical affinity between the substances thus brought into contact, the direct union of them is impossible. Nor can it be effected by allowing them to remain in contact for any length of time. The interven tion of a solution of nitrate of mercury is therefore used. The solution is, by the ope ratives, termed quick-water.' When a piece of copper or brass is immersed in or brought in contact with this solution,its surface is im mediately converted into an amalgam. To this amalgamated surface mercury and gold amal gam closely adheres, by means of the mole cular attraction of the particles of the fluid metals for each other.
The manner in which this agent is applied in practice varies according to the description of articles to be gilt. If they are small, strong, and to be gilt all over, as copper buttons, buckles, and rings, a quantity of them, three or four pounds in weight, is put into a deep glazed earthen pan, or ' jowl ;' to these are added about three or four tea-spoonfuls of the quick-water,' together with the requisite portion of amalgam. The whole is then thoroughly stirred with a brush or stick, till the amalgam entirely covers the surface of every article. When they are com pletely covered, they are by some gliders rinsed in cold water and dried by shaking in a bag of warm sawdust; while by others this part of the process is postponed to a later period of the operation, and they are put in their wet state, with the generated nitrate of copper still hanging about them, into the cage.
The ' gilding-cage' is made in a cylindrical form, and is generally about 18 inches in length by 9 or 10 in diameter. It is formed of coarse iron-wire gauze, supported by an external framework of iron, and furnished with a solid iron door at one extremity. It is provided with an axle, which extends to a length of about three feet from the end at which the door is placed, and is then termi nated by a winch, and to a distance of five or six inches in the opposite direction. The articles under process of gilding are placed in this cage, and the door of it securely fastened; it is then suspended by its axlo on two sup ports in an iron cylinder. The cylinder being previously heated by a coal fire beneath it, to such a degree as to be red-hot over a large proportion of its inferior surface, the cage is introduced, and the doors of the cylinder closed. The heated air contained within the cylinder soon raises the temperature of the substances immersed in it, and, as the cage is kept continually revolving, they have all an equable share of heat, and allow of a nearly equal evaporation of the mercury from all their surfaces. The hinder part of the cylinder is connected with a chamber and flue so con structed as to carry off the deleterious fumes of the mercury.
After the cage, with its contents, has been in the cylinder for a length of time, varying, according to the temperature at which it has been kept, from five minutes to a quarter of an hour, the mercury will be found to have entirely evaporated from the gilt surfaces. Another application of quick-water; and allother heating in the cage, are sometimes necessary to bring the surface to a proper state. The gilt articles are then ' heightened,' which is done by continuing to revolve them at the same high temperature within the cylinder, occasionally taking out the cage and shaking them together, that they may all have an equal share of the heat. The end in tended to be effected is a partial oxidation of the surface of the gold. This partial oxida tion occasions a slight difference of colour, which is perceptible by an experienced eye, and confers on the gold a degree of thatporange colour which is so generally admired in golden and gilt articles.