It is usual for the gilder to have a small iron pan, into which he puts a few of the pieces he intends to gild, and places it upon the fire ; and he takes them one by one, with the tongs, out of this pan, to brush them, and then returns them to heat, whilst he brushes others : at last, to finish the operation, he holds them in the tongs over the hottest part of the charcoal ; this completes the evapora tion of the mercury, and leaves the gold of a dull yellow, or gold colour. Small articles of which there are a great number of the same size, such as buttons, rings of watch chains, &c. are put all together into an iron frying pan to heat, and when the mercury begins to flow, they are all together thrown out into a cup, in which they are shaken up, and well stirred about with a painter's brush. This operation effects the spreading of the gold with very little trouble. It should be repeated several times, and the heat brought on very gradually, which the pan admits of doing more conveniently than any other means. As the heat continues to increase, the mercury begins to evaporate, and soon leaves the amalgam, or rather the gold, fixed upon the surface. The work being now dried off, pre sents only a dull scarfy appearance; but being brushed by a few strokes of the scratch-brush, it assumes a polished surface. The brushing is done with small beer or ale grounds, which however contribute nothing more to the operation than to soften the strokes of the brush, and pfe vent the work having any appearance of scratches. If a thicker coat of is required, the operation is repeat ed once or twice, and every time the colour will approach nearer to that of fine gold. The scratch-brush must be applied to clean the work between each gilding.
The simplest mode of heightening the colour, is to hold the work over the charcoal of a small stove, similar to the gliders stove, and heat it till the colour increases. During the heating, if any spots appear of a different colour, they are touched with a stick clipped in aquafortis, which re stores them. It is then thrown into a weak solution of aquafortis and water, which will discover any spots where the coating of gold is deficient. This method is supposed to act by making a perfect dissipation of the mercury ; but we think it rather operates to produce a farther alloy or amalgamation of the gold with the metal upon which it is spread. The work being again polished with the scratch brush is finished, unless a very high polish is required : it must be produced by burnishing with a blood-stone, using water to prevent the gold being rubbed off.
When a higher colour is required, the work is covered with a composition called gilding wax ; and being held over the fire till the wax smokes, and is on the point of inflaming, it is then suddenly cooled in water, and the scarf of the wax is cleared off by the scratch-brush and beer.
Gilding wax is compounded of bees wax and red chalk in equal quantities, with French verdigris and alum of green vitriol, which are added in small portions. The use of the wax seems to be only to flow, and carry the other in gredients to every part of the surface, and to determine the proper degreee of heat to be applied. There is another preparation of sal ammoniac, nitre, green vitriol, and ver digris, in equal parts, which they use after the wax is clear ed off, and they desire to produce a very high colour. It is spread over the work in a state of paste, to which it is mixed by urine, and heated till it smokes in the same man ner as the wax. This composition may be used instead of the gilding wax ; or, by repeating the application of the gilding wax two or three times, it will produce the same colour, which is a deep and brilliant copper colour, but ve ry superficial, and wears off long before the gilding is worn away.
Dead yellow is a preparation of gilding which presents a frosted surface, without any polish, and of a beautiful yellow colour. It is produced by a saline preparation like that above, but some yellow colouring matter is employed instead of the green verdigris. When the work is heated with this upon it, it is thrown into aquafortis and water, and the ebullition which this occasions produces the dead colour.
The operation of drying off, as formerly practised, was very prejudicial to the health of the workman, who always inhaled the fumes of the mercury. The application of a glass front to the chimney in which the stove is placed, has obviated this inconvenience ; but still the stove is not judi ciously constructed with a view of producing a gradually increasing heat. A considerable improvement might be made by employing an iron box, or oven, set in brick-work, over a close fire, and surrounded by flues, in the same manner as a boiler. It should be open in front, and have two or three shelves within it. The whole stove should be inclosed within a chimney, with a glass window in front, and a sufficient draft up the chimney window to carry off all the fumes. In using this stove, the work should at first be laid on the top of the box, where it will be gradually warmed; then the workman removing it to the upper shelf, it would become more heated, and fit for brushing.
When this is done, he should place it on the second shelf, which would produce a greater heat. Being again brushed, and afterwards placed on the bottom of the box or stove, the heat would be sufficient to complete the drying off.
Iron or steel may be ornamented by wash-gilding, if the surface is first covered with copper, by dipping the iron into a strong solution of blue vitriol, (sulphate of copper.) Upon this the amalgam can be applied; but such gilding presents a very indifferent colour, and is not durable.