Foils are thin plates or leaves of metal that are put under stones, or compositions in imitation of stones, when they are set.
The intention of foils is either to increase the lustre or play of the stones, or more generally to improve the color, by giving an additional force to the tinge, whether it be natural or artificial, by that of a ground of the same hue, which the foil is in this case made to be.
There are consequently two kinds of foils; the one is colorless, where the effect of giving lustre or play to the stone is produced by the polish of the surface, which makes it act as a mirror, and, by reflecting the light, prevents that deadness which attends the having a duller ground under the stone, and brings it by the double refraction of the light that is caused, nearer to the effect of the diamond. The other is colored with some pigment or stain of the same hue as the stone, or of some other which is intended to modify and change the hue of the stone in some degree; as, where a yellow foil may be put under green, which is too much inclined to the blue, or under crimson, where it is desired to have the appearance more orange or scarlet.
Foils may be made of copper or tin, and silver has been sometimes used, with which it has been advised, for some purposes, to mix gold; but the expense of either is needless, as copper may be made to answer the same end.
_To Prepare Copper for Foils._
Where colored foils are wanted, copper may therefore be best used, and may be prepared for the purpose, by the following means:
Take copper plates beaten to a proper thickness, and pass them betwixt a pair of fine steel rollers very close set, and draw them as thin as is possible to retain a proper tenacity. Polish them with very fine whiting, or rottenstone, till they shine and have as much brightness as can be given them, and they will then be fit to receive the color.
_To Whiten Foils._
Where the yellow, or rather orange-color of the ground would be injurious to the effect, as in the case of purples, or crimson red, the foils should be whitened, which may be done in the following manner:
Take a small quantity of silver and dissolve it in aqua-fortis, and then put bits of copper into the solution, and precipitate the silver; which being done the fluid must be poured off, and fresh water added to it, to wash away all the remainder of the first fluid; after which the silver must be dried, an equal weight of cream of tartar and common salt must then be ground with it, till the whole be reduced to a very fine powder, and with this mixture, the foils, being first slightly moistened, must be rubbed by the finger, or a bit of linen rag, till they be of the degree of whiteness desired; after which, if it appear to be wanted, the polish must be refreshed.
The tin foils are only used in the case of colorless stones, where quicksilver is employed; and they may be drawn out by the same rollers, but need not be further polished, as that effect is produced by other means in this case.
_Foils for Crystals, Pebbles, or Paste, to give the Lustre and Play of Diamonds._
The manner of preparing foils, so as to give colorless stones the greatest degree of play and lustre, is by raising so high a polish or smoothness on the surface, as to give them the effect of a mirror which can only be done, in a perfect manner, by the use of quicksilver, applied in the same general way as in the case of looking-glasses. The method by which it may be best performed is as follows:
Take leaves of tin, prepared in the same manner as for silvering looking-glasses, and cut them into small pieces of such size as to cover the surface of the sockets or the stones that are to be set. Lay three of these then, one upon another, and having moistened the inside of the socket with thin gum-water, and suffered it to become again so dry that only a slight stickiness remains, put the three pieces of leaves, lying on each other into it, and adapt them to the surface in as even a manner as possible. When this is done, heat the socket and fill it with warm quicksilver, which must be suffered to continue in it 3 or 4 minutes, and then gently poured out. The stone must then be thrust into the socket, and closed with it, care having been taken to give such room for it that it may enter without stripping off the tin and quicksilver from any part of the furnace. The work should be well closed round the stone, to prevent the tin and quicksilver contained in the socket from being shaken out by any violence.
The lustre of stones set in this manner will continue longer than when they are set in the common way, as, the cavity round them being filled, there will be no passage found for moisture, which is so injurious to the wear of stones treated in any other way.
This kind of foil likewise gives some lustre to glass or other transparent matter, which has little of itself; but to stones or pastes that have some share of play it gives a most beautiful brilliance.
_To Color Foils._
Two methods have been invented for coloring foils: the one by tingeing the surface of the copper of the color required by means of smoke, the other by staining or painting it with some pigment or other coloring substance.
The colors used for painting foils may be tempered with either oil, water rendered duly viscid by gum Arabic, size or varnish. Where deep colors are wanted, oil is most proper, because some pigments become wholly transparent in it, as lake, or Prussian blue; but yellow and green may be better laid on in varnish, as these colors may be had in perfection from a tinge wholly dissolved in spirit of wine, in the same manner as in the case of lacquers, and the most beautiful green is to be produced by distilled verdigris, which is apt to lose its color and turn black with oil. In common cases, however, any of the colors may be, with least trouble, laid on with isinglass size. in the same manner as the glazing colors used in miniature painting.
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