For the best work, the aquafortis pickle is used cold, and the cleaning is performed by the friction of a scratch brush, formed of fine brass wire instead of hairs. The brush is sometimes made of a circular form, and mounted on a spindle, turned rapidly by the motion of a foot-wheel and treddle, in the manner of a lathe, whilst the work is presented to the brush. But for delicate work, such as the wheels and other parts of watches, a very delicate hand brush is used ; and here it must be noted, that when the steel arbors of the wheels, or any other part of steel, are attached to the work, it must not be dipped into the pickle, but the pickle should be made stronger, and only the scratch-brush dipped into it, before the work is brush ed with it. The steel must be frequently dipped into clear water during the brushing, to prevent the action of any acid which may touch it.
To spread the amalgam perfectly upon the surface of the article which is to be gilt, an acid, called the quicken ing, is used. This is made of aquafortis, (nitrous acid) with a small quantity of mercury dissolved in it, so as to give it a milky whiteness. The article being dipped into this, attracts a coat of the solution of mercury, over the whole surface, in an instant ; and with this preparation, the amalgam is applied with the pencil. This is a piece of flattened copper wire, fixed in a handle : it is occasion ally clipped into the quickening, then touching the amal gam with it, it will take up a small quantity, which is rubbed upon the work, and spreads or flows in an instant over all the parts which have been touched by the quickening. The action of the quickening appears to arise from the greater affinity of the nitrous acid for copper than for the mercury; in conse quence of which, the acid takes to the copper the instant it is applied upon it, and leaves the mercury precipitated upon the surface. This coating of mercury facilitates the spreading of the amalgam when it is applied, although it may be spread without it, by merely dipping the pencil into the aquafortis, then into the amalgam, and afterwards applying it upon the surface of the work. This is the me thod employed fur delicate works, particularly where there are any parts of steel. Another method of applying the amalgam, is to mix it up in a cup or jar, with mercury and aqnafortis, to the consistency of thin cream. Into this the articles arc dipped, and become at once perfectly coat ed with the amalgam. We have seen this method used by a button manufacturer, and it succeeded very well, with scarcely any trouble; but it is only applicable on a large scale, for this amalgam will not keep long, because the acid turns the mercury black, by an oxide which will not pass off in the drying. They can only mix up, in this way, as much amalgam as they intend to use in a very short time. Without acid, the amalgam will keep any lenzth of time.
The next step in the operation is the drying of, or eva porating the mercury. For this a small charcoal stove or
oven is used: it is merely a square opening, surrounded by brick on every side, except the front, and a flue,pro ceeds from the top. It is made of a size proportioned to that of the work intended to be dried in it. As it is not intended that the draught of air shall be rapid, the stove may be placed in the lower part of a chimney, which is open all round the stove, so as to admit part of the cur rent of air to ascend into the chimney without passing through the tire ; and the charcoal being laid upon a small stone hearth, without any grate, it does not burn away very rapidly. The front of the chimney should be closed by a window-sash, which draws up and down, leaving just room enough beneath for the gilder to put his arms and hands into the chimney, while he sees his work in the stove through the glass, which prevents his inhaling the perni cious vapour of the mercury. A drawing of an apparatus of this kind is represented in Fig. 9. of Plate CVII. BUT TON' MANUFACTURE, with an additional apparatus to con dense and preserve the mercury dissipated by the evapo ration. The gilder is generally seated before this stove, and holds the work in a pair of iron pincers over the burning charcoal, till he sees it change colour from the white silvery appearance produced by the amalgam, to a gold colour, which gradually comes on as the mercury passes off.
At the first application of the heat, before it is suffici ent to make the mercury evaporate, the amalgam will be seen to grow more fluid, and flow as it were upon the sur face. In this state it is liable to collect more to one part of the work than others, probably because the heat is not equal in all parts, but renders the amalgam more fluid in some places than in others.
To counteract the effect of this tendency, a brush of soft hog's hair is frequently drawn over the work, to spread the amalgam perfectly equal over the whole sur face. This is a part of the process which requires much attention from the workman; for if he does not spread it well, the parts where the mercury accumulates will form whitish spots when dry ; and these spots will afterwards tarnish, and at length turn black, having very little of the gold laid upon them. A great deal depends upon the heat being very equally and regularly applied, until the mer cury begins to dry off, and the amalgam becomes fixed. The common gilder's stove is very ill calculated to attain this equality of heat. If the work has any cavities or hol low parts, into which the amalgam is liable to be accu mulated by the brush, it must he dislodged by shaking the work. The hair of the brush must be very soft and pointed, by rubbing it upon a piece of pumice-stone; and where there is any extent of surface to be gilt, the brush must be drawn in one direction, because it leaves a sort of grain, which will have a disagreeable appearance if it is not laid straight.