A cheap but tedious bronze is made by dissolving half a pound of sal ammoniac in a quart of dilute nitrous acid, say one part of aquafortis to two of water; this takes about two hours before the colour is fully developed. A better and quicker bronze is made with one ounce of corrosive sublimate dissolved in one pint of vinegar; this requires about a quarter of an hour to produce the colour. The best and most rapid bronzing liquid, but the most expensive, is however the nitromuriate of platinum, called chemical bronze; with this liquid the required effect is produced in two or three minutes. The solutions are all employed in the same manner; the work having been thoroughly cleaned, is equally wiped over with the bronzing liquor, and this is allowed to remain until it ceases to act on he metal, which should then appear nearly black. Sometimes to assist the action of the acid the work is slightly warmed, and if necessary a second or third coat of the bronzing liquid is applied. The work is then dusted over with common black lead, and brushed like a stove to give it a good gloss.
With the view of rendering the action of the bronzing liquid as uniform as possible, small articles are sometimes dipped in the acid; for larger articles, the bronzing liquid is sometimes dabbed on plentifully with a piece of rag tied to a stick, in order to avoid the appearance of streaks, which sometimes occur when the bronze is applied with straight strokes. In some cases, as soon as the nearly black colour appears to be sufficiently developed, the work is rinsed in clean water to prevent the further action of the acid on the metal, which is afterwards dried and cc2 blackleaded. In other cases the bronzing liquid is plentifully covered with black lead, in order to distribute it more equally, and the work is then allowed to remain until nearly dry before it is brushed.
The work is finally lackered in the ordinary manner; but the green lacker, coloured with tumeric, as mentioned in page 1396, is employed to produce the green tint usually seen on bronzed works. The colour of the bronze depends in great measure upon that of the lacker, and should the latter be too green, the colour is modified by the addition of pale lacker in different proportions, according to the tint required. But although the green lacker will communicate a green tint, it is quite essential that a nearly black surface should have been previously produced by the bronzing liquid, and upon which the depth and perfection of the colour primarily depend.
The quality of the metal has also some influence on the colour of the bronze, and on this account it is rather difficult to make the colour uniform when the works consist partly of cast and partly of sheet metal, as the latter does not readily take so dark a tint from the bronzing liquid.
In works partly dipped, and partly bronzed, the latter is the final process.
Hard-wood lacker, or polish, is applied to turned works in the following manner. The work having been turned as clean and smooth as possible, and rubbed with fine glass paper, as mentioned at page 1123, a thin rubber is made of three or four thicknesses of soft linen rag, merely laid over each other, and a few drops of the hard-wood lacker prepared as directed at page 1392, are placed on the center of the rag either by a brush, or by covering the mouth of the bottle with the rag and shaking them together; a single thickness of rag is then put over the lacker, and a drop or two of linseed oil is placed on the center of the rag immediately over the lacker.
The rubber is then applied with light friction over the entire surface of the work while revolving in the lathe, never allowing the hand or the mandrel to remain still for an instant, so as to spread the lacker as evenly as possible, especially at the commencement, and paying particular attention to the internal angles, so as to prevent either deficiency or excess of lacker at those parts. The oil in some degree retards the evaporation of the spirit from the lacker, and allows time for the process; it also presents a smooth surface, and lessens the friction against the tender gum. When the lacker appears dry, a second, third, or even further quantities of lacker are applied in the same manner, working, of course, more particularly upon those parts at all slighted in the earlier steps. After a little practice this will be found a quick and easy process, but when convenient it is always desirable to repeat the process after the expiration of a few days, as the lacker will by that time be partly absorbed, especially in the end grain of the wood, which is more porous, and should therefore receive a larger proportion of lacker in the first application.
For common works, the lacker and oil are frequently both placed on the same surface of rag. In this case the oil is first applied to the rag, and the lacker is then added; but this method, although more rapid, does not produce so even a surface as when the lacker is covered with a single thickness of oiled rag, through which the clearer portions alone percolate gradually.
French polish is applied to flat surfaces in nearly the same general manner as hardwood lacker is applied to turned works.
As previously mentioned, the only difference between ordinary French polish and hardwood lacker is, that the former contains rather a larger proportion of spirit, which is adopted principally in order that the French polish may spread more easily, and dry less rapidly upon flat surfaces, which are generally larger than turned works, and, therefore, require more time in polishing, especially as the friction is derived exclusively from the motion of the hand.
The rubbers used in French polishing are made in a variety of methods, according to the fancy of the polisher, and the size is of course proportioned to that of the work, but they seldom exceed three inches diameter. The small cloth or list rubbers mentioned in Article 5, p. 1090, are very generally employed for French polishing, especially in laying on the first coat; and it is mostly preferred that the list should be torn off the cloth, as this makes the edge softer than if it were cut. Sometimes the rubber is covered with a piece of linen rag upon which the lacker is applied; at other times the rag is omitted, and the lower face of the rubber itself is saturated with lacker, in order to soften that which may remain in the rubber from previous use, and the excess is squeezed out before commencing the polishing. These rubbers often serve for several months' use.