Silver takes a fine surface of gold, and of a yellow co lour, unless it is gilt twice over, and coloured afterwards.
Iron and steel will not take the amalgam, as the mercu ry has no affinity for these metals.
Tin, zinc, lead, or other of the baser metals, are never gilded by amalgam : they would deteriorate the gold too much, nor would they bear a sufficient heat.
To amalgamate the gold for wash-gilding, a small quantity is put into a crucible, or an iron ladle, with about six times its weight of mercury. The ladle or crucible should be rub bed on the inside with whitening, to prevent any waste; and being put upon a charcoal fire, and gently heated, the mer cury will soon dissolve the gold. To assist the solution, it should be stirred with an iron wite. The heat should not be sufficient to evaporate the mercury, at least not until the solution of the gold is nearly effected ; the heat may then be increased for a moment, till a vapour is seen to rise from the crucible. The amalgam being made, is thrown out into water, where a small portion of mercury will be seen to separate itself. This contains very little if any gold ; but the real amalgam forms a small lump of paste, of nearly the same consistence with butter ; but it will be observed to have particles of mercury intermixed with it. To remove these, the amalgam is twisted up in a piece of fine wash leather, and gently pressed with the finger and thumb : t he mercury will pass through the pores of the leather, and leave the amalgam fit for use. In this state it is the colour of silver, without any appearance of gold, and does not contain more than twice the weight of mercury to that of tile gold. The mercury which has passed through the leather, should be reserved for making future amalgam, as it will probably contain a small proportion of the gold.
Some workmen vary this process by heating the gold to redness, and heating the mercury to the point of evapora tion ; then, throwing the hot gold into the mercury, the amalgam is quickly effected. We do not know any differ ence of effect which should give the preference to either of these processes.
It is essential that both the gold and the mercury should be pure. The mercury of commerce is almost always adulterated with lead, which is very prejudicial to the gild ing. It should be separated by distilling the mercury in
an iron retort, with a gentle heat ; this ‘N ill leave the lead in the retort ; or the mercury may be revived from cinna bar in a very pure state. Nothing can be gained by em ploying gold of an inferior quality, as it will always require a greater proportion to produce the same covering, and the brilliancy will be much impaired.
It has been recommended to grind the amalgam on a marble stone, or in a glass mortar, and to wash it continu ally with a solution of common salt in water. This will carry away a blackish colour, occasioned by the oxide of the mercury ; and the washing is to he continued till it ceases to colour the salt and water. We have never met with any artist who practises this washing ; for if the gold and mercury are pure, and the amalgam is not overheated, so as to oxidate the mercury, it will be unnecessary.
The first circumstance to be attended to in wash-gilding is, to make the surface of the metal perfectly free from all ,!,curf, tarnish, or oxidation ; for the covering of gold upon the metal unites therewith by a kind of amalgamation, and forms a superficial alloy, which would be discoloured by the smallest coat of tarnish.
The cleaning is performed by pickling the metal, in a solution of aquafortis and water: the strength of the so lution is immaterial, except with respect to the time that the metal must remain in it. A quantity of acid, equal to one-fourth of the water, will clean the metal in a few mi nutes; but if weaker, is equally effective in a longer time : and if the article is accidentally left in the pickle for a long time, it will not be injured. For cheap and coarse ar ticles, a strong solution of sal nixon, or sal ammoniac, is employed ; and to render it more active, the metal is neat cd and plunged into it : this raises a black or brown oxide over the whole surface. it is then dipped ill the pickle of aquafortis, which throws off a slight sc,,le Iron) the surface, and renders the metal clean at once; and it is pur into clean water, to prevent the farther action of the acid from pro clueing a tarnish before the gilding is performed.