As we have said, there are many methods by which to give the proper patina to metals, and a good deal of mystery, some fakers and imitators claiming to be in possession of unrevealed secrets.
When exposed to the air for a long time, copper and bronze acquire a fine brown or green patina which, as every collector knows, greatly enhances the merits of an artistic piece in these two metals. A perfect imitation of the result of a long process of time is not an easy matter, in fact an almost impossible task.
Formerly the patina of a bronze was in a way the final test of authenticity, but nowadays there are modern imitations of so deceptive a character that the best connoisseurs are taken in.
One of the best known methods by which old patina is imitated on copper and bronze, is to follow as closely as possible the process by which the genuine patina is produced. Thus the action of rain, interment, immersion in some permeating substance that will generate hydrosulphuric acid are called into service by those willing to wait a comparatively long time for the desired effects. Others accelerate the above process by increasing the proportion of the natural conducive elements. The objects are also treated with water containing ammonia, carbonic acid, etc., exposed to the intense and direct action of vapour or vaporized acid in order to produce those basic salts that form a certain patina.
To obtain the malachite kind of patina that generally characterizes objects found in the ground, the imitator generally brushes the metal over with a very weak solution of cupric nitrate to which a small quantity of common salt in solution may be added. When completely dry it is again brushed over with a liquid consisting of one hundred parts of weak vinegar, five of sal-ammoniac and one of oxalic acid, and the application is repeated after the first has dried. In about a week’s time the metal will have acquired a green-brown colour that may be polished with encaustic if the patina is to have a shiny appearance.
Such is the leitmotiv, more or less, of the processes for obtaining the green or brown-green patinæ. Some dip the object in cupric acid and then place it in a room in which an excess of carbonic acid is produced, by others preference is given to one or the other element according to the tone and colour desired.
Brass articles are coated with green patina by a solution containing 150 parts of vinegar to which has been added ten parts of copper dissolved in twenty of nitric acid. An application of this liquid is generally made on the object.
The brown patina usually characterizing old medals is obtained in many ways. One is by heating the medal at the flame of a spirit lamp and then brushing it with graphite. To colour a number of medals at the same time, some imitators dissolve thirty parts of verdigris and thirty parts of sal-ammoniac in ten of water, adding water to the solution till a precipitate is no longer formed. Then the medals are placed in a shallow dish without touching one another and the boiling solution is poured over them. The medals are allowed to remain in the solution till they have acquired the desired tint, which should be a fine brown.
Green or bluish patinæ may also be given to bronze or copper by triturated copper carbonate used as a paint with a pale spirit varnish, shellac or sandarac, and applied with a brush.
Verdigris generally gives a bluish tint and crystallized verdigris a pale green tint. The two tones can be mingled to obtain some special hue.