Finally a memoir by Villenoisy[52] should be noticed, the first portion of which is devoted to a proof that the patina of ancient bronzes is due to natural causes and is not the result of the art and methods of the metal-workers of the ancient world. The second portion deals with the various kinds of patina and their formation, as the following excerpts will show:
The following substances may be mentioned as capable of attacking alloys:—Ordinary oxygen, which has but a slight action on copper in the dry state but a more vigorous action in the presence of moisture, or as ozone; sulphur also, ammonia, carbonic acid, and organic substances. Water has no direct influence, but acts as a solvent. The metals or metalloids of the alloys can unite independently with oxygen, sulphur, or carbonic acid, etc. to form oxides, sulphides, or carbonates; or again they can react among themselves and produce copper stannate or lead stannate. Ammonia will form ternary compounds or play a catalytic part. Whatever processes may result in the formation of patina, the changes which occur are too slow to allow their imitation and examination in the laboratory. The four metals which are found in ancient bronzes, viz. copper, tin, zinc, and lead, are particularly liable to certain changes. Copper forms chiefly cupric and cuprous oxides. The first of these is soluble in ammonia; the latter combines with ammonia to form a substance which is colourless, but which becomes blue on exposure to air. Tin forms stannic acid which probably produces stannates with copper and lead. Zinc becomes zinc oxide, lead is converted into oxides. Sulphur, as sulphuretted hydrogen, causes the formation of metallic sulphides. Ammonia has a threefold action, viz. it causes and furthers hydration, it is an energetic solvent, and it forms double salts. This last-mentioned action is particularly important in the formation of patina. Carbonic acid in the presence of moisture attacks copper, lead and iron, and, as a carbonate, exists in every metallic oxide which is exposed to the air. Several combinations of copper with carbonic acid are known, while lead is readily converted into lead carbonate by oxidation. The part played by the carbon compounds resulting from the decomposition of animal and vegetable substances has hitherto received little attention, but this decomposition of organic material is probably the chief cause of the beautiful blue patina. The action of oxygen will depend upon the composition of the metal, upon the locality, and upon numerous other circumstances, while the colour of the patina will vary accordingly.
Villenoisy proposes to classify patina into three groups:
(1) Blue patina, with grey to blue-green and apple-green tints.
(2) Dark green patina.
(3) Black patina.
1. The blue patina produced by the action of ammonia upon the products of previous oxidation does not destroy the outer form of the bronzes, but is nevertheless unfavourable to the preservation of the metal, since the substratum of the patina is a porous mass, consisting of lead stannate and lead carbonate mixed with ammoniacal copper carbonate. The specimen has frequently an intact appearance, as if covered with a thin layer of oxide only, whilst in reality all traces of metal have already disappeared, and slight pressure often suffices to break the bronze into pieces. The nearer the colour of the patina approaches to grey, the less solid is the bronze likely to be, a result which is no doubt caused by the presence of lead carbonate. This type of patina has often a yellowish colour, especially on prominent parts, where, being porous, it has retained in its superficial layers substances which were in suspension in the subsoil water. The occurrence of a pale fine-grained patina of a uniform colour is in almost all cases due to the scaling off of patina belonging to this type.
2. Whilst blue patina is generally formed on bronzes which have been buried in earth, the dark green patina is formed both in the earth and also in the open air. The presence of lead seems to be an obstacle to its formation. This dark green patina consists of variable proportions of basic copper hydrate and copper carbonate. The green layer frequently rests upon one of a red colour, a circumstance which proves that the dark green patina is almost always the result of two successive reactions: cuprous oxide is first formed and subsequently takes up water and carbonic acid. Tin is present as copper stannate. The cuprous oxide, which is generally regarded as unaffected by air, is perhaps drawn into further reaction through the agency of ammonia. In those situations where there is a flow of rain water a certain translucency of the green patina is often produced, and this is also possibly caused by ammonia. Unlike the blue patina, the dark green variety assists the preservation of bronze.
3. Black patina is probably due to a variety of circumstances. The substances which enter into its composition are cupric oxide, lead oxide, lead peroxide, copper sulphide and lead sulphide. If bronze does not contain lead it is blackened only by the action of sulphur. The rarity of black patina is no doubt due to the rapid oxidation of the copper on the originally rough, unpolished surface, which leads to the formation of a green patina.
These extracts show how little value can be attached to a classification of bronzes from the character of the patina present: the views upon the subject are so divergent, while the actual composition of the incrustations which form the patina and their external appearance are so widely different. In fact only two groups of bronzes may be distinguished, i.e. those which show patina and those from which patina is absent.