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.
The first group comprises almost all the bronzes which are found in peat, which show, with rare exceptions, a metallic, often somewhat darkened, surface. Their state of preservation depends upon the nature of the peat in which they are found, but the metal surface has, in the majority of cases, become somewhat rough and etched, although all the details are clearly distinguishable. More rarely one side retains the original polished surface while the other side is much corroded. If a much corroded bronze is found, the peat in which it has lain has probably contained free sulphuric acid (see also p. 13). All bronzes found in water must be included also in this group. The second group will then comprise all bronzes with an oxidized patina.
The classification given by Villenoisy seems entirely unsuitable, for it does not by any means exhaust all the kinds of patina which may occur. Thus no mention is made by him of the frequent occurrence of a patina which contains chlorine. If we separate the dark brown and the blackish patina, in so far as these two colours are pure, from those of a green colour, the first two varieties cannot be regarded as groups, because the tones of colour differ too much, and because, as Villenoisy himself observes, widely different patinas often occur on one and the same bronze. The durability of a patina upon a bronze cannot be judged either by the outer appearance or by the chemical composition alone. The fact that there has been no alteration in the outward appearance for many years offers no guarantee against further changes taking place. Thus a Minotaur[53] in the Berlin Museum, which for many years had shown no sign of change, was eventually found to be completely covered with numerous bright green spots over its entire surface. My own opinion is that the only patina which is really stable is that which consists of combinations of oxygen, hydrogen and carbonic acid with the metal, somewhat similar to those analysed by Schuler (see page 24), and by Arche and Hassack (see page 27). The presence of sulphides, and even of sulphates, does not seem to be injurious.