Villenoisy proposes to classify patina into three groups:
(1) Blue patina, with grey to blue-green and apple-green tints.
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.