The following extract is taken from the section dealing with patina in Bibra's "Bronzes and Copper Alloys[35]":
"The conditions under which Patina is formed, or rather the conditions under which copper alloys are gradually decomposed, are variable in the extreme. The four main factors which may be instrumental in determining the chemical changes may be thus stated:
(_a_) The composition (qualitative and quantitative) of the particular alloys.
(_b_) The mode of smelting and the original manipulation of the components, such as a good or poor mixing, fine or coarse grain, etc.
(_c_) The locality in which the alloy has lain.
(_d_) The length of time during which the alloy has been exposed to the particular conditions.... Marked differences may appear in the extent and nature of the chemical changes shown by the same alloy; thus one fragment while underground may have been enclosed in an urn containing bone ash and dry sand, while another fragment may have been in contact with decaying animal matter."
From what has been said above, the variations in the composition of patina may be readily explained. The composition has been found to be:
(beta) Basic carbonate and sulphide of copper.
(gamma) Malachite (normal carbonate of copper), with occasional admixture of cuprous oxide and azurite (acid carbonate of copper) [Stolba].
(delta) Crystalline cuprous oxide, according to Wibel[36] a reduction product of the carbonate of copper, by the action of the copper of the bronze.
Lastly, copper chloride has been occasionally found in patina [Haidinger][37]. This is only to be expected from the varying character of the localities in which the statues or bronzes are found. The author has himself noticed on board ship, how objects of copper and brass, which are exposed to the salt spray, develop a durable coating of copper oxychloride[38] (atacamite).
In conclusion, reference may be made to a statement of Chevreul[39], who, after examination of both hollow and solid specimens of Egyptian statuettes, states that the bronze is of an excellent quality and that it occurs in four different conditions. He describes these four conditions, three of which are undoubtedly patina or altered copper, as follows: