The first in this category represents the ultimate stage of decomposition of bronze and forms the outer incrustation of the statuettes. It is a compound of copper chloride and copper oxide and water in the same proportions as in Peruvian copper oxychloride (atacamite); the blue parts contain water, carbonic acid and cupric oxide. It is in fact the blue hydrated copper carbonate.
(β) The blood-red substance consists chiefly of cuprous oxide with an admixture of tin oxide. It contains chlorine, apparently as cuprous chloride, sometimes in considerable quantity.
(γ) The reddish colour seems to be due to the tin undergoing more alteration in the course of time than the copper.
(δ) The well-preserved bronzes are remarkable for the excellent quality of the alloy.
Chevreul continues:
“Copper and tin have thus undergone gradual changes from without inwards into chlorides, oxides and carbonates; the tin has been converted into oxide, the outermost layer of copper into oxide and chloride, while the layer in contact with the unaltered bronze beneath can only be oxidised into the suboxide.”
In a fissure in a statuette he found crystals of blue basic carbonate of copper, chloride of lead and hydrated oxychloride of copper.
Bibra himself examined the patina of several bronzes and found it to consist mainly of sulphate and carbonate of copper.
To complete the quotation from Chevreul’s work we may observe that he finds the cause of the formation of the patina to be the action of air, of water containing salt, and of carbonic acid. It is interesting that Chevreul succeeded in restoring a small bronze containing chlorine by reduction in a stream of hydrogen.
In the year 1865 M. A. Terreil[40] published the analysis of a bronze patina containing chlorine. The result is as follows: