In another volume of the series[28] von Fellenberg states that basic copper chloride occurs as a constituent of patina.
A few lengthier quotations may be conveniently given here, in part verbatim, in part abstracted from literature which is not readily accessible.
Reuss[29] states that it has been hitherto generally assumed that copper is first converted into cuprous oxide which is then converted into a green hydrated oxy-carbonate which is separated from the metal by a thin layer of cuprous oxide. The specimens examined by him, however, showed no such dividing layer, the metal being either directly in contact with the malachite[30], or else separated from it by a black or bluish layer of cupric oxide. He further draws attention to the occurrence of irregular knobs two to three lines in height which consist, in part, of azurite[31]. Neither oxides of tin nor chlorine were found. The alteration of the bronze he explains by the prolonged oxidising action of water containing carbonic acid.
In an exhaustive memoir Wibel[32] describes the various kinds of patina as malachite, copper-oxychloride, and azurite, with admixtures of tin oxide, silver, iron oxide, lead chloride and copper chloride. He discusses also the occurrence of the cuprous oxide layer which is said to have been described by Sage as early as 1779. After detailing the observations of Davy, Hünefeld, and Picht, that the metallic copper exists partly in alloy and partly free as crystals in the layer of cuprous oxide, he continues as follows[33]:
"The process of decomposition in bronzes has been regarded as a slow oxidation, in which cuprous oxide marks the first and incomplete stage, while the carbonates represent the later completed phase. The formation of both these substances was assumed to be due to moist oxidation, on bronzes as well as in those superpositions of copper, cuprite, and malachite, so frequently found in minerals. Indeed, no other process of formation of the carbonates is conceivable; moreover cupric oxide, if really present, would be naturally regarded as a product of oxidation. The other substances, such as tin oxide, which are occasionally found, would be produced in part by similar simple processes, in part by the simultaneous action of particular salts, the chlorine compounds, for instance, by the presence of water containing sodium chloride. Similarly the production of cuprous oxide was usually attributed to an incomplete oxidation of the copper, although it might very well be the result of an inverse process, viz. the reduction of pre-existing cupric oxide."