Copper and its alloys are subject to the same far-reaching changes as iron, but the action is less rapid. Bronzes of widely different composition have to be dealt with to ensure their preservation, and to a less extent, copper also[26]. According to von Fellenberg[27] bronze objects may be classified according to the material in which they have been found, i.e. peat mud, water, or earth.
“(1) Bronzes from peat mud are covered with a black earthy mass, which can be easily removed by water and brushes, the alloy then assumes its metallic lustre and the characteristic colour of bronze. The complete preservation of the pure metallic surface of the bronzes, in the same condition as they were when they were submerged, is easily accounted for by the enclosure of the metal in mud of organic origin under several feet of water which effectually excludes the oxygen of the air.
(2) The bronzes found in water, as for example in the beds of lakes and rivers, are less perfectly preserved. They have usually a thin coating of a calcareous deposit, which however often allows the lustre and colour of the metal to appear in places. When such bronzes have dark or green coloured patches or spots, the layer is very thin and may be removed by treatment with acids, which allows the metallic colour to become visible. Bronzes preserved in water still retain the same definite edges and points which they possessed when they entered the water. If bronzes which are markedly incrusted with verdigris are found in water in all probability they had lain in the ground a considerable time before being covered with water, and oxidation had penetrated deeply into the metal before immersion.
(3) Bronzes found in the earth or in graves appear covered with a fine green crust of verdigris which may be either light or dark in colour and which often has a vitreous lustre. This is generally known as Patina.
This crust varies in thickness from that of writing-paper to several millimetres. If the green crust be filed away, or better, removed by dilute nitric or sulphuric acid, the bronze is found to possess a reddish colour; below the crust of cupric carbonate is found a layer of cuprous oxide, which may be removed by ammonia, thus revealing the metal with its characteristic colour and lustre. This condition is characteristic of the slow oxidation of bronze in moist earth. The layer of cuprous oxide between the pure metal and the external crust of copper carbonate has been shown by the examination made by Dr Wibel to be a product of the reduction of copper carbonate by the metallic copper of the bronze. Bronzes belonging to this category have often lost their former metallic properties, and if of small diameter have often been completely converted into cuprous oxide, surrounded by a lustrous green or blue crust of carbonates. If a metallic core remains, it is found to be crystalline, brittle, and non-coherent, that is, it flies to pieces under the blow of a hammer. Fine ornamentation and sharpness, whether of edge or of point, have often disappeared. This does not occur with bronzes preserved in water.”
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.”