From the following considerations Wibel thinks that he is justified in his assumption that the layer of cuprous oxide is the result of reduction. Firstly, by no means all bronzes which have been dug up, even though from the same excavation, show the layer of cuprous oxide. Secondly, the cuprous oxide layer is in the crystallized state. Thirdly, 'all the facts of chemistry show that the formation of cuprous oxide can only take place by reduction, given the ordinary conditions of temperature and pressure.' Finally, in addition to oxygen and carbonic acid, many salts, those of ammonia for example, occur in the spots where bronzes are found and favour the formation of copper salts. Wibel also quotes in support of his views the experiment of Bucholz[34], that a strip of copper, the upper half of which is immersed in a layer of distilled water, and the lower half in a concentrated neutral solution of copper nitrate carefully poured beneath it, becomes coated with copper and cuprous oxide.
"Bronze objects are attacked by waters which contain oxygen, carbonic acid and a greater or less percentage of salts. Such soluble salts as are formed are removed by solution, while the bronzes become covered, according to circumstances, with an insoluble layer either of carbonate or of oxide, whereby the form of the objects is preserved. The water then penetrates by capillary action through the porous coating into the interior, and attacks further portions of the metal, forming a layer of soluble cupric salt; a portion of which is able to pass by diffusion through the external layer. For the same reasons the liquid, bounded as it is on one side by the metal and on the other by the almost insoluble crust, shows varying degrees of concentration: thus all the conditions necessary for the Bucholz process are fulfilled. If the water is rich in salts, a concentrated copper solution is formed and even metallic copper may be deposited from it (i.e. the 'copper crystals' of bronzes); but if, as is usually the case, the water contains only small quantities of salt, cuprous oxide crystals only are formed. The fact that the process takes place chiefly in the pores made by the water itself is readily understood, because of the comparative quiescence of the liquid; and that it causes a marked progressive change in the object arises from the continual exchange of a portion of the copper solution already formed with fresh solvent from outside. Where the absence of carbonic acid or other circumstances hinder the formation of an almost insoluble crust, the reactions detailed above may, under favourable conditions, take place directly upon the surface of the bronze; if, on the other hand, there is a too rapid change of liquid (as for example in very wet localities), the process may altogether fail to set in, since the necessary conditions of rest, etc. are wanting. Since the absence of the necessary conditions may arise from a number of purely accidental causes, it will be easily understood, that bronzes from one and the same grave may show the same percentage of carbonates, but very dissimilar percentages of cuprous oxide. In short all actually observed conditions in which bronzes are found are accounted for by the explanations given above."