“Copper objects, which have been buried in the earth for several centuries, are found to be covered with a green patina and with an earthy layer of varying thickness which has the same colour. The metal itself is to a greater or less depth converted into cuprous oxide. After removal the patina returns; in other words, the metal shows further growths, and when in contact with the atmosphere of our climate is in all cases by degrees converted into dust. These facts are well known to every collector and archaeologist, who designate the specimens thus affected ‘métaux malades’.... Analysis shows that the superficial green layer consists in great measure of atacamite (cuprous oxychloride) agreeing with the formula 3CuO, CuCl2, 4H2O. There are also found traces of sodium salts. The changes which have been observed are produced by salts from the soil, especially sodium chloride, held in solution by water. In fact a few drops of salt water placed upon a copper plate are sufficient for the formation of oxychloride.... This reaction is the result of the simultaneous action of the oxygen and of the carbonic acid of the air upon the copper and upon the sodium chloride in the presence of moisture, as is represented by the following equations:
4Cu + 4O = 4CuO
4CuO + 2NaCl + CO2 + 4H2O = 3CuO, CuCl2, 4H2O + Na2CO3.
Thus the continuous transposition which, under the influence of a salt-containing water, often acting in large volume, converts the metal into oxychloride, is readily explicable: while the process whereby the small quantity of sodium chloride originally present in an excavated bronze may cause its destruction after it has been placed in a museum is the following:
When the reactions given above have resulted in the formation of a certain amount of copper oxychloride, it is to be supposed that a small quantity of sodium chloride comes into simultaneous contact with the oxychloride and with the metallic copper. A slow reaction takes place and a double compound of cuprous chloride and sodium chloride is formed. The remaining portion of copper is converted into cuprous oxide:
3CuO, CuCl2, 4H2O + 4Cu + 2NaCl = Cu2Cl2, 2NaCl + 3Cu2O + 4H2O.
The solution of the double salt is also in turn oxidized by the air which penetrates the whole mass. The result of the reaction is therefore sodium chloride, atacamite, and copper chloride:
3Cu2Cl2 + 3O + 4H2O = 3CuO, CuCl2, 4H2O + 2CuCl2.
The copper chloride which remains, if in contact with air and copper or even cuprous oxide, is similarly converted into oxychloride:
CuCl2 + 3Cu + 3O + 4H2O = 3CuO, CuCl2, 4H2O.
The cycle is thus complete, and its constant recurrence under the influence of oxygen and moisture is the cause of the destruction of those objects containing copper which are imbedded in earth, and even of those which are preserved in our museums.”