2Ag + O + (n + 2)NaCl + CO2 = 2AgCl, nNaCl + Na2CO3.
But this reaction differs from that which takes place in the case of copper in that it does not proceed continuously except in the presence of a considerable quantity of salt water only, as for instance in the sea. In museums the alteration goes no further than corresponds to the minute quantity of sodium chloride contained in the object. On the other hand in an earth which contains salts, the continued presence of water can bring about a more or less marked change, and in some cases even a stable silver subchloride may be formed.”
Lead.
Objects of lead have always a white appearance due to the formation of lead carbonate, as has been already mentioned above in connection with bronze. The carbonate is also often mixed with oxide.
Tin.
Objects made of tin[77] are frequently found in pile-dwellings in a good state of preservation. They are, however, occasionally covered with white or brown layers of hydrated tin oxide, while in some cases oxidation has advanced so far that no trace of metallic tin is left in the hard grey masses of oxide which result.
Gold.
Gold is found to be unaltered, or there is at most a thin layer of silver chloride, which is the result of the action of sodium chloride upon the silver which the gold usually contains. Gold objects often have a red coating, which has been found to consist of ferric oxide, and is due to extraneous deposits which have been fixed by the silver chloride. I have not been able to prove the presence of gold chloride[78], and it does not appear possible that water containing sodium chloride can have the power of acting upon gold. If the ferric oxide is removed mechanically, some of the gold will naturally be removed with it, and this can be readily ascertained on analysis.
The degree of brittleness in objects of gold depends upon the changes which have taken place in other metals, especially silver, which are mixed with it.