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.
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 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.
Ancient glass, which is for the most part lime-soda silicate, exhibits a dull, rough surface with the well-known iridescence. The alkali is removed from the glass by the action of moisture, oxygen and carbonic acid, while the silicic acid remains in the form of minute scales, which cause the iridescence by interference. According to Bunsen the chemical action of the gases of the atmosphere on glass is facilitated by the condensation of water upon its surface; for the water thus condensed absorbs large quantities of carbonic acid. In certain circumstances almost the whole of the alkali is withdrawn from the glass. An analysis of glass of this kind, together with a discussion of the chemical reactions involved, is given in Muspratt's "Chemistry[79]."
Glass objects which are markedly iridescent undergo gradual decay even under museum conditions; this is probably due to the continued action of carbonic acid.