Glass.
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 In certain circumstances almost the whole of the alkali is withdrawn from the glass.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.