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The Assyrians used also opaque glass, which they colored, sometimes red, with the suboxide of copper, sometimes white, sometimes of other hues. They seem not to have been able to form masses of glass of any considerable size; and thus the employment of the material must have been limited to a few ornamental, rather than useful, purposes.
The pigments used by the Assyrians seem to have derived their tints entirely from minerals. The opaque white is found to be oxide of tin; the yellow is the antimoniate of lead, or Naples yellow, with a slight admixture of tin; the blue is oxide of copper, without any cobalt; the green is also from copper; the brown is from iron; and the red is a suboxide of copper.
So, again, sugar is a well-known antidote to poisoning by salts of copper; and sugar reduces those salts either into metallic copper, or into the red suboxide, neither of which enters into combination with animal matter. Now diluted sulphuric acid has the property of decomposing all compounds of lead with organic matter, or of preventing them from being formed.
It is uncertain whence the coloring matter was derived; perhaps the substance used was the suboxide of copper, with which the Assyrians are known to have colored their red glass. The blue of the Assyrian monuments is an oxide of copper, sometimes containing also a trace of lead.
A red precipitate of suboxide of copper is at once formed, and by the time the mixture cools to 167 deg. Fahr., the precipitate will have settled. A suitable wooden sieve is placed in the vessel, and on this the polished articles are laid. In about one minute the sieve is lifted up to see how far the operation has gone, and at the end of the second minute the golden color is dark enough.
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