vitrifaction

Mercury, was given an azure tint by vitrifaction, the whole stage having been subjected to an intense heat after it was erected, whereby the bricks composing it were converted into a mass of blue slag; the seventh stage, that of the moon, was probably, like the fourth, coated with actual plates of metal.

Chaldea From the Earliest Times to the Rise of Assyria 1879

Mercury, was given an azure tint by vitrifaction, the whole stage having been subjected to an intense heat after it was erected, whereby the bricks composing it were converted into a mass of blue slag; the seventh stage, that of the Moon, was probably, like the fourth, coated with actual plates of metal.

The Seven Great Monarchies Of The Ancient Eastern World, Vol 4. (of 7): Babylon The History, Geography, And Antiquities Of Chaldaea, Assyria, Babylon, Media, Persia, Parthia, And Sassanian or New Persian Empire; With Maps and Illustrations. George Rawlinson 1857

Their _character_ is peculiar, exhibiting by no means complete fusion, but rather semi-vitrifaction by roasting; the ore retaining, not unfrequently, a large measure of its original weight and form, explained, as Dr. Percy kindly informs me, by charcoal being the fuel employed, and not necessarily arising from want of skill in the operatives.

Iron Making in the Olden Times as instanced in the Ancient Mines, Forges, and Furnaces of The Forest of Dean 1846