Lead Silicates

When litharge is fused with silica it forms a yellow, vitreous silicate which enters into the composition of the lead glazes used in pottery, and also of flint glass, which, on account of its high refractive power, is employed for optical purposes and for making artificial gems. The solubility of lead oxide in lead glass increases with temperature, and the yellow colour of glass rich in lead is attributed to the thermolytic dissociation of lead silicate, thus:

PbO.xSiO₂ ⇔ PbO + xSiO₂.

According to Mostowitsch, a number of lead silicates exist, in which the proportion of lead oxide to silica rises from PbO:SiO₂ to 6PbO:SiO₂.

By the study of the freezing-point curves of mixtures of lead oxide and silica, Hilpert and Weiller find evidence for the existence of only PbO.SiO₂, and probably 2PbO.3SiO₂. According to Hilpert and Nacken, however, 3PbO.2SiO₂ exists, and probably 3PbO.SiO₂.

Cooper, Shaw, and Loomis, from a study of heating curves, show the existence of Pb₂SiO₄, melting at 746° C., as well as PbSiO₃, melting at 766° C.

Lead silicates are of importance in pottery manufacture, because; being less soluble than "raw lead" in dilute acid such as occurs in the gastric juice, they are less harmful to the workers. Thorpe and Simmonds have examined the solubilities of various lead silicates in dilute hydrochloric acid, and find that when the molecular proportion of acidic to basic oxide falls below 2:1 the silicate is readily attacked by the dilute acid; but when larger proportions of silica are combined with lead oxide there is little action. This is in harmony with the observation of Faraday that glass made of equal weights of silica and lead oxide does not become dull when exposed to the action of hydrogen sulphide; but that a glass made by fusing 8 parts of this glass with 1 part of potash is so tarnished. In the former case the ratio (acidic molecules)/(basic molecules) is 3.7, ih the latter about 2. It is well known, basic molecules not only that lead glass is easily fusible, but that it is easily reduced and blackened in the inner blowpipe flame. To account for this reducibility, Simmonds, who recognises the four simple silicates Pb₂SiO₄, PbSiO₃, Pb₂Si₃O₈, and PbSi₂O₅, as well as various double or complex silicates, introduces certain novel constitutional formula for these compounds, containing oxygen chains and linked silicon atoms.

In a further communication, Simmonds claims that the blackening of lead glass when heated in a reducing flame or a current of hydrogen is due not to the separation of metallic lead, but to the formation of a reduced silicate or "silicite" of lead. It was found, indeed, that powdered flint glass, reduced in hydrogen, contained no uncombined lead.