9 Lead Compounds 157
These coatings are applied to numerous products including china, vases, sinks, toilets,
and washing machines. Lead, in the form of litharge (PbO), is often added to glazes,
but not usually to enamels, because it reduces the viscosity of the glass, which in turn
provides a smoother, more corrosion-resistant surface. The higher index of refraction
that results is desirable for these applications. Lead-containing glazes typically have
a composition between 16 and 35wt% PbO [13–15].
Optical glass includes a wide variety of applications. Of these, lead oxide is most
often incorporated into optical flints, although it might also be added to optical crown
glass, ophthalmic glass (crown or flint), and optical filter glass [16]. For example,
products in which the presence of lead is valued include Cerenkov counters, magne-
tooptical switches and shutters, and the cores of fiberoptic faceplates [17]. One of the
reasons why lead is added to optical glass is it creates a high index of refraction, which
can facilitate total internal reflection. The lead content of optic glasses varies considerably:
Fig. 2Viscosity versus temperature characteristics for various glass compositions [7]. 1, Fused sil-
ica; 2, 96% silica; 3, soda lime (plate glass); 4, lead silicate (electrical); 5, high-lead; 6, borosilicate
(low expansion); 10. aluminosilicate
Table 6 Refractive indices of various glasses [8,9]
Glass composition Average refractive index
Silica glass, SiO 2 1.458
Vycor glass (96% SiO 2 ) 1.458
Soda lime silica glass 1.51–1.52
Borosilicate glass (Pyrex TM) 1.47
Dense flint optical glasses 1.6–1.7
Lead silicate glasses 2.126