Lead Glass - Properties

Properties

The addition of lead oxide to glass raises its refractive index and lowers its working temperature and viscosity. The attractive optical properties of lead glass result from the high content of the heavy metal lead. The high atomic number of lead also raises the density of the material, since lead has a very high atomic weight of 207.2, versus 40.08 for calcium. The density of soda glass is 2.4 g/cm3 or below, while typical lead crystal has a density of around 3.1 g/cm3 and high-lead glass can be over 4.0 g/cm3 or even up to 5.9 g/cm3.

The brilliance of lead crystal relies on the high refractive index caused by the lead content. Ordinary glass has a refractive index of n = 1.5, while the addition of lead produces a range up to 1.7. This heightened refractive index also correlates with increased dispersion, which measures the degree to which a medium separates light into its component spectra, as in a prism. Crystal cutting techniques exploit these properties to create a brilliant, sparkling effect as each cut facet reflects and transmits light through the object. The high refractive index is useful for lens making, since a given focal length can be achieved with a thinner lens. However, the dispersion must be corrected by other components of the lens system if it is to be achromatic.

The addition of lead oxide to potash glass also reduces its viscosity, rendering it more fluid than ordinary soda glass above softening temperature (about 600 °C), with a working point of 800 °C. The viscosity of glass varies radically with temperature, but that of lead glass is roughly 100 times less than that of ordinary soda glasses across working temperature ranges (up to 1100 °C). From the glassmaker’s perspective, this results in two practical developments. First, lead glass may be worked at a lower temperature, leading to its use in enamelling, and second, clear vessels may be made free from trapped air bubbles with considerably less difficulty than with ordinary glasses, allowing the manufacture of perfectly clear, flawless objects. When tapped, lead crystal makes a ringing sound, unlike ordinary glasses. Consumers still rely on this property to distinguish it from cheaper glasses. Since the potassium ions are bound more tightly in a lead-silica matrix than in a soda-lime glass, the glass when struck absorbs less energy. This causes the glass to oscillate, thereby producing its characteristic sound. Lead also increases the solubility of tin, copper, and antimony, leading to its use in colored enamels and glazes. The low viscosity of lead glass melt is the reason for typically high lead oxide content in the glass solders.

The presence of lead is used in glasses absorbing gamma radiation and X-rays, used in radiation shielding (e.g. in cathode ray tubes, where lowering the exposure of the viewer to soft X-rays is of concern).

The high ionic radius of the Pb2+ ion renders it highly immobile in the matrix and hinders the movement of other ions; lead glasses therefore have high electrical resistance, about two orders of magnitude higher than soda-lime glass (108.5 vs 106.5 Ohm·cm, DC at 250 °C). Lead-containing glass is therefore frequently used in light fixtures.