Page 26 - Curriculum Visions Dynamic Book

Lead and glass
Glass is one of the most common materials
we use. It is formed from sand with additional compounds such as soda (sodium carbonate), limestone and lead. It is heated to about 1300°C, when the reactants chemically combine to produce glass (and give off gaseous products).
The Venetians were the first to add lead to
the glass-making mixture. The lead allows the glass to be moulded over a wider range of temperatures. This, in turn, allowed the Venetians to make glass in more complicated shapes.
However, the lead-containing glass called crystal was developed in England in the 17th century.
The use of lead made the glass even clearer and therefore more transparent. Also, the lead changed the optical properties of the glass, giving cut glass a special sparkle. This helped to make London the foremost glass-making centre
of the 18th century.
The effect of lead additives
 Lead glass (crystal)
Most glasses are a form of silica made from a network of silica
molecules. In quartz these molecules are packed together in a regular framework; but when other atoms, such as sodium, calcium or lead, are added, the rigid framework is somewhat broken up. This is what makes glass less viscous than molten quartz, and why it also melts at lower temperatures.
Lead makes glass sparkle because it changes the optical properties of the glass, increasing the refractive index of the glass and thus increasing the reflectivity. A traditional lead crystal glass contains (by weight) about 56% silica, 30% lead oxide, 2% sodium oxide and 12% potassium oxide.
Optical glass, for example, glass used in lenses, has a broad spectrum of lead content, the amount varying between 3 and 70% depending on the refractive index required.
Lead also has the ability to absorb radiation, so optical glass with a high lead content (64% lead oxide, 27% silica) is used for remote viewing of X-ray facilities, hot cells in nuclear research, irradiation facilities for food preservation and medical sterilization installations.
 Lead glasses have about half the radiation absorption capability compared with lead sheet of
the same thickness, which is why the glass is usually made thicker than the remaining lead shielding.
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