Commonly known as lead crystal, lead glass is used to make a wide variety of decorative glass objects. It is made by using lead oxide instead of calcium oxide, and potassium oxide instead of all or most of the sodium oxide.
The traditional English full lead crystal contains at least 30% lead oxide (PbO) but any glass containing at least 24% PbO can be described as lead crystal. Glass containing less than 24% PbO, is known simply as crystal glass. The lead is locked into the chemical structure of the glass so there is no risk to human health.
Lead glass has a high refractive index making it sparkle brightly and a relatively soft surface so that it is easy to decorate by grinding, cutting and engraving which highlights the crystals brilliance making it popular for glasses, decanters and other decorative objects.
Glass with even higher lead oxide contents (typically 65%) may be used as radiation shielding because of the well-known ability of lead to absorb gamma rays and other forms of harmful radiation.
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Many glass making terms have entered the language: 'Shut yer gob': a molten lump of glass is called a 'gob' to which the glass blower attached a tube to blow the glass into shape. The blower had to blow hard which made his cheeks very large. Today someone with a big mouth is told they have a big gob.