Lead Glass (aka Crystal or Lead Crystal)
Tupperware, made from HPDE, became a product defined by a ‘burp’, the sound the material made when you peeled away the lid and air was sucked into the soft, waxy container. Lead crystal is also a material defined through sound, in this case the ‘ping’. Lead glass, or lead crystal as it is also known, signifies luxury, the resonance and ‘ping’ being the way most people assess its quality. It is associated with diamonds and pearls, ostentation and wealth. For anybody interested in chemistry, this is the result of the addition of lead oxide – 30 per cent for English full-lead crystal – which replaces the lime that is used in the more conventional soda-lime glass.
The introduction of lead into the recipe for glass results in the glass having a higher refractive index – brightness and sparkle – and a relatively soft surface in comparison to regular soda-lime glass. The resultant properties of clarity and softness, combined with the processes of grinding, cutting and engraving create the overall effect of a crystal-like glass. There are several ways of classifying the quality of lead crystal – which can be described under various terms, including crystal glass, pressed lead crystal, lead crystal and full lead crystal – that are dependant on the percentage of lead oxide; however, many people prefer to finger flick the glass and listen for that pure ring to test its quality.
Image: Stemware by Josef Hoffmann for Lobmeyr
•High lead content
•One of the heaviest glasses
•Low rigidity compared to other glasses
•High refractive index so high clarity
•Relatively soft, so easy to engrave
•More expensive than soda-lime
•The ‘ping’
•Not as good as borosilicate or glassceramic in thermal shock
Sources
Widely available.
Cost
Relatively expensive
Sustainability issues
The lower temperature at which this type of glass is formed means that it requires less energy to form. Although lead is a known health risk and can cause lead poisoning, which affects the nervous system, the lead (there must be at least 24% for a glass to be classed as lead crystal) in lead crystal is locked into the chemical structure posing no health risk.
Production
Lead glass can be processed using many of the techniques employed for soda-lime glass, including hand blowing, pressing and extrusion. However, its relative softness means that it is easier to work and one of the main processes used is cutting and engraving, which exploit its high clarity.
Typical applications
Because of its clarity, it is often the choice material for drinking glasses, vases, cut-glass decorative tableware and ashtrays. But its use is not limited to fine crystal tableware, it is also used in optical components and neon tubing. Lead glass, with a typical composition of 65% lead oxide, is used in glass applications that require radiation shielding.
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–High clarity Easy to engrave –Easy to work –Less energy intensive to produce than other glasses |
–Comparatively low rigidity –Relatively high cost –Poor thermal-shock resistance |