When I was a kid, we would get out the fine china and the the lead crystal for holidays like Thanksgiving and Christmas, and both names struck me as odd, because nothing about the nice plates seemed Chinese, and the nice glasses were obviously glass, not lead. Fine porcelain of course did come from China originally, and lead crystal does contain lead, even if that’s obviously a bad idea:
Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% (by mass) lead(II) oxide (PbO), while modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% PbO. Lead glass is often desirable for a variety of uses due to its clarity. In marketing terms it is often called crystal glass.
The term lead crystal is, technically, not an accurate term to describe lead glass, because glass lacks a crystalline structure and is instead an amorphous solid. The use of the term remains popular for historical and commercial reasons, but is sometimes changed to simply crystal because of lead’s reputation as a toxic substance. It is retained from the Venetian word cristallo to describe the rock crystal (quartz) imitated by Murano glassmakers.
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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. Lead also raises the density of the glass, being over 7 times as dense as calcium.
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The brilliance of lead crystal relies on the high refractive index caused by the lead content. Ordinary glass has a refractive (n) of 1.5, while the addition of lead produces a range up to 1.7[1] or 1.8.[6] This heightened refractive index also correlates with increased dispersion, which measures the degree to which a medium separates light into its component wavelengths, thus producing a spectrum, just as a prism does. Crystal cutting techniques exploit these properties to create a brilliant, sparkling effect as each cut facet in cut glass reflects and transmits light through the object.
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When tapped, lead crystal makes a ringing sound, unlike ordinary glasses. The wine glasses were always valued also for their “ring” made by the clinking glasses. The sound was better when large quantity of the lead oxide was present in the glassmaking material, like in the British and Irish wine glasses of the 17th-19th centuries with their “rich bell-notes of F and G sharp”.
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George Ravenscroft (1618–1681) was the first to produce clear lead crystal glassware on an industrial scale. The son of a merchant with close ties to Venice, Ravenscroft had the cultural and financial resources necessary to revolutionise the glass trade, setting the basis from which England overtook Venice and Bohemia as the centre of the glass industry in the eighteenth and nineteenth centuries.
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The amount of lead released from lead glass increases with the acidity of the substance being served. Vinegar, for example, has been shown to cause more rapid leaching compared to white wine, as vinegar is more acidic. Citrus juices and other acidic drinks leach lead from crystal as effectively as alcoholic beverages. Daily usage of lead crystalware (without longer-term storage) was found to add up to 14.5 ?g of lead from drinking a 350ml cola beverage.
The amount of lead released into a food or drink increases with the amount of time it stays in the vessel. In a study performed at North Carolina State University, the amount of lead migration was measured for port wine stored in lead crystal decanters. After two days, lead levels were 89 ?g/L (micrograms per liter). After four months, lead levels were between 2,000 and 5,000 ?g/L. White wine doubled its lead content within an hour of storage and tripled it within four hours. Some brandy stored in lead crystal for over five years had lead levels around 20,000 ?g/L.
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It has been proposed that the historic association of gout with the upper classes in Europe and America was, in part, caused by the extensive use of lead crystal decanters to store fortified wines and whisky.