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Concept
Lead glass
Summary
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 lead crystal remains popular for historical and commercial reasons, and sometimes simply crystal because "lead" seems toxic to consumers. It is retained from the Venetian word cristallo to describe the rock crystal (quartz) imitated by Murano glassmakers. This naming convention has been maintained to the present day to describe decorative hollow-ware.
Lead crystal glassware was formerly used to store and serve drinks, but due to the health risks of lead, this has become rare. One alternative material is modern crystal glass, in which barium oxide, zinc oxide, or potassium oxide are employed instead of lead oxide.
In the European Union, labeling of "crystal" products is regulated by Council Directive 69/493/EEC, which defines four categories, depending on the chemical composition and properties of the material. Only glass products containing at least 24% of lead oxide may be referred to as "lead crystal". Products with less lead oxide, or glass products with other metal oxides used in place of lead oxide, must be labeled "crystalline" or "crystal glass".
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.
Official source
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