Lead glass is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40 weight% 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 desirable owing to its decorative properties.
Originally discovered by Englishman George Ravenscroft in 1674, the technique of adding lead oxide (in quantities of between 10 and 30%) improved the appearance of the glass and made it easier to melt using sea-coal as a furnace fuel. This technique also increased "working period" making the glass easier to manipulate.
Technically, the term crystal is not applied to glass, as glass, by definition, lacks a crystalline structure. The use of the term lead crystal remains popular for historical and commercial reasons. It is retained from the Venetian word cristallo to describe the rock crystal imitated by Murano glassmakers. This naming convention has been maintained to the present day to describe decorative hollow-ware.
Due to the potential health risks of lead that it contains, true lead crystal glassware is rare nowadays. One material that is commonly used to manufacture glassware and referred to as "crystal" is lead-free crystal glass. In lead-free crystal glass, barium oxide, zinc oxide, or potassium oxide are employed instead of lead oxide. Lead-free crystal has a similar refractive index to lead crystal, but it is lighter and it has less dispersive power. 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 "crystallin" or "crystal glass".
Read more about Lead Glass: Properties, History, Lead Glazes, Lead Crystal, Safety
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