Lead glass

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 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 the 'working period' making the glass easier to manipulate. The term lead crystal is, by technicality, not an accurate term to describe lead glass, as being an amorphous solid, glass 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. 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 crystal glass, in which 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 '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 207.2, versus 40.08 for calcium. The density of soda glass is 2.4 g/cm3 (39 g/cu in) or below, while typical lead crystal has a density of around 3.1 g/cm3 (51 g/in3) and high-lead glass can be over 4.0 g/cm3 (66 g/in3) or even up to 5.9 g/cm3 (97 g/in3). The brilliance of lead crystal relies on the high refractive index caused by the lead content. Ordinary glass has a refractive index of n = 1.5, while the addition of lead produces a range up to 1.7 or 1.8. This heightened refractive index also correlates with increased dispersion, which measures the degree to which a medium separates light into its component spectra, as in a prism. Crystal cutting techniques exploit these properties to create a brilliant, sparkling effect as each cut facet reflects and transmits light through the object. The high refractive index is useful for lens making, since a given focal length can be achieved with a thinner lens. However, the dispersion must be corrected by other components of the lens system if it is to be achromatic. The addition of lead oxide to potash glass also reduces its viscosity, rendering it more fluid than ordinary soda glass above softening temperature (about 600 °C or 1,112 °F), with a working point of 800 °C (1,470 °F). The viscosity of glass varies radically with temperature, but that of lead glass is roughly 100 times less than that of ordinary soda glasses across working temperature ranges (up to 1,100 °C or 2,010 °F). From the glassmaker's perspective, this results in two practical developments. First, lead glass may be worked at a lower temperature, leading to its use in enamelling, and second, clear vessels may be made free from trapped air bubbles with considerably less difficulty than with ordinary glasses, allowing the manufacture of perfectly clear, flawless objects.