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Answered 2016-06-18 15:45:44

Crystal is defined legally by a UK British Standard BS 3828 of 1973 and the EU Crystal Glass products directive 69/493 EEC of the 19th March 2002, Gazette No 24700 . The Refractive Index by UK and EU law, must be not less than 1.520, and the density must be of at least 2.54 g/cm2. If lead is present, to be called "Crystal Glass" it must contain 10% at least of zinc, barium, or potash, alone, or together. (In the USA, only 1% is needed, which would easily be considered just plain glass by the UK.) Only Lead crystal, ( or Full Lead Crystal with over 30% lead) contains lead or other materials which modify the characteristics of the material. The same is true for other types of glass where the refractive index is changed by inclusion of other materials to make camera lenses etc. "Lead crystal" was a British invention discovered by George Ravenscroft and registered on the 16th May 1667, and financed by the Worshipful Company of Glass Sellers of London.

The addition of up to 30% lead oxide, ( though most manufacturers are reducing it to 24%, the legal minimum to be called lead crystal ), gives crystal glass a high index of refraction which makes it sparkle when cut at sharp angles, is softer to cut, and gives the glass a distinctive long lasting ringing tone when it is struck gently with a hard object like a fingernail. The lead may under very extreme conditions leach out into acidic beverages, like white wine or Orange Juice ( if stored over night in a fridge for instance). The glass is also more fragile and subject to temperature shock: Crystal glass will often break when cold water is poured into a hot glass or vice versa.

Glass is a liquid in the sense that it does indeed flow, but it flows over an extensive period of time. The difference between a crystal and glass, is that a crystal has a homogenous solid formed by a repeating, three-dimensional pattern of atoms, ions, or molecules and having fixed distances between constituent parts. Glass on the other hand, has molecules that are disordered but are rigidly bound.

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