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Glass or crystal?
It is the addition of lead to the silica that determines the nature of the material. The French Afnor standard distinguishes three types of crystal. Between 10 and 24% lead, it is crystalline. Above 24%, it is lead crystal. Finally, superior crystal, such as Lalique’s, exceeds 30%. Added in the form of an oxide, lead gives the glass brilliance. It also makes it more malleable to work with and lowers its melting point from 2,730°F (1,500°C) to 2,550°F (1,400°C). The addition of metal oxides or rare earths during the manufacture of the crystal colours the material. The crystal is ready to be worked 20 to 40 hours after being put in the furnace.
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