The level of lead in glass is a measure of its value. Glass made with lead is heavier and softer than ordinary glass and can be cut and faceted with intricate patterns. It also has a higher refraction index, which makes it sparkle and gives it more of a prism effect.
English glassmaker George Ravenscroft developed crystal glass in London around 1675. Many accounts of Ravenscroft suggest he discovered the effects of lead oxide as an additive to glass after a lengthy process of experimentation. However, glass historian David Watts suggests that Ravenscroft picked up the idea from Italian glassmaker John Baptiste da Costa, who was familiar with techniques used on Murano, a Venetian Island famous for its glass. However it happened, Ravencroft's introduction of the use of lead was a milestone in glassmaking history.
The commonly accepted levels of lead in the various categories of crystal were set by the European Union in 1969. Europe is home to some of the world's most famous crystal companies including Baccarat and Arc International of France, Royal Leerdam Crystal of the Netherlands, Swarovski of Austria and Waterford Crystal of Ireland. Although European standards require at least a 24 percent lead content for a piece of glass to be labeled lead crystal, many of the most famous brands use higher levels. Waterford Crystal has a lead level of 33 percent.
In the United States, any glass that contains more than 1 percent lead is labeled crystal. The U.S. standards and definitions of crystal are related to health concerns. Lead is a toxic metal that builds up in the blood over time. It can cause serious damage to the nervous system and kidneys and can be fatal in high doses. Several universities and organizations have done studies to determine if crystal glass is safe. The results from those studies found that hazardous amounts of lead from decanters and glasses leaches into fluids such as wine and whiskey in as little as 30 minutes. Over time, lead levels in stored beverages increases significantly, particularly in crystal with higher lead contents.
The National Safety Council does not make any distinctions among types of glass that contain different percentages of lead. Instead, the council warns anyone who uses crystal items not to store liquids in crystal decanters, not to drink from crystal glassware on a daily basis and not to feed infants from crystal glasses or bottles. Other groups also have added the warning not to store jams and jellies in crystal jars.