Antique Bohemian Art Deco Crystal Cut Table Lamp Circa 1920

"Crystal"—
More literally known as Lead Glass—is over 1000 years old, and cut crystal is close behind. Crystal differs from standard glass in a few key ways, none totally doctrinal but all functionally important. Where a common drinking glass is largely composed of silica + sodium oxide + calcium oxide, "crystal" adds lead. This has been seen from Venice to the Czech Republic, ancient Rome to Edo era Japan. Unsurprisingly, different countries and practices differ on the minimum lead content that earns a "crystal" classification—as low as 2% by weight in the States and twelve times that in Europe. Crystal drinkware generally contains 18-35% lead, and it goes even higher if the piece isn't for mouth use... but I'm not drinking out of crystal statues today, so we'll ignore those.

Overall, lead oxide and antimony are added in significant amounts because they allow a softer glass body, which makes deft shaping and cutting less difficult and less likely to blind the artisans with brittle chips. They also increase material clarity and heighten refraction of light, which is an important feature in good glassware that becomes amplified with facet cutting. Higher lead content also reduces viscosity, making the material more elastic and easier to work with at lower temperatures than standard glass. The possibility of lower working temps also means ability to eliminate air bubbles and other imperfections while melting and forming, which has helped build crystal's reputation as a flawless and pristine material.