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What kind of crystal are crystal drinking glasses made out of?
Category: Chemistry
Published: March 15, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
"Crystal" glasses are not made out of crystal at all. The naming of certain types of glass as "crystal" is a confusing and inaccurate historical tradition. A crystal is any material that has its molecules aligned spatially into regularly repeating patterns. Metals, ceramics, salts, ice, sugar, and rocks are all crystal. "Crystal" glass is not. In fact, the term "crystal glass" is a pure oxymoron. By definition, glasses are materials that have their molecules unordered. In other words, the very definition of glass is a material that is not a crystal.
So what is "crystal" glass made of if not crystal? Traditionally, "crystal" glass was just regular glass where the calcium is replaced with lead oxide. A more accurate name is therefore "lead glass". Adding lead to glass raises its index of refraction. The index of refraction measures the amount that a material bends light. Materials with a higher index of refraction sparkle more because they bend light more. One of the materials with the highest index of refraction is diamond. That is why diamond sparkles so much. Adding lead to glass makes it look more like diamond, hence lead glass became identified as crystal-looking glass, which got shortened to "crystal glass". In this way, adding lead to glass allows an artist to make a candle holder or a wine cup that looks like diamond without actually needing to use diamond, which would be prohibitively expensive. Adding lead also makes the glass easier to work with, so that more intricate designs can be cut into the glass. In our modern day, lead is recognized to be poisonous to humans, so "crystal" glass actually has barium or zinc now instead of lead.