Chemistry Studies in Siena 2012: Lead Oxide: Exploring the difference between glass and crystal

In light of our visit to ColleVilca where we learned about the chemistry of glass and glass blowing methods, I began to question what makes crystal more valuable than glass. From my understanding, what differentiates the two is based primarily on lead oxide content, crystals ranging from 24-27%. Anything slightly under this amount would be considered lead glass while anything higher could be potentially poisonous for us to use. What is intriguing though is that this small range in chemical composition is enough to make the difference between a prized item and one that is overlooked. If glass and crystal and virtually identical to the untrained eye then there must be some reason why crystal is so much more desired and expensive. So, I asked our guide what it was about crystals that made them so different from glass and why it is more valuable. I was hoping for some magical property or characteristic unique to crystals but, much to my dismay, she referred back to lead oxide content. She said lead oxide is very expensive to use so the more you have in your product, the more valuable it is. A little research, however, and I find there is a lot more to lead content than I thought.

Crystals are a type of glass, meaning all crystals are glass but not all glasses are crystals. The concentration of lead necessary to constitute something as a “crystal” varies from region to region. In the United States, having 1% lead oxide incorporated into production of glass is enough to give a product the “crystal” name. In Europe, there is a standard for making glass/crystals but the range is much larger than the one set by ColleVilca. Here is a breakdown of lead content and what this means in terms of defining glass in Europe.

glass ware= 4-10% lead content

lead-glass= 8-10% lead content

crystals= 10-30% lead content

lead crystal= >30%

The presence of lead oxide in important because it hardens the glass, making it easier for artisans to exquisitely cut and engrave it. More importantly, lead has intrinsic properties that make objects sparkle. Glasses with more lead oxide content have more sparkle, meaning they can diffract light better. Regular glass with little to no lead content cannot diffract light, whereas fine crystals are capable of casting “color spectrums though a whole room” in the presence of light. Certified Vilca crystals, which are guaranteed to have over 24% lead content, are made with a very high refractive index of over 1.56. The presence of lead adds more weight to the final product, making crystals heavier than regular glass. This is not to say that, however, they are sturdier. Incorporating “lead into glass reduces the ‘give’ in the glass,” actually making it more fragile and brittle. In regard to atomic structure, crystals are arranged in a highly ordered and repetitive manner whereas arrangement in glass is random. This is why the sound produced when you flick a crystal produces a much more dainty and long-standing "ding".

Below is an image highlighting the structural difference between glass and crystal:

So, like I had thought, there is more to lead oxide than it’s price. The presence of lead gives crystal physical properties that make it more desirable than glass. Higher levels of lead oxides means better cuts, more intricate design, more sparkle, more weight, and of course, more delicacy. The art of glass-blowing at ColleVilca was indeed a sweaty and arduous process but the result? Priceless.