Is Glass A Liquid?

The subject of whether glass is a liquid comes up often when I am teaching glass fusing workshops.  In fact, I incorporate a little bit about glass structure in the information I give out on my full fusing experience day – because it partly explains the properties of glass, and why we can do what can with it.

It’s actually an ‘amorphous solid’ – its molecules don’t set a regular crystalline structure like many elements and compounds (MIT refer to it as a ‘highly resilient elastic solid’ – read here) – although elastic solid does sound like a bit of an oxymoron doesn’t it?

 

 

So why do people – and I’ve sadly heard other glass artists say this – think glass might be a liquid?   The most common ‘evidence’ cited is medieval glass windows being thicker at the bottom.  The glass must have flowed down, right?  Wrong.  They made it that way.

Prior to the invention of float glass in the mid 20th century by Pilkington Glass,  window glass was hand made.  Two commonly used methods were cylinder and crown glass.   Read more about crown glass here, but essentially, it results in a disc of glass that is of uneven thickness – and sometimes, that thicker bit is at the bottom!  Yes, it is that simple.

 

Here’s picture of Crown Glass being made

 

You can lose yourself down a rabbit hole with the whole history of glass, the development of the window (it’s that exciting, I wrote my dissertation on it!) – this is just a quick look, and an attempt to explain that variance.

 

 

Are You Absolutely Sure, A Tour Guide Definitely Said The Glass In Church Windows Moved?

Yes!   If glass flows downwards in matter of a few hundred years, why aren’t the ancient glass in the British Museum just a puddles on a shelf?  They predate the Middle Ages by a millennia and more.   The Portland Vase dates back around 2,000 years.   This mosaic dish is even older at circa 300BC

 

Find out more about the Portland Vase here, it’s got an interesting history (And no, Natalie from Great Pottery Throwdown, Josiah Wedgwood wasn’t the creator of the original cameo vase!)

Also in the British Museum – this dish is around 2,300 years old.

 

Conclusion.  Glass is NOT a liquid.  It doesn’t move measurably over time.  Pretty much the only way to make glass flow is to get it warm…. like 600ºC and above, warm… which is what I do!