Explaining Coefficient of Expansion in Glass
How do I know what glass to buy for my first painting (fired) project? What does Coe 96 or Coe 90 mean? How about thickness?
In physics terms, Coefficient of Expansion, or COE, describes by how much a material will expand for each degree of temperature increase.
So what does this mean in relation to glass? Glass expands when it gets hot and contracts when it cools, which results in a change in density. Measured in a laboratory, this figure would be miniscule and very, very long, so for ease they are shortened to whole numbers – COE 90 or COE 96, for example.
Each type of glass has a different COE – most Bullseye glass, for example, is said to have a Coefficient of Expansion of 90. Spectrum, has a COE of around 96, while standard window glass, has a COE that is around 84-87, depending on the manufacturer.
This isn’t so crucial with regard to painting, as temperatures aren’t as high as fusing or other warm glass techniques (eg slumping), but it is best to anneal (cool down slowly) any glass when firing, as it will allow the molecules to realign and prevent stress which leads to breakage.
Any thickness of glass can be used for painting, but if you’re going to use the lead came technique, then 3mm is best as it fits easily into the lead cames. Whichever glass you choose, ask the supplier or manufacturer for their recommended firing schedule.
I hope that helps, and good luck with your painting projects.
How can you tell the difference with various glass sheets
A friend gave me a bunch of glass.The glass is mixed COE90 and COE96.How do I tell which one is which?
Hi Jerry, you can’t unless you can recognise manufacturers’ glass. I always advise labelling every time you make a cut on a sheet – so both halves etc.
You could try firing in kiln… if it doesn’t crack, compatible!