We often get asked how to fuse different types of glass together, here are a few tips to ensure your glass fusing goes without a hitch caused by compatibility problems!
All glass is fusible, however, not all glass is compatible. Compatible means that two or more glasses (from different sheets) can be successfully fused together, without risk of breakage.
Different types of glass can absorb different amounts of heat, and expand and contract at different rates. If you fuse two pieces of glass that ‘move’ differently in the kiln together, when they cool and contract the friction between the two pieces will result in stress. This stress can cause breakage. Unfortunately it is not always the case that the piece breaks in the kiln, it can happen days, weeks, even months later.
Glass designed for fusing are given a number called a Co-efficient of Expansion, or COE. This number represents by how much glass expands and contracts in the kiln. It is a measurement. The larger the COE number the softer the glass, and the more it expands when heated, the lower the COE number, the harder the glass and the less it expands when heated. Mixing different COE glasses and fusing them together means that they will pull apart when cooling.
You can test for stress with polarisation filters, put one over a light source, the glass in between and the other filter at 90 degrees over the top. Stress will show up as a milky white halo around affected areas.
It is good to test glass that isn’t tested compatible on test strips. Then risk is minimal, and so is cost! Make up a test strip and fire it to see what happens. It means that should something go wrong, you will only waste a small amount of glass and time, and not your whole project.
If you stick to a range of tested compatible fusing glass rather than attempting to mix different types of glass in the kiln, you will have successful results without the stress. Choose a range for your project and dont mix them. Either use Spectrum System 96 Glass or Bullseye Glass, these are our two favourites.