材料
Use glass with the same COE
Japanese: 同じCOEのガラスを使う
For any given layout, always use glass with the same COE (coefficient of thermal expansion). Glass with different COEs will inevitably crack during cooling. This is the most important rule.
The basic rule for fusing glass is to match the coefficients of thermal expansion (COE). Use pieces with a COE of 90 with each other, and pieces with a COE of 96 with each other.
If they're not, they'll pull against each other as they cool and crack.
If you remember the codes for each major manufacturer, you won't get confused. Bullseye is 90, and Wissmach, Spectrum, OGT, and System 96 series are 96. Just knowing this will keep you from having trouble in most situations.
If you're unsure, check the product number written on the back of the glass or look it up in the manufacturer's catalog. To be on the safe side, avoid mixing glass that doesn't have a COE listed.
When trying out a new type of glass, the surest way is to fire a small test piece first. Even if the COE matches, residual stress can sometimes remain.
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.