Why is my fused glass piece cracking ?

First, incompatibility or fit of the glass being fused together. Next, variations in thickness of the pieces being fused together. And finally, problems in your kiln set up ranging from shelf placement to the kiln schedule itself. It would not be wise to think these are the only reasons, but these are the most common. 

When we investigate cracking problems, compatibility is the first thing you should check. In short, is all the glass used in your piece made from the same COE (coefficient of expansion) glass. The two most common glass COE’s are COE 90 (produced by Bullseye Glass) and COE 96 (produced by Oceanside Glass, Wissmach Glass, and Youghiogheny Glass). Putting a piece of COE 90 glass into your COE 96 project (or vice versa) will almost always crack during firing or at some point in the future. If it does crack, it will let you know which are the possible culprits because the crack will start where the COE mis-match exists. Sometimes all you will see is an area around the suspect piece that shows the stress it is under and where it will likely crack some time in the future. This crack occurs because one piece of glass is expanding (when heating up) or contracting (when cooling down) at a different rate than the pieces next to it. If you have a crack because of mismatch, you may be able to cut out the problem glass, replace with the proper COE glass, and fuse it a second time. That’s not always possible, so sometimes the best approach is to scrap the glass and start over. 

Often artists stack glass in varying thicknesses in order to achieve their desired result after fusing. When a piece is 1/8 inch thick in some places, 3/8 inch thick in others, and ½ inch thick in still others, the thicker the area, the longer the heat will take to penetrate or leave those areas of glass. As a result, the warmer, thicker area is still expending or contracting while the cooler, thinner area is now closer to done expanding or contracting. The interface of thick and thin areas in a fused glass art piece is a very high stress area and prone to cracking unless the heating and cooling rates are lowered. In addition to slowing down the heating and cooling rates, it may be appropriate to increase the annealing time so the thick and thin areas achieve the same temperature before their final cool down. 

The next thing we should look at when your fused glass cracks is tied to your kiln. Heating or cooling too fast puts a lot of stress on the glass. If you heat too fast, the top of the glass being fused is hotter than the bottom of the glass that is touching the shelf. Since the shelf is an insulator, it takes time for heat to come through the shelf to raise the temperature of the glass. It could be as simple as the top of the glass is hot and the bottom of the glass is cold (and you probably experienced what happens with you put a hot glass into cold water – it breaks). In this simple case, slow the rate of heating or cooling to allow the glass to come up to temperature more evenly. Sometimes your shelf simply can’t get hot fast because it has no air flow around it to heat from the bottom of the shelf up into the glass. If your shelf fits in your kiln so tight that you can’t get your fingers between the sides of the shelf and the kiln walls, then not enough heat can get around it and you should use a slightly smaller shelf. If your kiln shelf is sitting directly on the bottom of your kiln, you should put ½ to 1 inch kiln posts under the shelf to raise it off the bottom and allow hot air to flow around the shelf. Interestingly enough, you can tell based on the crack itself whether the glass failed while heating or cooling. If the crack has soft, rounded edges, the failure occurred while heating since the crack was exposed to the hottest temperatures and softened after the crack occurred. If the crack is sharp, it happened during cool down since the crack never had a chance to soften. This information will help you adjust your kiln schedule before running your next fusing. There are certainly other phenomenon going on when we mix art with science in glass fusing, but these three reasons for cracking fused glass cover a lot of ground for the newer glass artist to understand. One thing you can count on is that it is far more likely to have a piece of glass fail because it was heated or cooled too fast than by going to slow (there are some other failures that could be encountered by going way too slow, but we’ll save that for another blog post when we discuss devitrification). 

For now, happy creating and check back for more posts on fused and stained glass art by SW Art Glass.