We've been seeing quite a few broken pieces happen during the soldering process due to heat and I think it's time to pass this tip along in the blog. Heat cracks happen due to stress points in the glass. These occur naturally and are a by-product of cutting glass. The only way to reduce these stress points is to make sure that you grind the edges of all of your glass thoroughly. Just a quick pass over the grinder wheel is not enough. Below you can see that although this piece of glass has been touched by a grinder wheel, it has only been ever so slightly ground. The blue arrows point to the few areas that have been ground with only the arrow on the left being ground correctly. The white, matte edge is what should appear COMPLETELY around the glass. That matte finish reduces glass stress considerably. Also in the picture below are green arrows pointing to deep chips that have been created by pressing too hard on the cutter while scoring your glass. These 'deep chips' may be less than a 1/16th of an inch deep but these are chips which easily become cracks while soldering.
Here is the same piece of glass after being ground with just a fair amount of pressure along the entire edge against the grinder wheel. Note the matte finish along the entire edge. People ask me how hard should they press against the grinder and the answer is simple: Enough to make your edges appear as shown below.