To quote the glass artist Michael McNerny, “Glass doesn’t lie”. This means the pattern of your break can usually tell you the cause of the breakage. Analyzing the kinds of cracks can give you evidence as to “who done it” Because when you find your glass has cracked, you had best find out why lest you repeat the crime.
Obviously, glass incompatibility should be your #1 suspect. Different glass types can have different rates of expansion when they get hot. This is expressed in one of those weird mathematical logarithms as the COE (Coefficient of Thermal Expansion). A glass with a COE of 90 means and expansion or contraction rate of .000090 or 9 X 10-6 which is a lot more information than you need to keep your glass from cracking.
However, you have probably used varying COE’s to your advantage when trying to open the lid from a glass jar. Traditional wisdom is to hold the top of the jar under hot water. The metal lid expands more and faster than the glass, so it gets a wee bit larger and therefore makes opening the jar easier.
That’s great in the kitchen but when blowing or fusing glass, 2 different rates or amounts of expansion will lead to cracking. So, if you are a fuser and notice that the crack in your glass started along the line where 2 different pieces of glass meet, or if you work with hot glass and your piece shatters, you can pretty well expect that incompatibility was the culprit.
Ok, you opened up the kiln only to find that your glass artwork was broken. Why did this happen? Basically, if the edges of the crack are rounded, the break occurred during the first part of the annealing cycle, while the piece was being raised in temperature, * the edges then had a chance to soften during annealing. If the edges are sharp, you can bet it broke as the glass was on the cooling side of the cycle, when the temperature was coming down.
Listed below are three more ways to figure out why your piece broke.
1. The piece is in 2 or 3 pieces, along gently curves (sometimes straight lines) that curve sharply toward the edges. It may not have been annealed long enough or at a high enough temperature.
2. Pie shaped pieces with rounded edges (usually). This cause was probably thermal shock, often happening around 400F on the way up. Fire more slowly.
3. BB shot pattern. Looks like a spider web – small connecting cracks. Probable cause is a ding in the shelf or kiln wash. Repair the shelf and/or re-prime.