Diagnosing Cracks in Fused Glass

One of my least favorite things in the world is opening the kiln and finding a cracked piece of glass!  Today we are going to look at a few of the causes of cracks in fused glass, and what we can learn from them.
One of the most common cracks that you see in recycled glass is incompatibility.  Incompatibility cracks are typically circular, sometimes a full circle, sometimes an arc.  They usually start at or circle around the edge of the incompatible glasses.  Here’s an example:
In this first example, it’s just incompatible, and the take away is don’t mix these glasses (float and dark green bottle). Here is what an incompatibility crack would look like in a frit casting:
See how the crack is arcing away from the edge of the blue frit?  Definitely incompatible. This type of cracking can often be avoided by using smaller frit sizes when mixing glass.  This frit was larger than rice grains, which is pushing the frit mixing envelope.  Sugar size or smaller is better.
Another fabulously fun kind of cracks happen in conjunction with molds.  There are a bunch of ways that molds can wreck our glass, one is lack of kiln wash.  It’s always easier to add another thin coat of wash on dubious pieces rather than chip off the glass.  Here’s an example:
The all over crazing of the glass is an indicator that the glass is sticking to the mold, as are the needle like pieces of glass that remain stuck after the glass is chipped off.  (this is art glass)  The message here?  Be sure the mold is kiln washed!
Even if the mold is kiln washed, it can still cause cracks.  Particularly on home-made molds, there is a hazard of the glass hanging up on the more vertical edges of the mold.  Often you can pinpoint the problem areas by looking where the verticals and the glass cracks coincide. Here’s an example:
 You can see where there are hang ups.  This is not a lost cause, however.  If I were to sand down those edges (wearing a respirator, of course) I could re-kiln wash this mold and try again. (this one is also art glass)
Additionally, we have the ever popular thermal shock.  Thermal shock cracks usually go through the center of a piece straight and then veer off towards a side.  Here’s an example  (the corner piece went missing in the studio):
This is a very thick cast piece using tempered glass chunks and Float Fire.  I know this happened on the cool down for a couple of reasons:  1.  tempered glass chunks are small enough and separate enough on the ramp up that they won’t shock.  2.  The edge of the break is perfectly square.
If the shock had happened on the ramp up, this edge would be rounded like a full fuse edge.
Lastly, the infamous Relativitus Break.  The sad occasion when a well meaning relative bangs your glass on a hard surface and breaks it.  Sadly, the only cure is to disown them and try to acquire more graceful relatives.