Do you know which crack is annealing and which crack is thermal shock?

When you see a thermal shock crack (left pic) you will notice that the glass has become rounded. Thermal shock can happen when the glass is heated up quicker then it can handle. It can also happen by opening the kiln when the glass is vulnerable to temperature changes. Beware when you are wanting to peek into the kiln before the glass has gotten close to room temperature. Peeking too early can sometimes cause a crack!

Annealing cracks (right pic) are never rounded and usually follow an S curve. Make sure if your piece has lots of ups and downs or is thicker than a couple layers that you anneal the proper amount of time plus a little more to avoid these cracks.