Is this an incompatability crack?

[SuzyQ]

When you see a hairline fracture near the hole running down towards the other hole? At first I thought it was thermal but one by one the beads are each starting to show the fractures. So which is it? BTW this is a bead design I make often and I have never had the problem.

[chrisdd]

Suzy, everything I've read here says that cracks that run hole to hole are caused by thermal stress. Are you using a press to make them? I sometimes have problems if the press is cold.

[kbinkster]

It may also be from the glass being to thin. A good rule of thumb is to have at least a mandrel's width thickness of glass around the mandrel.

[SuzyQ]

Ah, I'm glad it isn't imcompatablitly. I was skeptical. I guess it could be the stamp I used to impress on it after it was formed. I'll keep it on my kiln and try a couple more.
Good to know on the glass thickness. I just checked and they are pretty thick where they cracked, but it could very well happen to me. Thanks.
Thanks ladies!

[alexm]

A crack that is straight and runs vertically is usually from thermal stress.

Incompatibility cracks are often jagged and come in multiples instead of one or two cracks.

[SuzyQ]

Thanks Alex. I'm still trying to figure out how I got them too cold this time...

[Karen Hardy]

I think it was Val that told me that the straight cracks are the thermal stress ones and the "spiderwebby" cracks are from incompatability. Haven't found anything that tells me different.

If you're using a press - one thing that I do to avoid the thermal cracks is to make sure that AFTER I press, I get that bead red hot and glowing again before I put it in the kiln. Hot enough to get the temperature soaked through the bead, but not hot enough to melt and distort it. Works good so far, I've only had cracks in my FFFF beads (fiddle, fiddle, fiddle f%$#ck!). :rolleyes: