Cracking in pieces after glazing

[Bonnie Hornsby]

I threw a chip and dip set.  I used a buff clay from Highwater.  I fired to cone 04.  The two pieces went through the bisque firing fine.  I used a combination of Amaco Potters Choice glazes which are very stable glazes.  I fired to cone 5.  When I unloaded the kiln, the larger piece had a large crack down the middle.  What would cause that?  Any help would be much appreciated.  Thanks so much!!!!

[Norm Stuart]

Bisqued ware fired to ^6 which cracks, with or without a glaze, is usually relieving stresses, from throwing or hand forming, as the clay becomes more plastic at temperature. Photos of the crack and where it is on the bowl would be helpful for others to suggest a cure.

Highland Clay does not post COE numbers for their clay, so it's just a guess to think there's a COE mismatch between glaze and clay.  But I think this is unlikely to be the problem just based on the way your describe the damage to the body rather than the glaze.

[Bonnie Hornsby]

Norm,

Thanks for responding.  I showed the piece to someone locally and they said it was because the clay was marbled.  It cracked where the two types of clay were joined.  I recycle clay all the time and that was the first time that happened.  Thanks again!

[Norm Stuart]

Mixing clays with two different COE or shrink ratios can certainly build up enough stress in the kiln to develop cracks.  Sometimes even before firing this stress can develop cracks.

I think you found your problem.