Cement Mold Issue

[donb]

Hello,

I am using using the cement mold mix from Boyce Lundstom's glass fusing book 1. I am getting a bowing in flat molds that are 9 ish inches and longer.
Does anyone know why this is happening and how to reduce/eliminate it?

Mold mix formula:
6 parts calcium aluminate
3 parts sand
2 parts vermiculite
1 part fire clay

Any ideas whould be appreciated.

Thanks,

Don

[Barbara Cashman]

I would suggest forgetting the formula and just purchasing a castable refractory for furnaces. Cheap, doesn't shrink. Weighs a ton but lasts forever. - Barbara