Oct 22, 2026: We are working on protecting our servers against the PHP2Shell vulnerability recently discovered. Servers were upgraded ahead of schedule on Oct 21. This could cause some issues. Please contact us if you encounter any.
Pyrax (Pyrophillite) is a mineral having a very low thermal expansion. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock cracking. To that end, I mixed it with only kaolin (ball clay would add some quartz that would increase thermal expansion). I made slip-cast pieces of similar thickness for testing. I fired them to cone 2 (after finding that by cone 4, shock-resistance begins to decline). As you can see from the video, the addition of grog actually harms the performance! The higher the Pyrax, the better. To get a real appreciation for how well this body endures differential heat stress, wait till the end to see how a glazed porcelain piece compares.
| Materials |
Pyrophyllite
A refractory aluminum silicate mineral often used in clay body recipes to lower thermal expansion, control fired maturity, mullite development catalyst, etc. |
| Glossary |
Thermal shock
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant. |
![]() PayPal | No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me. |
Buy me a coffee and we can talk