As a fellow who does pottery, I'd just like to say that although heat does cause clay to harden, this is only *after* the heat has been applied. When its still hot it's pretty weak and can be semi-liquid/liquid (that's how you get those pretty glazes.) However once you have done this once, it is much more heat resistant the second time around.
oh, one more thing, fired clay can be pretty strong. 'Fragile like a crystal' is misleading. Since this is the same process as that which makes glass or rocks and even refined metals you are partly right - some materials can be real fragile (glass or fine china) *but* some materials can be really strong (like granite or steel...). A good stoneware clay (~1200 *C) can be pretty strong without being really fragile... you can bust up heads with it anyway...
It's all in the recipe you know.
I am back. My idea was that maybe a fireborn could get physical attacktype at higher level (maybe as his mental power increases he can use this to "concentrate his essence" (I lack a good English description for it)).
Do not meddle in the affairs of wizards
for they are subtle and quick to anger.