[Sca-cooks] pottery braziers, cooking in pottery

[Louise Smithson]

One of the other major differences between modern produced reproductions 
(normally) and the period ones is firing temperature. 

Most modern potters fire to cone 5 or 6 which is a stoneware fire. Most european 
(or at least English) pottery in period was fired to earthernware temperatures, 
cone 05 or 04.  This makes a difference to how a clay body behaves.  

When you fire to stoneware you vitrify, i.e. make the clay glass like.  If a 
crack starts in a vitrified body unless there is heavy grog (sand, fired clay 
particles or shell) the crack will run through the pot and you get a crack. 

In earthernware the body is not vitrified, this means that the cracks don't tend 
to run very far at all.  Think of it as the difference between a terracotta 
planter (you put water in and it leaks through the body) which is earthernware 
and a ceramic vase or mug (you put water in and it stays on the inside, no 
weeping) which is stoneware.  

One way to make stoneware more resistant to cracking is to increase the amount 
of grog (such as is often found in Raku clay) or to incorporate sand or other 
material in.  Something that will "interupt" the crack. 

Helewyse 
(another potter).