[Clayart] Wide range clay

sumi sumi at herwheel.com
Sat Jan 22 01:59:51 UTC 2022


I think you'll find we are pretty united on this question - there is no 
such thing as a clay that is mature throughout that range. Food safe is 
a wishy washy term, though. It's safe to put food on a pot that isn't 
vitrified, simply being porous won't poison you. But it might not be 
safe to use it in the microwave - once it has absorbed water due to its 
porous nature, it will get excessively hot in the microwave. And it 
might not be safe to leave, say, an unvitrified teapot full of tea on a 
nice piece of furniture - it will leak and ruin the furniture. And it 
won't be as durable. The glaze will craze, perhaps not immediately, but 
eventually as it absorbs moisture. There is definitely debate about 
whether a crazed glaze will harbor unsafe levels of bacteria, but crazed 
glaze will weaken the ware. Personally, I think if you wash it in the 
dishwasher, the bacteria is not an issue.

What absorption rate is acceptable can't be answered definitively - I'd 
say 2-3% at cone 6, other people insist on less than 1%. And plenty of 
traditional functional ware is earthenware, which is never vitrified, so 
you can't even say that unvitrified clay is not "safe" - it's just used 

> Any thoughts from all you experienced potters about those clays that are marketed as a wide ranging temp clay, cone 6-10…..are they really fully vitrified at cone 6? I hear very different opinions about this…..what absorption rate is necessary for functionally food safe pots?  ( continental clays B clay, Buff etc…. )
> I have always fired these clays to cone 10 but some people suggest that these clays are suitable for both cone 6 and 10 firings. Thoughts?
> Vanessa Greene

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