How temperature affects the porosity of our bisque

We start with bone dry greenware, which is porous. So all the water – the physical water – has evaporated from the clay body before the bisque firing. Then we put it into the bisque, where all of those particles sinter (fuse) together, but the clay body remains porous throughout the bisque firing.

As the temperature goes up more of the particles (the fluxes in the clay body) start to melt. Eventually, once we get to our glaze firing temperature, the clay body has become vitrified, which means that it has close to zero porosity. (This is in reference to stoneware and porcelain. Earthenware does not vitrify.)

So, as temperature rises, a clay body goes from being very porous to not porous at all. The higher your bisque temperature is, the less porous your bisque will be.

Porosity goes down as bisque temperature goes up.

Should I change my bisque temp?

I don’t recommend changing your bisque temperature unless you are specifically trying to adjust your bisque porosity for a particular reason. There isn’t really a correct bisque temperature. You could be bisque firing to cone 08, 06, 04 etc. Generally, for the average clay body, cone 04 is on the high side for bisque temperature, but it’s also a very common bisque temperature.

If you’re having issues where your glazes are taking forever to dry or, because your bisque is becoming saturated with water too quickly, then you may consider lowering your bisque temperature. Just know that it’s going to affect the way your glaze applies to all your pots. Lowering by one cone isn’t going to make a huge difference to your glaze results, but could be helpful for your application process. So going from 04 to 05, you could test that out and see if that helps.

I was bisque firing my porcelain to cone 04 because when I was in school, we were bisque firing to cone 04 and I just continued that. Then I switched to cone 06 once I understood how bisque temperature relates to the glazing process. I lowered my bisque temperature to cone 06, which helped my thin porcelain pieces to be more absorbent for the glazing process.

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