Bisque firing is the first of two firings. It takes fragile dry greenware and turns it into bisqueware: a porous, hard, handleable ceramic that takes glaze evenly. Almost everything that goes wrong in a bisque is caused by heating too quickly at the start.
What the firing does
Below about 600 degrees, chemically combined water leaves the clay, and above that the body begins to sinter into something hard and permanent. It stays porous, which is exactly what glazing needs: a porous body draws water out of the glaze and leaves an even coat behind. A fully vitrified body would take glaze badly, which is why the two firings exist.
Why the start is slow
Any water still in the clay turns to steam and expands violently, and a piece that looks bone dry can hold moisture in a thick base or a handle. A slow climb to around 600 degrees, often with a hold, gives that water time to leave gently. Firing dry work slowly is the whole of bisque technique.
Packing a bisque
Bisque ware can touch and can be stacked, because nothing is molten and nothing sticks. Bowls nest, small pieces go inside larger ones and the kiln can be packed densely, which is why a bisque firing is far more economical per piece than a glaze firing. Keep the packing even so heat reaches everywhere, and put a cone where you can see it.
What temperature to bisque to
Common practice is a lower bisque than the glaze firing, so the body stays porous, with the exact figure depending on the clay and on how absorbent you want the ware for glazing. Too low a bisque leaves the body fragile and prone to pinholing later; too high leaves it too dense to take glaze evenly. The supplier's guidance for the body is the place to start.
Questions people ask about bisqueware
What is bisqueware?
Clay that has been through the first firing: hard, permanent, still porous, and ready to take glaze evenly.
Why is pottery fired twice?
The first firing makes fragile greenware handleable and porous so it takes glaze well; the second melts the glaze and matures the body.
Can bisque pieces touch in the kiln?
Yes. Nothing is molten in a bisque firing, so work can be stacked and nested, which makes it economical per piece.