BISQUE (1) Clay after the first firing.
Bisque is the name given to all types of clay which have been fired without a glaze. The lower the temperature of firing, the greater the porosity the piece has as it burns away all carbonaceous matter. The temperature for firing clay into bisque is between 650° C and 1050° C. Refractory can be fired up to 1300° C. The higher the temperature of firing, the less porosity it has and the stronger it becomes but it is still absorbent to liquids. When you use products in the bisque state that have been industrially made or handmade they can vary in porosity so the drying time of a glaze-base can differ.
Tiles can be bought from the same firm but each size may be made using a different method and this varies the drying times. The glaze-base on a bisque tile 15×15cm can take up to 12 hours to dry and on one 7.5×20cm only 3 hours. This is not related to size but to the methods and products used to make them.
The advantages of bisque firing:
1) The work becomes stronger, less likely to break and easier to handle.
2) Pieces can be piled up on top of each other and stored indefinitely.
3) If broken and the cracks can be seen or heard, they can be thrown away, so time is not wasted decorating them.
4) Sometimes cracks cannot be seen, so sounding the piece before decorating is important, to check that it is not cracked.
5) A flat sound can mean that the bisque has been under-fired or is broken.
6) When you buy clay, you must check to what temperature it can and should be fired. Over-firing clay may turn it into a liquid and destroy your kiln.
7) Clay while being fired shrinks. If you fire clay to its correct temperature, shrinking will stop, but if you under-fire it, when fired again, shrinking will continue.