Bisque, also biscuit, is the name given to all types of ceramic ware which has been fired and is unglazed. The firing of the greenware changes clay into a bisque article causing permanent chemical and physical changes. The lower the temperature of firing, the greater the porosity in the product, the clay turns into bisque at 650°C but should generally be fired to 1000°C to burn away all carbonaceous matter. Refractory can be fired up to 1300°C. The higher the temperature of firing makes a harder and more resilient article with less porosity but is still absorbent to liquids such as water and glaze. Firing, to start with, should be very slow to dry off any moisture retained in the wares to avoid cracking and warping particularly in work that is thick and if a piece varies in size. The drawing are to show how clay dries the first is damp clay and the second, from the outside looks dry as all the parts touch dry but in-between are small spaces that can retain dampness.
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 or colors 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 15×15 cm can take up to 12 hours to dry and on one 7.5 × 20 cm only 3 hours. This is not only related to size but to the methods and products used to make them.
The advantages of bisque firing: Editar
- The work becomes stronger, less likely to break and easier to handle.
- Pieces can be piled up on top of each other and stored indefinitely.
- If broken and the cracks can be seen or heard, they can be thrown away, so time is not wasted decorating them.
- Sometimes cracks cannot be seen, so sounding the piece before decorating it is also important, to check that it is not cracked.
- A flat sound can also mean that the bisque has been under-fired.
- 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.
- 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.
: Diccionario Editar
- Bisque. Wikipedia.