Bisque vs. Biscuit Firing

What are bisque and biscuit firing?

When making ceramics, the items are very often fired twice – one time without glaze, and then a second time with. The former is known as bisque or biscuit firing. Nowadays, “biscuit” is a lesser used term, bisque is often used to refer to both, but they are sometimes used interchangeably. This is a shame, because although they both refer to the first firing of clay, there are important differences between them.

What is the purpose of the first fire?

Firing removes water and impurities from clay, and creates chemical changes to produce ceramic. These changes produce waste-products in the form of escaping gasses. The purpose of the first firing is to remove all these gasses before the glaze is applied, so that they don’t interfere with it during the glaze fire. Also, the first firing weeds out any items that are prone to cracking, so that you don’t waste time and glaze on them during the glaze firing.

What is the difference between bisque and biscuit?

Properly, bisque firing refers to a first firing process at a  higher temperature than the subsequent glaze firing, and biscuit firing refers to a fist firing process at a lower temperature than the subsequent glaze firing. There are important differences between the two.

Firing at a higher temperature removes more water and impurities, and introduces more of the chemical changes that transform the clay into ceramic. This means that high-temperature bisqueware is stronger than the lower temperature biscuitware. However, this does come at a price. Because water-removal acts to draw the individual clay particles closer together, bisque fired ceramic is less porous and will therefore absorb less glaze, leading to a less impressive finish, and to a higher likelihood of imperfections such as streaks in the finished product.

How to choose between them.

The tradeoff when choosing between the higher-temperature bisque process and the lower-temperature biscuit process is one of strength vs beauty. Therefore, to simplify somewhat, bisque-fire when strength is more important than beauty, and biscuit-fire when beauty is more important that strength. This might map onto ornamental ceramics vs. items that will be put to practical use – ceramics that are put to everyday use will likely need to be stronger than those that are purely decorative.

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