biscuit firing
The initial firing to which pottery is subjected, before it is glazed.
biscuit firing: pottery's first trip through the kiln
Biscuit firing (also called bisque firing) is the first high-temperature exposure of unfired clay ware. It converts raw greenware, which is fragile and water-soluble, into a porous, rigid ceramic body that can accept glaze. The firing typically reaches 1000 to 1150 degrees Celsius, depending on the clay type and intended final product. At this temperature, chemically bound water leaves the clay permanently, and the mineral structure reorganises into a stable ceramic state.
The ware emerges from biscuit firing still absorbent enough to accept liquid glaze without immediate runoff, yet strong enough to handle without breakage. This intermediate state is essential to the production sequence. Without bisque firing, raw greenware would collapse or slump if glazed and refired, or glaze would sit on the surface like paint rather than fuse to the body. Industrial potters fire biscuit ware stacked closely together, sometimes with stilts or saggars, since warping and glaze adhesion are not yet concerns at this stage.
The kiln atmosphere during biscuit firing is typically oxidising (plenty of oxygen). Cooling must be managed carefully: too rapid cooling can induce cracking, particularly in thicker sections. Many industrial kilns hold the ware at 200 to 400 degrees Celsius for several hours during the cooling phase. Shrinkage of 10 to 15 percent by linear dimension occurs during this first firing, so dimensional accuracy is established here, not before.
Common defects and variables
Bloating, pinholing, and black-core defects can occur if firing is too fast or the clay contains iron sulfides or carbonaceous material. Stoneware and earthenware bisque at different temperatures: earthenware often bisques lower, around 1000 degrees Celsius, while stoneware may reach 1150 degrees or higher. Porcelain is sometimes single-fired (glazed greenware, then fired once to full maturity) rather than bisque-fired, because its low porosity makes it difficult for glaze to adhere after bisque firing.
Industrial producers distinguish bisque firing from glost firing, the second firing that melts and hardens the glaze. The bisque kiln is often a tunnel kiln, roller kiln, or periodic downdraft kiln designed for high throughput. The term "biscuit" itself has no universally agreed origin, though some attribute it to the French "biscuit," meaning twice-cooked, reflecting the two-fire process in classical ceramics.