As the clay is slowly heated, this water evaporates out of the clay. If the clay is heated too quickly, the water will turn to steam right inside the clay body, expanding with an explosive effect on the pot. By the time the boiling point of water (212 F and 100 C at sea level) is reached, all of the atmospheric water should have evaporated out of the clay body. This will result in the clay compacting and some minimal shrinkage. This chemically combined water’s bond loosens when heated. Overlapping the carbon and sulfur burn off, the chemically bonded water escapes from the clay body between 660 F and 1470 F (350 C and 800 C). If the water heats too quickly, it again can cause the explosive production of steam inside the clay body. It is due to all these changes (and more) that the firing schedule must allow for a slow build up of heat. The change in crystalline structure will actually cause the pottery to increase in size by 2 percent while heating, and ​lose this 2 percent as it cools. Ware is fragile during this quartz inversion and the kiln temperature must be raised (and later cooled) slowly through the change. Bisque firing is usually done at about 1730 F (945 C) after the ware has sintered but is still porous and not yet vitrified. This allows wet, raw glazes to adhere to the pottery without it disintegrating. Vitrification is a gradual process during which the materials that melt most easily do so. They dissolve and fill in the spaces between the more refractory particles. The melted materials promote further melting, as well as compacting and strengthening the clay body. It is also during this stage that mullite (aluminum silicate) is formed. These are long, needle-like crystals which act as binders, knitting and strengthening the clay body even further. It is also imperative to note that different clays mature at different temperatures, depending on their composition. A red earthenware contains a large amount of iron which acts as a flux. An earthenware clay body can fire to maturity at about 1830 F (1000 C) and can melt at 2280 F (1250 C). On the other hand, a porcelain body made of pure kaolin might not mature until about 2500 F (1390 C) and not melt until over 3270 F (1800 C).