Chemistry of Ceramics

Clay undergoes two basic stages in firing--dehydration and vitrification. After the excess water molecules which are combined in a moist, plastic clay with clay molecules containing what is referred to as chemical water have been transported to the surface of the ware by capillary action and have been driven off by evaporation during the drying process, the pot may be fired. At about 350° C, the molecular or chemical water, which cannot be removed by any amount of drying below that temperature, begins to be driven off. Basically, the chemical composition of clay is A1₂0₃·2Si0_2·2H₂0, or two molecules of water to each two molecules of silica and each molecule of alumina. During dehydration to two molecules of water (H20) are driven off. By percentage, this water amounts to about 14%. The dehydration process is essentially complete at about 500° C. Beyond this point incipient sintering begins; the molecules of silica and alumina begin to collapse together, partially filling the voids left by the escaped water and creating a relatively firm bond which gives the fired clay body its hardness and strength. This is the earliest stage of vitrification. At this point, the pottery will no longer absorb water chemically and can never return to its original plastic state.

The clay body is held together firmly, but not rigidly, by the collapsed molecules of alumina and silica which intermesh like a basket weave. This is the key point at which the African firing ceases. Above this temperature vitrification continues with the melting of some of the impurities in the clay, chiefly beginning with iron oxides. These combine and melt to form a glassy substance which flows around the remaining alumina molecules, gluing them together in a rigid bond. At higher temperatures mullite, an alumina silicate characterized by long, needle-like crystals, begins to grow and becomes interwoven with the glassy substances in the clay body. Glassification and the growth of mullite combine to form an extremely rigid and hard crystalline structure in the clay body which gives stoneware and porcelain its characteristic clear ring when tapped.

Both dehydrated and vitrified clays have relatively high coefficients of expansion when heated. However, the loose, non-glassy structure of low-fired (i.e., dehydrated). African earthenware allows the clay body to expand and contract at different rates in different areas of the pot without building lines of extreme stress. The bonds between clay molecules give in certain areas and absorb the stresses so that when the clay pot is placed over an open fire different areas can expand at different rates without shattering the pot.

In the case of high-fired, thoroughly vitrified wares the rigid bond created by glassification and the formation of mullite crystals does not allow for this "give." As one area of the pot is heated more rapidly than another on contact with an open flame the pot expands unevenly and internal stresses build, quickly shattering the vessel.

These problems have been overcome in the United States only in the last fifty years by the application of the most sophisticated and complex technological developments at Corning Glass to produce inexpensive flame proof ceramic ware. The African potter, on the other hand, has been producing thoroughly serviceable cooking pots for millennia using only the raw materials and techniques at her immediate disposal.

The American or European potter who travels to Africa soon makes his way to the pottery section of local markets. Often his first gesture in examining he pottery he finds it to lap the rim of each pot to test its hardness or degree of vitrification and thus the quality of the firing to which the pot has been subjected. He is surprised by a low, earthy thump rather than the high bell-like ring to which he has grown accustomed in his work with high fired stoneware and porcelain, and he immediately concludes that his contribution to the spread of modern western technology in Africa will be to teach the African potter how to fire his or her pots to the point of vitrification, thus improving their strength and durability, creating a much more serviceable product.

What the western potter fails to understand is that the African has consciously rejected the techniques that produce hard, vitrified pottery and has chosen instead to limit the temperature of the firing to the point where the clay is thoroughly dehydrated and oxidized but has not yet begun to melt, flow, and form the new crystalline structure characteristic of vitrified wares: about 600° C plus or minus as much as 100° C depending on the particular clay body used. This low firing temperature contributes two important qualities to African pottery that are notably lacking in almost all high-fired western products: porosity, which allows for transpiration and the cooling of the contents of the pot, and the ability to withstand, without shattering, uneven and rapid heating and cooling when pottery is used for cooking over an open fire.

The fact that the African potter consciously chooses to fire her or his pots at a low temperature is proven by statements by potters that the use of too much fuel or too long a firing will weaken the pots and cause them to break in use.