Science behind the art.
Everyone is familiar with the shiny black Native American pottery and the shiny burnished red pottery, but how many know what is going on in the clay to produce these lovely pieces?
Most earthenware clays have Iron oxide as a principle ingredient. This accounts for the red color seen in terra cotta pieces and similar earthenware pieces fired in an ordinary fire with plenty of oxygen. The ancient potters discovered as early as 6000 bc that by firing in a smoky fire, one starved for oxygen, that the red clay pottery would turn black. What causes this effect? The common iron oxide found in earthenware clay transforms into red hematite (Fe2O3 with 1.5 oxygen atoms for each iron atom) in a fire with plenty of oxygen and thus stays red, however, in a reducing atmosphere, one starved for oxygen, the iron oxide loses oxygen to the starving fire and is transformed into iron magnetite (Fe3O4 which has only 1.33 oxygen atoms for each iron atom) which is black. Thus the red pottery turns black, however, if the right temperature is again achieved in an oxygen rich atmosphere the iron magnetite will again turn into iron oxide and the pottery will again turn red. The chemical compositions of both iron oxide and iron magnetite are stable at room temperatures so the colors are also stable in whichever form they are in. But if the pottery is again raised to the temperature at which the chemical composition becomes volitile the color will change depending on the amount of oxygen available in the atmosphere at that time.
The highly polished surface that is found on many of these pieces is functional as well as decorative. The polished surface is the result of rubbing a smooth stone on the surface at the leather hard stage. Clay is made up of tiny flat plate like particles. At the leather hard stage the pressure and action of burnishing is sufficient to rearrange and align the particles and densify the surface. Because of the way light reflects more from the flat surfaces of the tiny plates than it does from their edges and because of both the aligning of the plates and the compacting of the surface a shine results and the improved density results in a lower porosity on the surface than in the underlying ware. The compaction and particle orientation also accelerates the sintering process and the consolodation of the layer during firing. These results are further enhanced when a slip of tiny particles that contains extra alkali is applied to the surface. This is known as terra sigilata, which is formed by mixing the clay in water to settle out the coarse particles. Sometimes ash is added which improves the suspension of the tiny particles and also increases the alkali of the mixture contributing to a denser, lower porosity material with a glossy surface. The extra alkali forms a more liquid phase that aids in consolodation during firing. As the firing process reaches 1000 C the burnished effect will be lost. However, at around 900 C most earthenware clays have reached the state of the sintering process where the major components of the clay body are beginning to melt so they are rarely fired past this point anyway. This does become a factor for high firing bodies such as porcelain and stoneware, however, and as a result you will not see high fired pottery with a burnished finish. Although they can be burnished and they do not melt at 1000 C the burnish goes away around that temperature and will not stay through till they are mature. This is undoubtly a result of the changes that are occuring once that temperature is reached.
The majority of the information in this article was gleaned from the book "Ceramic Masterpieces" By W. David Kingery and Pamela B. Vandiver This book is recommended to those who wish to learn more about the science behind the art of ceramics.