Clay
Clay is the fundamental material in ceramics. It is composed mainly of hydrated aluminum silicates, with chemical formula Al₂O₃-2SiO₂-2H₂O. This mineral is formed through the decomposition of feldspathic rocks, such as granite, which undergo a process of alteration by the action of erosion and weathering, both by atmospheric agents and by water and wind. This process transforms the original minerals into a finer and more malleable mixture that is clay, which has unique characteristics of plasticity and moldability, making it ideal for manual work and the creation of ceramic pieces.
Depending on their mineral composition, clays can vary significantly in their properties. Their plasticity depends on the amount of water they retain, as well as the proportion of other components such as silica, alumina and metal oxides. In addition, clays can have different colors, ranging from white and gray tones, as in kaolin, to more red, yellow and brown, which are due to the presence of iron oxides. The firing temperature of clay also varies according to its composition; some clays require higher temperatures to vitrify and form a solid structure, while others sinter at lower temperatures.
There are two main types of clay: primary and secondary. Primary clays, such as kaolin, are purer and are found close to their source of formation, usually where the original rock has been broken down. These clays are whiter and have fewer impurities, making them ideal for the production of porcelain and other fine pieces that require high purity and firing at elevated temperatures. Secondary clays, on the other hand, have been transported by wind, water or ice from their place of origin, resulting in a higher presence of impurities such as minerals, metal oxides and organic matter. These clays tend to be more varied in color and texture, and are more common in the production of utilitarian ceramics, such as bricks and stoneware.Interesting Fact
Primary clays such as kaolin are very pure, which makes them ideal for porcelain production, while secondary clays, such as ball clays, are more plastic due to their impurities.Helpful Tip
If you are looking for a clay suitable for making sculptures, choose a mixture with grog, which will add strength and reduce shrinkage during firing.
