The clay used in papermaking is usually refined from white kaolin clay, which in turn is mined from kaolinite. Kaolinite is a product of the decomposition of feldspar, which occurs when feldspar comes in contact with the atmosphere.
The addition of clay to paper pulp is done to increase one or more of a paper's properties, in particular such optical and structural properties as opacity, brightness, smoothness, and ink receptivity. The brightness imparted to paper as a result of the addition of clay is not as high as that imparted by other fillers such as titanium dioxide and calcium carbonate. Clay is, however, the most abundant and least expensive of the three.
The addition of fillers such as clay, however, can negatively affect other paper properties, such as strength, bulk, and stress resistance. (See also Fillers.)