Kaolin, or China clay, is nearly white in color. It is distinguished from other industrial clays based on its fine particle size and pure coloring. Its ability to disperse in water make it an ideal pigment.
The primary constituent in kaolin is the mineral kaolinite, a hydrous aluminum silicate formed by the decomposition of minerals such as feldspar.
The name kaolin derives from the Chinese and means high ridge. High ridge is a reference to the hill in south-eastern China where the clay was originally discovered and used. In the 7th and 8th centuries, the Chinese were the first to use kaolin to make porcelain. It was not until centuries later that other areas of the world could duplicate the process.
The white color of the mineral can either be naturally occurring, or can result after processing which removes minerals and other color-bearing compounds. The small particle size of the mineral contributes to its white color by scattering light.
Although kaolin is found throughout the world, deposits with suitable whiteness, viscosity and other favorable characteristics are rare.
Paper Industry
The primary use of kaolin is in the paper industry. It serves as a paper coating which improves appearance by contributing to brightness, smoothness and gloss. It also improves printability. Additionally, it is used by the paper industry as a filler reducing cost and the use of tree-based resources.
China, Porcelain and Tableware:
Many people are under the mistaken impression that the only use for kaolin is in the manufacture of china. This is not true, and its use by the paper industry far exceeds its other uses. However, kaolin still serves as a valuable component in china and other tablewares. Its color, gloss and hardness are ideal characteristics for such products.
Ceramics
Kaolin has a wide range of application is the ceramic industry, and is commonly used in whiteware products, insulators, and refractories. In whitewares, kaolin viscosity improves the molding ability of pre-heated materials, offers a stronger finished product, and a smooth surface finish such as those found in tile and porcelain. Kaolin is becoming more prevalent in porcelain insulation due to its high dielectric strength and as an alternative to the scarcer resources high in potash feldsparic materials. Kaolin is used in refractories due to its extremely high fusion point, low water content, and high green strength. Ceramics produced with kaolin have a wide variety of uses, mainly as a metal substitute.
Rubber
Kaolin is an ideal additive to rubber due to its "booklet" particle structure which adds strength and stiffness to the product through enhanced stress transfer. Kaolin is a low cost substitute for silica, and improves properties of calcium carbonate found in rubber solutions. Fine kaolin with smaller particles produces a stiffer rubber due to increased surface area, while providing more abrasion resistance.
Cements
Kaolin serves an important function in the cement industry as highly calcinated, pulverized kaolin adds compressive strength, flexural strength and water permeability to cement. In general 5-15% activated kaolin mixed with cement creates a superiorly strong mortar when hardened. Water permeability is useful in prolonging the durability of concrete and reduces weakening as a whole; calcined kaolin adds flexibility, which is often preferred to the usually brittle finished product.
Cosmetics and Soaps
Kaolin is widely used in the cosmetic industry in mud masks for its absorbent qualities and its ability to control rheology. Calcined kaolin when applied to the face in a mud mask actually draws out impurities and toxins from the skin. This is possible because of the insolubility of kaolin which retains its absorbent qualities when mixed with water.
