What Is China Clay?
China clay is a type of clay first used by the Chinese for making chinaware and porcelain. From the hill, Kao-ling, close to the famous pottery-producing center of China, comes the clay’s other name of Kaolin. Samples of kaolin were first sent to Europe by a French Jesuit missionary around 1700 as examples of the materials used by the Chinese in the manufacture of porcelain.
Kaolin is produced by the disintegration of muscovite-biotite-granite rock which is easily decomposed by atmospheric agents into its present form. It is composed of 46 per cent silica, 40 per cent alumina and 14 per cent water.
One of the most famous and largest of china clay deposits is to be found in the far west of England, in Cornwall and West Devon. This clay has been produced by ascending hydrothermal solutions from the inside of the earth’s crust, such as superheated steam, boron and florine compounds, or carbon dioxide, as opposed to descending atmospheric solutions.
The uses of china clay are varied. By far the largest amount goes into the manufacture of paper. It is also used in the production of pottery, some chemicals and cosmetics, and in pharmacy. Kaolin also makes a good poultice, as it retains heat.
Approximately 40 percent of the kaolin produced is used in the filling and coating of paper. In filling, the kaolin is mixed with the cellulose fibre and forms an integral part of the paper sheet to give it body, colour, opacity, and printability. In coating, the kaolin is plated along with an adhesive on the paper’s surface to give gloss, colour, high opacity, and greater printability. Kaolin used for coating is prepared so that most of the kaolinite particles are less than two micrometres in diameter.
Kaolin is used extensively in the ceramic industry, where its high fusion temperature and white burning characteristics makes it particularly suitable for the manufacture of whiteware (china), porcelain, and refractories. The absence of any iron, alkalies, or alkaline earths in the molecular structure of kaolinite confers upon it these desirable ceramic properties.
In the manufacture of whiteware the kaolin is usually mixed with approximately equal amounts of silica and feldspar and a somewhat smaller amount of plastic light-burning clay known as ball clay. These components are necessary to obtain the proper properties of plasticity, shrinkage, vitrification, etc., for forming and firing the ware. Kaolin is generally used alone in the manufacture of refractories.
Substantial tonnages of kaolin are used for filling rubber to improve its mechanical strength and resistance to abrasion. For this purpose, the clay used must be extremely pure kaolinite and exceedingly fine grained. Kaolin is also used as an extender and flattening agent in paints.
It is frequently used in adhesives for paper to control the penetration into the paper. Kaolin is an important ingredient in ink, organic plastics, some cosmetics, and many other products where its very fine particle size, whiteness, chemical inertness, and absorption properties give it particular value.