Kaolin

Kaolin or kaolin is a kind of clay obtained from granite rocks. It is used in the construction of some ceramics and porcelains. also known as killer bees in Turkey. It is a white and soft soil type.

In a typical porcelain construction, there are 30% feldspar, 20% flintstone, and 0% kaolin, excluding 10% kiloliters, by weight. Since it is not easy to dissolve, feldspar is added to porcelain dough to provide a lower temperature range. Also, the flintstone prevents warping during the burning event.

Kaolin - what it is:

The chief constituent of kaolin is the clay mineral kaolinite, a hydrous aluminium silicate, Al4Si4O10(OH)8, formed by the decomposition of aluminium silicates, particularly feldspar. 

Kaolin - where it is found:

Kaolin is now mined primarily in Malaysia and in Cornwall, England. China clays were first discovered in England in the 1740s. In England the china clays are found near St Austell, on western flanks of Dartmoor and on the western and southern parts of Bodmin Moor. The quarrying technique is unusual. High pressure hoses are directed at the wall of the clay pits. The fine clay forms a slurry and is washed down. Most of the impurities are left behind. 

Kaolin - how it is used:

China clays have poor plasticity so they are often used in conjunction with additives - usually ball clay and bentonite. As a general rule china clays are quicker to cast than sedimentary clays. They are highly valued for their whiteness, hence their use in bone china. Shrinkage of clays fired at 1300ºC is about 12%.

China clay

 
china clay, one of the purest of the clays, composed chiefly of the mineral  usually formed when granite is changed by hydrothermal  Usage of the terms china clay and kaolin is not well defined; sometimes they are used synonymously for a group of similar clays, and sometimes kaolin refers to those obtained in the United States and china clay to those that are imported. Some authorities term as china clays the more plastic of the kaolins. China clays have long been used in the ceramic industry, especially in fine porcelains, because they can be easily molded, have a fine texture, and are white when fired. France's clays are made into the famous Sèvres ) and Limoges potteries. These clays are also used as a filler in making paper. In the United States, deposits are found primarily in Georgia, North Carolina, and Pennsylvania; china clay is also mined in England (Cornwall) and France.

 

CHINA CLAY - A RAW MATERIAL FOR EVERYDAY PRODUCTSPart 2: A wide range of applications

By calcination, a thermal process (> 1100 ° C), the characteristics of the china clay are modi- fied.

The calcination process does not only cause dehydration but particles agglomerate. These irregular particles are not broken by dispers- ing. Hardness and whiteness degrees and con- sequently also the abrasion resistance of coat- ings increase. Due to the irregular structure many small air bubbles adhere onto their sur- faces. Their refractive index is lower than re- fractive index of the surrounding coating film. That is why they scatter light and increase opacity. A special form of calcination is the so- called "flash calcination". In this process china clay is heated up very quickly. Within the china clay platelets small voids and edges are formed where air bubbles can settle. The resulting higher opacity does not only provide higher opacity in the dry film but also in wet film. Cal- cined china clays are used for paints and coat- ings, mainly in matt systems.

CHINA CLAY - A RAW MATERIAL FOR EVERYDAY PRODUCTSPart 2: A wide range of applications

China clay, “filler” for paper industry

Over a hundred years ago it has been frowned upon to add fillers to paper. One even spoke of fraud. But by and by it was found that fillers are not only fibre replacements, but they also provide positive characteristics to paper such as drainage acceleration and constant rheolog- ical behaviour. Paper became non-transparent, whiter, received a closed, smooth surface. Moreover it became softer and more flexible. The main requirements for fillers are: low price, white colour, water insolubility and op- timum fine dispersion. China clay has all these properties and additionally it is resistant to changes of pH-value. At the beginning of the papermaking the process pH-value was acidic; the current trend is to neutral or alkaline pro- cess pH-values. This allows an increased use of calcium carbonate, which is cheaper than chi- na clay, but decomposes in acidic media. Therefore china clay is more and more dis- placed from its dominant position, but it still will continue to play a significant role in paper coating.

China clay and ceramics

Already in prehistoric times people had dis- covered that the "special Earth" became fire resistant by burning. At high temperatures the mineral particles are sintered. That is, particles are softened superficial and stick together without melting and losing shape.

The major component of porcelain, probably the finest "special earth", is china clay. Porce- lain differs from other ceramics as it does not form crystalline sintered structure, but it vitri- fies due to feldspar content. Therefore, even with thin wall thicknesses porcelains may have a certain semi-transparency. The manufacture of porcelain has been developed in the sev- enth century in China and kept secret for a long time. In the early 18th century Johann Friedrich Böttger and Ehrenfried Walter von Tschirnhaus had invented the manufacture of porcelain in Europe and further developed. Soon famous porcelain manufacturers such as Meissen, Hutschenreuther, Arzberg, Villeroy & Boch, Augarten, Herend, Seltmann and many others were established in Europe. Today, not only decorative objects and high quality table- ware are made of porcelain, but also a lot of technical parts like insulators. In order to achieve certain properties china clay is added to many ceramic pastes originating also from local mining areas. China clay is especially used for higher sized products such as washbasins, stove tiles and large tiles, or for particularly dimensionally stable parts such as inserts of catalysts, turbine blades or ceramic bearings.

 

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