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.