Moulding clay

In the press process the clay is pressed into a mould. Clays with low plasticity can be used and mostly sand-struck (paving) bricks... 

The most important practical applications of the science of lyophobic colloids are doubtless to be found in the domain of mechanical and rheological properties. This becomes immediately obvious when one realizes that the soil, moulding clays, paints, varnishes, pastes arc lyophobic systems. Unfortunately the theoretical knowledge in this field is still rather restricted, although a wealth of empirical data is available. 

Clays have plate-like molecules with charges on their surfaces (Chapter 16). The charges draw water into the clay as a thin lubricating layer between the plates. With the right moisture content, clays are plastie they can be moulded, extruded, turned or carved. But when they are dried, they have sufficient strength to be handled and stacked in kilns for firing. 

In slip easting a thin slurry, or suspension, of clay in water is poured into a porous mould. Water is absorbed into the mould wall, causing a layer of clay to form and adhere to it. The excess slurry is tipped out of the mould and the slip-cast shell, now dry enough to have strength, is taken out and fired. The process allows intricate shapes (like plates, cups, vases) to be made quickly and accurately. 

The first stage in the conversion of iron ore to steel is the blastfurnace (see Panel), which accounts for the largest tonnage of any metal produced by man. In it the iron ore is reduced by coke, while limestone removes any sand or clay as a slag. The molten iron is run off to be cast into moulds of the required shape or into ingots ( pigs ) for further processing — hence the names cast-iron or pig-iron . This is an... 

Since 1960, the inner core has been made from c/5-poly-butadiene by the compression moulding technique. This replaced the earlier material made from a suspension of barytes or bentonite clay in water and glycerine or the winding of rubber threads made from t /5-polyisoprene, either from latex or a dry rubber compound. A typical thread recipe is given Table 4. 

Control shrinkage after moulding. Any filler will decrease shrinkage most commonly used are silica, clay, calcium carbonate, alumina, talc, powdered metals and lithium aluminium silicate. 

Swollen clay materials are soft and easy to mould. They serve to produce ceramic materials. High quality fire-clay has a high kaolinite content. Upon firing, the intercalated water is removed first at approximately 100 °C. Then, beginning at 450 °C, the OH groups are converted to oxidic O atoms by liberation of water, and after some more intermediate steps, mullite is formed at approximately 950 °C. Mullite is an aluminum aluminosilicate, Al(4 )/3[Al2 Si,05] with x 0.6 to 0.8. 

The discovery of the working properties of clays must have resulted in one of humankind s first expressions of representational art, roughly contemporaneous with the discovery of the colouring properties of natural pigments and their use in cave art. The additional discovery that the result of the manipulation of this art form could be rendered permanent by the use of fire must indeed have been a source of wonder. The earliest fired ceramic so far known is a small moulded figurine from Dolni Vestonice in what was Czechoslovakia, dated to approximately 26000 years BP (Vandiver et al., 1989). By approximately 10000 years ago, simple utilitarian vessels were being produced in the Near and Far East. 

Many gums and resins that ooze out of the trees also behave like plastic. Sealing Wax when warmed becomes plastic can be impressed with a seal. Glass and clay are also plastic material of mineral origin. By early nineteenth century, man had learned to mould many articles from natural gums and resins and make protective films of varnish from Shellac. 

The principal use of calcined clay in thermoplastics is in polyamide moulding compounds, for which purpose it is normally coated with an amino functional organo-silane. 

The moulding is now ready to receive the gold-size, which consists of pipe-clay, rod chalk, black lead, suet, and bullock s blood. Of this mixture, which is sold to the gilder in a tenacious mass, rather softer than butter, a portion is taken and melted with common clear thin size and whfle still slightly warm, is laid on with a brush, and with great nicety of hand. . 

This manipulation requires that the mass to he formed be of sufficient tractability to be easily introduced into the cavities of the mould by kneading, and that, when pressed, It be of sufficient tenacity as not to bend or yield upon removal. It must, therefore, acquire consistency, and dry in the mould—a condition which Tenders it absolutely necessary that the moulds he constructed of substances of a porous nature, otherwise the pieces would adhere and lose their shape upon removal. The moulds are, therefore, generally formed of gypsum—plaster of Paris—and sometimes of burned clay, and are of every variety of size and pattern, a complete set of pattarns being required for evory new design, and for every size of the same pattern. Moulds for plates, dishes, and other shallow articles, consist of only one pioco, while for jugs, vases, and more elaborate forms, several pieces or moulds are employed. 

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