Rich clays cannot be used alone on account of their eminently plastic properties. They stick too much to the moulds or cylinders of machines, and the paste they form with water falls in and loses shape after working. Its very tenacity prevents equal drying in all parts of the mass, hence cracks are formed going from the drier surface to the damper interior. If the pieces have parts of different dimensions, the less thick parts dry more quickly than the thinner ones, causing deformations. Firing increases these faults still more.
The only way to avoid these accidents is to mix closely with the rich clays, substances which will thin them sufficiently to be easily worked, uniformly dried, and satisfactorily baked. These substances are called antiplastics, shortening, or thinning materials.
Their function is to diminish the plasticity of rich clays. When mixed closely with the latter, they form within the mass a kind of skeleton which has two uses: to support the plastic paste and prevent it from losing shape, and afterwards to assist drying by acting as a system of drainage. Thus unequal drying and warping are avoided, for the anti-plastic skeleton is so identified with the clay that it follows its contraction and maintains the regular shape of the piece.
First of these substances is sand, which is an excellent shortening matter. According to its nature it will communicate special properties to the paste: if it is silicious (quartz) it will diminish the fusibility of fusible clays; it will, on the contrary, increase it if of a felspar, iron, or limestone nature. If the sand contains large grains, it will be passed through a sieve, and if necessary crushed.
The pulverisation of sandstone gives a sand which may advantageously be substituted for ordinary sand, but which is naturally dearer on account of the cost of pulverisation.
Carbonate of lime and chalk may be used as shorteners, but uh great care. In fact, if in firing, the lime formed by decomposition of the limestone does not combine entirely with the silicate of alumina, and in order to attain that object a temperature is required not usually reached in firing, it will be found scattered about in the mass, and by its abundance will make the products fall into powder. In faience pastes it is used for reasons which we explain on p. 21.
The clay marls, and in general thin clays, are all indicated for shortening potter's clays; the reverse is equally true, and the addition of a certain amount of rich clay to a thin one assists the working of it noticeably. A marl clay should not contain more than 10 to 20 per cent, of limestone to be used direct, but if used as an antiplastic, this proportion may rise to 30 per cent., as long as it does not exceed equal parts in the mixture.
These substances have a double use; like sand they are excellent antiplastics, but they have this additional advantage, that they burn in firing in the heart of the mass, thus spreading the heat evenly, and economising fuel.
They are most commonly used in the manufacture of bricks and pottery; the products obtained are less dense in consequence of the numerous hollows contained in them; but if well baked they are not porous, ring well, and have excellent qualities which cause them often to be preferred to other better-looking pieces.
Too large cinders are taken out by the sieve. The coke dust has the same properties as cinders and is used for the same purpose.
These are formed from the pulverisation of fragments of bricks, tiles, and pottery, and possess antiplastic properties similar to those of sand. They would be too expensive for use with ordinary products on account of the cost of pulverisation, but they are commonly used in working refractory substances and other ceramic products of fairly high price. Naturally fragments are also used of out-of-date or defective pieces such as bricks, crucibles, glassware, retorts, saggers, etc.
Antiplastics of vegetable origin such as sawdust, husks, and straw can only be employed occasionally on account of their high price in our countries. As they disappear in firing and leave large hollows, they are used for objects which require great porosity, like water-coolers, or those intended to resist sudden changes of temperature, like enameller's plates.
When pulverised these substances form excellent shorteners; they are, however, less economical than cinders in spite of their lower cost, because their pulverisation is difficult, and they have little or no calorific power. The machines used to pulverise these substances for use as shorteners vary according to their hardness. For some, like sand and cinders, crushing mills such as we have described suffice; but for hard and large bodies like fragments of pottery, Carr machines are necessary.