That is to say that clays are able to preserve the shape which is given them; a valuable quality utilised in the manufacture of ceramic productions. This property varies with the nature of the clays, being highly developed in some, such as potter's earths, and slight in the thin or limestone clays and lehm.
It is generally admitted that the plasticity of clays varies with the proportion of combined water which they contain. This water enters into the very composition of the clays, which contain as much as 18 or 19 per cent, of it, and only leaves them at a red heat, while the hygroscopic water, or quarry water, which does not play the same part in the properties of the clays, mostly leaves them when dried in the air. The 2 or 3 per cent, remaining after this drying are expelled by a heat of 100° C.
But the clay, so heated and deprived of its hygroscopic water, recovers its properties when suitably moistened, whereas, if deprived of its water of combination, it acquires new properties: hardness, sonorousness, and inability to regain its former plasticity.
The testing of the plasticity of a clay is not an easy matter; daily practice alone can teach it.
For plastic clays cannot be worked alone on account of their adhesiveness and of the change of shape, accompanied by cracking, which they undergo while being dried.
Clays as they come from the pit contract when dried; if warmed, they continue to contract more or less sensibly according to their nature. The contraction becomes considerable when the clay has been made into a paste with water, and may be as much as a quarter of the linear dimensions. Usually, in the case of brick-clay, the total shrinkage, in drying and firing, amounts to 5 to 15 per cent, of the linear dimensions. Shrinkage takes place twice: during the air-drying, and during firing. It varies according to the mode of the latter process, and the method of manufacture.
Plastic and fusible clays undergo most contraction, but for different reasons; the former because they contain a great deal of water. They experience the greatest shrinkage at the moment when they lose this water; that is to say, at about 110° C. A higher temperature causes little contraction, because the infusible molecules cannot get closer to one another. Fusible clays, on the other hand, undergo shrinkage at that period of the firing, because the molecules weaken and approach one another to form a homogeneous whole, of a more or less close texture according to the temperature.
The shaping of the paste plays an important part in the shrinkage. Thus articles made of soft clay, that is to say of clay soaked in water, undergo more contraction than those formed from hard or semi-hard clay, that is to say from clay merely moistened with water.
Articles made by expression undergo more contraction than those manufactured by simple compression. In the latter, inequalities of pressure cause inequalities of shrinkage, hence there is warping when the pieces are fired. This is especially noticeable in tiles and other articles which have a large surface in comparison to their thickness.
Shrinkage takes place perpendicularly to all the surfaces of the pieces; that is to say, they shrink simultaneously in length, breadth, and thickness. But sometimes it happens that it acts unevenly in consequence of pressure on the piece when being baked. Thus pieces placed in the lower part of the kiln, and resting one upon another, so supporting the weight of all the upper layers, may increase in length and breadth while diminishing in height. These deformations are especially noticeable in highly baked bricks; they bear no trace of external fusion, but their fracture has a compact texture resembling that of stoneware, and evidently due to the drawing together of the molecules under the action of heat and pressure.
A thorough knowledge of the shrinkage of clays is of gre importance in the manufacture of pottery, not only to give the proper dimensions to moulds and dies, but also with regard to the adhesiveness of glazes, as we shall show later on Experience alone can teach us, since contraction varies with the clay. In expression-machines we allow an average of 10 per cent. difference between the brick issuing from the die and the same brick when baked. For the moulds in presses, working with unblended clay, only 5 per cent, is allowed.
A clay is said to be refractory when it supports without melting a temperature of about 1800° C., which is that borne by china-clays. The most refractory clays are those which most resemble pure unadulterated silicate of alumina. The presence of metallic oxides, such as the oxide of iron, lime, potash, soda, etc., makes clays fusible by reason of the formation, at high temperatures, of complex silicates, which are all more or less fusible. Lime, which when pure is very refractory, makes an infusible clay fusible if mixed with it. The carbonate of lime, or limestone, added to a clay in the proportion of one-half or three-quarters, renders it fusible.
The oxides of iron which colour clays with various tints make them fusible by reason of the formation, during baking, of silicate of iron.
To summarise, then, foreign bodies in clays tend, at a high temperature, to combine with the silicate of alumina, the base of clay, to form with it complex silicates which are generally more fusible. Hence the necessity of never subjecting ordinary pottery to too high a temperature. Nevertheless the heat must be sufficient to form complex silicates, yet not great enough to melt them. In this way we obtain an impervious, hard, compact substance, similar to stoneware.
If the temperature is pushed too high, the complex silicate melts like glass, and binds together the different articles being fired. Sometimes, in a badly managed kiln, a sudden access of heat softens the pieces and binds them into one single mass, which has to be removed from the kiln with the tongs. In a continuous kiln, the walls of which are not made of refractory substances, we have seen a part of the vault become welded to the pieces being fired under a tremendous access of heat, and break away from the arch under the action of shrinkage; these are accidents easily avoided by careful watching and regular stoking.