The preparation which clays must undergo with a view to their use in pottery depends upon their nature, the process of manufacture, and the kind of article to be made from them. We shall only give here a few general remarks, as we intend later on to refer in a more detailed manner to this preliminary work, when we speak of the manufacture of the various products.
We have said that the preparation which clays have to undergo depends upon their properties and the process of manufacture. If, for instance, brick-earth (lehm) is used for what are called native bricks, it is an exceptional case; all other products are usually made of mixtures of different clays or even of foreign substances which have to be incorporated in them to make a homogeneous whole. The physical nature of clays enters into the choice of mixtures; rich clays must be thinned by the addition of anaplastic or thinning bodies, - sand, thin clays, cinders, etc., - while, on the other hand, thin clays must, for certain products, be enriched by loams, etc.
In fact, with the exception of the above-mentioned case, clays coming from the pit require to be treated, in order to remove the hard portions and foreign matter contained in them. Besides this, they have to be mixed with water uniformly, and the whole must be well mixed to form a sufficiently homogeneous paste, well fitted for moulding, drying, firing, and consequently for making good products.
Dry clays also are treated by reducing them to dust in order to mix them with others, or even to use them as they are by compressing them under powerful pressure followed by firing at a high temperature.
These various operations may be summarised under two heads-
1. Disintegration and division of the mass by cutting and crushing; separation of the hard bodies or their crushed remains so that they may mix with the rest of the clay.
2. Addition of foreign substances necessary for the formation of paste, water and antiplastics if necessary; and making a close mixture by pugging or blending. The processes by which these results are obtained are -
Disintegration and division of the mass
Weathering and decomposition.
Washing and removal of stones.
Crushing and pulverising.
Addition of foreign substances. - Pugging
Thinning, and treatment with antiplastics.
In order to disintegrate the clay after it comes from the pit, it has been customary for ages to extract it in the autumn and to spread it in heaps of various heights on a flat surface. If there are several kinds of clay, they are placed in alternate heaps, so as to make a mixture when the heaps are demolished.
Winter and its climatic changes - rain, snow, frost, thaw - cause a physical alteration which breaks the hard pieces and splits up the mass throughout all its thickness.
Weathering, always a costly operation, has almost entirely-disappeared with the introduction of machinery, except for manufacture by the Walloon method, and even for that it has been abandoned in many works.
Certain plastic clays experience, besides the disintegration caused by winter changes, another phenomenon called decomposition. These clays take a grey then a black colour, setting free sulphuretted hydrogen. On contact with the air the black colour disappears, carbonic acid is given out, and sulphate of iron is formed. The presence of organic matter appears to be one of the causes of decomposition, for it can be hastened by sprinkling the clay with liquid manure, marsh water, etc.
This special phenomenon gives rise, in the midst of the mass of clay, to complex chemical reactions which last for several years. It is then quite different from what is called weathering.
And besides, it is independent of weather changes, since it can take place in a covered space, provided the clay is kept constantly damp. Water and certain substances are thus indispensable for the decomposition of clays, and so is time. Also, experience seems to show that the oldest pastes are the best. The decomposition of clay is peculiar to the porcelain manufacture, and even for that purpose many important factories have abandoned the process, so we will not lay stress upon it.
We will bear in mind only that decomposition and weathering are different processes, and that both of them, although they have useful effects on the physical qualities of the clay, are being more and more neglected under new economical conditions.