150 



some clays may be mixtures of gels whilst others are mixtures of 

 colloidal and amorphous alumino- silicic acids. 



The customary arrangement of the paste in the souring shed is 

 to fill a large clean floor with the paste to a height of about 5 feet, 

 cover it with wet sacking and keep the latter moist for the desired 

 duration of the souring. As fresh clay is added to one part of the 

 floor, the soured clay is removed from another. Where the output 

 is sufficiently large, it is convenient to use an annular building with 

 12-20 entrances as the addition of fresh paste and the removal of 

 the soured paste may then proceed continuously with a minimum 

 of trouble and space. In some works, the clay is stored in the form 

 of balls or blocks about 3 feet diameter. If the paste is allowed to 

 sour in wagons, each holding about 1 ton, a considerable amount of 

 labour is avoided, though the gain is largely counterbalanced by 

 the cost of the wagons. 



During the souring period, the paste must be kept moist and special 

 care should be taken not to allow a dry crust to form on its surface. 



Air as well as moisture is necessary for the effective souring of 

 some highly plastic clays, as the desired decomposition of the organic 

 matter cannot occur in the absence of air. 



It is very important to cover the clay to be soured, with wet 

 sacking, as if this is not done the plasticity wiU be decreased instead 

 of increased. In time, the union of the particles and also the growth 

 of larger particles at the expense of the smaller ones may occur. 

 Both may lead to the enlargement of the interstices between the grains 

 and to the solidification of the gel residue, by which the total volume 

 of gel remains fairly constant under constant tension. Crystallisa- 

 tion and other processes which enlarge the interstices during ageing 

 also increase the vapour tension. During ageing the total surface 

 of the particles is reduced, but the interspaces grow larger. This 

 indicates that ultramicrons grow at the expense of the amicrons. 



The duration of the souring jDcriod varies greatly in different cases. 

 In some cases, only a couple of days is allowed ; six to eight weeks 

 is much more valuable when practicable, and the ancient makers of 

 some Chinese porcelains are understood to have kejDt their paste for 

 a hundred years ! It is by no means unusual for the clay to be soured 

 as much as six months and in excejitional cases, several souring 

 periods are arranged. In making glass-house pots, for instance, the 

 clay, is allowed to sour after a preliminary treading; it is then 

 re-trodden, again allowed to sour and finally is re-pugged before it is 

 ready for use. 



Increasing the Souring Effect. — As the results which occur when 

 the paste is allowed to sour are so beneficial, it is obviously advantageous 

 to develop them as much as possible. The chief methods of doing 

 this are : — 



(i) To increase the amount of organic matter in the paste 

 and to secure a more intense souring action. For this purpose, 

 various putrefactive organic solutions such as wine, old vinegar, 

 sewage, peat extract and also solid organic substances such as 

 chopped vegetable matter, farmery waste, and even molasses 



