137 



has apparently been completely removed. Disastrous results have 

 frequently been known to occur in the steaming operations in th-e 

 kiln for want of sufficient care in this particular. Manufacturers 

 frequently adopt a very wise precaution in having such goods 

 stamped with the date of making, and in holding their workmen 

 responsible if they are rendered unsound by being burned before 

 the lapse of a stated period of drying. 



In order that the goods may not twist or warp when drjdng, it 

 is essential that they should shrink very little. This means that 

 only a limited proportion of plastic clay can be used in the material, 

 although spme is necessary to bind the particles together and to 

 give it the general characteristics of '" clay." With a carefully 

 compounded mixture, the contraction of the paste prior to entering 

 the kiln should not exceed 1 inch in 16 (or | inch per foot). If it 

 does so, more non-plastic material must be added. The lower the 

 contraction, the better the chance of the ware coming '" true " out 

 of the drjdng rooms ; hence, as much non-shrinking material as 

 possible should be used in the clay mixtures, so as to keep the contrac- 

 tion at a minimum. The addition of a non-plastic material to a 

 clay enables less water of formation to be used, and so reduces the 

 shrinkage, but Aron has shown that if the amount of water is kept 

 the same as for the clay alone, the addition of non-plastic material 

 will increase the contraction which occurs on drying, until a certain 

 point (that of maximum density) is reached. After this, the more 

 " grog " added to the clay the less will it shrink, and the greater 

 will be the porosity. The nature of the non- plastic material added 

 will also affect the shrinkage to some extent, and will exercise a 

 considerable influence on the amount of water which must be mixed 

 with the clay. Thus, a porous, burned clay will absorb more water 

 than will sand. Provided the non-plastic material is of a nature 

 suitable to the clay (this must be determined by actual experiment), 

 it may be added in any desired proportion so long as it does not too 

 seriously reduce the strength of the mass, as it will do if more is added 

 than the binding power of the clay can accommodate. 



The j)orosity of a dehydrated clay appears to be due to the 

 capillary structure of the material. 



Other properties ^^•hich clays in the plastic state possess in common 

 with colloids are : — 



Unctuousness, or a smooth, almost greasy, " feel," is a characteristic 

 of some clays, a few being so oleaginous that they may be saponified 

 by treatment with caustic alkali, the plasticity being thereby increased. 

 In most cases, however, such treatment makes the clay more fluid. 



Toughness, or cohesion, is closely allied with (1) extensibility, or 

 the ability of clay to stretch when pulled, which is measured by 

 ascertaining the fullest extent to which a clay test-jiiece of a given 

 size will stretch without breaking ; (2) torsion, or the extent to which 

 a piece of clay can be twisted, which is measured by clamjiing one 

 end of a bar of clay as rigidly as jDossible and rotating the other slowly 

 by means of a screw, counting the number of complete revolutions 

 which can be made before the bar breaks ; (3) bending moment, or 



