THE PHYSICAL PROPERTIES OF CLAY. 91 



SHRINKAGE. 



All clays shrink in drying and burning, the former loss being 

 termed the air shrinkage, and the latter the fire shrinkage. 



Air shrinkage. 1 — When a mass of clay is mixed up with water, 

 each of the grains of clay can be considered as being surrounded 

 by a film of the liquid, which may prevent them from coming into 

 close or actual contact. As soon as the wet, molded mass is set 

 aside to dry, however, evaporation of the water contained in the 

 pores of the clay begins, and, as it passes off, the particles of clay 

 draw closer together, causing a shrinkage of the mass. This 

 will continue until all the particles come in contact, but since 

 they do not fit together perfectly, there will still be some spaces 

 left between the grains, and these will hold moisture, which 

 cannot be driven off except by gentle heating. The air shrink- 

 age, may, therefore, cease before all the water has passed off. 

 This fact can be told by the loss of weight which takes place in 

 the clay, if put in a hot-air bath (at ioo° C.) after the air shrink- 

 age ceases. The amount of air shrinkage is usually low in lean 

 clays, and high in very plastic clays, for the reason that the 

 latter absorb considerable water in mixing, which they then give 

 off in drying. At the same time, however, it must be noted that 

 all clays which require a high percentage of water in mixing, 

 do not show a high air shrinkage, as is shown by the diagrams on 

 Plate XIII A. 



In these, the horizontal lines indicate percentages; the num- 

 bers at the top of each diagram represent the locality, arranged 

 according to the percentage of air shrinkage which they showed, 

 the lowest being placed at the left end, and the highest at the 

 right. 



Taking the diagram for the Cohansey clays, the first one is 

 No. 219 (Lab. No. 699). A point was marked indicating its air 

 shrinkage, viz., 2.5, 2 and above this a second point was marked. 



1 Whenever the term air shrinkage is used in this report it refers, unless 

 otherwise stated, to the linear shrinkage, expressed in terms of the length 

 of a freshly molded brick, and measvired on its greatest dimension. 



! The percentage of both air and fire shrinkage is expressed in terms of the 

 length of the freshly molded bricklet. 



