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water at their outer ends and begin to form spaces in the clay, these 

 spaces being filled Avith air and water vapour. Contraction still 

 occurs throughout this second stage until the substance is so far 

 solidified that the individual particles can no longer slip over each 

 other at all. The third stage is then reached in which capillary action 

 and shrinkage cease entirely. Evaporation now takes place entirely 

 within the mass, and spaces are formed exactly corresijonding to the 

 water lost. That shrinkage ceases before the clay is completely 

 deprived of water is shown by Aron and Brogniart to be characteristic 

 of many, but not of all, clays. Aron supposed that the clay shrinks 

 until the particles are practically in contact with each other, so that 

 any further water which may be driven off does not make any notable 

 difference in the volume of the clay; but supporters of the colloid 

 theory argue that the heat used in drjdng really cause the colloid 

 particles to shrivel, thus reducing their surface and increasing their 

 density. Aron has further sho^\Tl that the " pore space " is constant 

 for each kind of clay, and is independent of the amount of water of 

 formation added to the clay, though this last statement is only true 

 of the purer clays. 



If, now, the pastes made with varying amounts of water of 

 formation are subjected to exactly the same conditions of drying, 

 the rate is not proportional to the water added, but is slower in 

 proportion for those with less water. It takes, approximately, the 

 proportional time in the first two stages of drying, but the more soUd 

 the mass the longer it takes to eliminate the last portions of the 

 water. It follows also from this, that want of uniformity in the 

 substance of a mass of clay, such as must exist in bricks made by 

 hand, and in a less degree in those made in a press or die, causes a 

 corresponding want of uniformity in the shrinkage and the rate of 

 drying in different parts of it. This is one cause of the warping or 

 twisting of bricks in drying. 



In the second stage of drjing, all clays lose water more rapidly on 

 the outside than on the inside, the angles and arrises in their turn 

 drying more rapidly than the faces. The consequence of the greater 

 shrinkage of the outer layer is a frequent cause of cracking, and it 

 is, therefore, necessary to pursue this stage with great caution and 

 to effect the drying with air already heavily charged ^^'ith moisture. 

 It is also essential, for this reason, to avoid the excessive prominence 

 of any part of a complex-shaj^ed article, and it is advisable to follow 

 aiy projections on the exterior with hollows on the interior, so as 

 to maintain an approximately regular thicloiess of material throughout. 

 The frogs or indents on both sides of a common brick are serviceable 

 in drjdng for the same reason. In a re-pressed brick, they serve the 

 additional purpose of rendering the cors'.stencj' more even throughout. 

 For objects of reasonable size, the rate of drpng is apjiroximately 

 proportional to the ratio of surface to volume. Objects of large size, 

 however, take much longer to dry, and require the application of 

 considerable heat to complete the removal of all the water of manu- 

 facture from the interior. Many large goods made of fireclay and 

 stoneware clay require extremely careful treatment, and have to 

 be kept in a heated atmosphere for several days after the moisture 



