148 



Weathering. — The reduction of large arid hard masses of clay and 

 shale is often greatly facilitated by exposure to weather, i.e., to the 

 action of air, sunshine, frost and rain. When so exposed, many clay 

 materials disintegrate rapidly and may afterwards be made into a 

 plastic paste much more easily than by any mechanical process of 

 grinding and mixing with water. Different clays and shales are 

 affected differently by exposure ; some disintegrate after a few hours' 

 exposure on a warm day, whilst others 'appear to require a succession 

 of frosts and rainy periods. In most cases, the most feasible explana- 

 tion of the physical changes which occur is that the conditions of 

 exposure result in the partial peptisation of the colloidal cementing 

 material which binds the particles of clay together. It is well known 

 that sand grains soaked in concentrated glue and then suitably dried, 

 form a hard rocky material which, on exposure or soaking in water, 

 falls'to powder as the colloidal element absorbs water, swells, and is 

 no 'longer able to hold the particles together. It is suggested — 

 though no definite proof is available — that'^when natural clays are 

 exposed to weather a similar absorption of water by the colloidal 

 matter occurs, and is followed by a corresponding disintegration of 

 the mass. In the case of some clays, a certain amount of chemical 

 change such as the fermentation of organic matter, or the oxidation 

 of the p5Tite, &c., also occurs and may also facilitate the disintegration, 

 but the chief cause of the reduction of the material to a more or less 

 pulpy mass, bears a much closer resemblance to the softening of the 

 colloidal cementing mass than to any ordinary process of oxidation 

 or other obviously chemical reaction. 



Commercially, the weathering of indurated clays is of great import- 

 ance, as it not only reduces the cost of grinding and mixing, but the 

 weathered product is much more homogeneous and the water present 

 is far more uniformly distributed than when the treatment of the clay 

 is purely mechanical. 



According to W. Taylor, the colloids produced during the 

 weathering are not amorphous alumino-sihcates, but mixtures of single 

 gels produced by the mutual precipitation of positive and negative 

 sols. 



The ancient practice of storing clay in cellars for a long time, and 

 known as maturing or ageing, is now seldom practiced to anything 

 like the extent to which it was formerly thought necessary. Where 

 hollow goods of very fine quality are made there is an undoubted 

 advantage in thus storing the clay before it is made up into goods, 

 but the keeping of clay in air-tight boxes for several years, as practised 

 by the Chinese and more recently by Wedgwood and other famous 

 potters, is no longer considered essential, though its beneficial effect 

 on the clay cannot be denied. In Germany, the use of sumps, in 

 which the clay and water remains in contact with each other for a 

 considerable time, is still regarded as necessary. 



In freshly pugged paste, there is only a limited amount of colloidal 

 matter in an active form. Its amount may be increased by subjecting 

 the paste to conditions under which any dry and horny colloidal gel 

 wiU absorb water, swell, and form a soft friable jelly, and the same 



