CLAY 811 



and the softest clays, and many of tho most valuable clays occur in a semi-indurated 

 condition, are mined by the process of blasting, and brought to the surface in hard 

 rock-like masses. These, by exposure to atmospheric changes and alternations of 

 wet and drought, frost and thaw, are speedily, by tho process known as weathering, 

 disintegrated and reduced to the pla&tic condition. 



' The state of mechanical subdivision is of no little importance in the applicability of 

 clays to the various purposes of ceramic manufacture. Every gradation exists between 

 an almost impalpable condition and a mixture of coarse and fine matter, the coarse 

 residue of which sometimes forms as much ns 10 or 20 per cent, of the entire weight. 



4 Bearing in mind that most clay-strata result from the mechanical disintegration 

 of older rocks, it will be easily understood that their state of subdivision has been 

 dependent for the most part on the transporting and sorting agencies of water, 

 carrying away and separating the finer particles held longer in suspension than the 

 coarse matter. 



' The origin of some of the finer white clays must, however, be attributed to the 

 chemical dissolution of calcareous rocks by tie agency of carbonated water, the in- 

 soluble silica, alumina, magnesia, &c., associated with carbonate of lime in chalk and 

 limestone in the finest state of subdivision, being left behind as an impalpable 

 residuum. The writer, in a paper on ' The sources of the materials composing tho 

 white clays of the Lower Tertiaries,' published in the 'Quarterly Journal of the 

 Geological Society' (vol. xxiii. p. 387), suggested such a derivation from the chalk, of 

 the smooth clays of Bovey Tracey and Newton Abbot, and of similar clays from the 

 Lower Bagshot beds of Wareham, and it seems scarcely open to question that the 

 white clays resting on the carboniferous limestone of North Wales, Derbyshire, and 

 Tipperary, are the remnants of the subaerial dissolution of the limestone.' 1 



Tho chemical composition of plastic strata varies as much as their mechanical condition. 

 They may be generally described as an association of silicate of alumina, alumina, 

 free silica, and magnesia, with more or less water of combination. Clays and marls 

 scarcely ever occur entirely free from iron, to which their colour is mainly due ; it 

 exists in various states of combination, further referred to below. Carbonaceous 

 matter, especially in the tertiary and carboniferous clays, is frequently associated in a 

 fine state of subdivision, and tho alkalis are generally present in variable proportions 

 both as silicates and carbonates. 



Contraction in Burning. This character is of so much importance in all branches of 

 ceramic manufacture that it may be of interest to notice one or two features that the 

 experiments exhibit. 



The amount of contraction in burning, due partly to the loss of water of combination 

 and of the carbonic acid in the carbonates, when present, and to the ignition of any 

 carbonaceous matter contained in the clays, but more especially to the drawing to- 

 gether of the particles in the production of vitreous silicates, is very variable, and 

 depends both on the chemical and mechanical composition of the clays. The presence 

 of the alkalis and iron tends to complete vitrification, which is always accompanied 

 by a great amount of contraction, and the production of a glass-like body with a bright 

 conchoidal fracture. On the other hand, in clays containing much free silica or even 

 silicate of alumina without the accompaniment of the fluxing alkalis, a small amount 

 of contraction takes place and an open porous ' body ' is the result. 



The amount of contraction is not less due to the state of mechanical subdivision of 

 the constituent particles. Clays in a coarse state of subdivision and containing a 

 large proportion of gritty matter, especially of silicious sand, invariably contract 

 loss in burning than those of smooth and fine texture, in which the constituents 

 are in an impalpable state of subdivision ; this will be at once seen by a comparison 

 of the amount of contraction of slabs composed of the coarse clay in its natural 

 state with those moulded from the clays from which the coarse particles have been 

 removed ; and the larger the proportion of coarse matter in the native clay the 

 greater is the difference between the amount of contraction of the clay in its natural 

 and refined states. The average contraction observed in a considerable number (122) 

 of burnt slabs composed of the native unrefined clay amounted to 6'01 per cent., and 

 of the burnt slabs of refined clay to 7'53 per cent, of the original moulded size. This 

 appears due to two causes : first, that a mixture of large and small particles is, to 

 begin with, actually more dense than a mass of particles of equal size, 2 and therefore 

 admits of less contraction in the drawing together of the particles in vitrification ; and, 

 secondly, that the coarser subdivision and less intimate contact seem to hinder the 

 recombination of the constituents as vitreous silicates in the process of burning. 



1 See papers on this subject by the writer at pp. 241 and 299, vol. iv., of the ' Geological Magazine.' 

 * Illustrated by the fact that a busncl of shot weighs the same as a bushel of bullets, but is 



exceeded in weight by a bushel of mixed shot and bullets, the small shot filling the vacancies 



between without displacing the larger spheres. 



