812 CLAY 



Few clays produce a perfectly vitreous and unabsorbent body. Some burnt slabs 

 approach to n glassy texture, while others are so highly silicious and coarse in grain 

 as to be held together by a very slight cohesion. These are in the burnt state 

 open and spongy, and have undergone but little contraction in the kiln. Such clays, 

 as for example those from the North Worcestershire coalfield, of which the celebrated 

 Stourbridge fire-bricks are made, are from their refractory character eminently suitable 

 for the manufacture. 



The great majority of clays are intermediate in character between these two 

 extremes, and after the process of burning, form a compact but slightly porous body, 

 subject to a moderate amount of contraction, and are available for general pottery 

 purposes. 



The Colouring Matter of Clays. No native clay is entirely free from the presence 

 of iron which occurs in aluminous earths in various proportions and states of combina- 

 tion. Those most free from iron are the white lower tertiary clays of Devon and 

 Dorset, largely exported from Teignmouth and Poole for the manufacture of white 

 earthenware ; for this purpose the absence of iron is a matter of great importance, as 

 it imparts to the ware a yellowish tint, to obviate which it is found necessary to cloak 

 and neutralise the natural cream colour of the burnt clay by the admixture of very 

 small proportions of cobalt blue. 



Iron in the white and grey tertiary clays occurs principally in the form of grey 

 carbonate of protoxide, generally in association with finely-divided carbonaceous 

 matter, in proportions varying from a mere trace up to 4 or 5 per cent. 



Iron, which is so prejudicial in clays employed for white pottery, is an essential 

 colouring matter in those used in the manufacture of terra cotta, encaustic tiles, bricks, 

 and all common pottery. 



These may be considered separately as (a) Grey clays. (6) Yellow clays, (c) Eed 

 clays. 



(a) Grey clays, so largely developed as ' clunches ' and fire-clays in the carbona- 

 ceous beds of the coal-measures, owe their colour, in addition to the presence 

 of carbonaceous matter, to carbonate of protoxide of iron in a fine state of 

 subdivision, and occasionally to the presence of finely-divided pyrites, or 

 bisulphide of iron, which also occurs in the London clay, gault, &c. 



A pale grey clay, almost white, from the base of the Ashdown Sands 

 (Wealden), near Hastings, contains a much larger proportion of iron 

 than its colour would seem to indicate, from its occurring in the form of 

 the comparatively colourless basic sulphate, of which there is T68 per cent, 

 present. 



(6) Yellow clays are coloured by hydrous sesquioxide of iron, and generally occur 

 as surface deposits, or where red and gray clays have been subject to 

 weathering, as on exposed surfaces or along lines of jointing. They 

 occasionally occur interstratified with red and grey beds, but appear more 

 generally to be the result of a kind of rusting process. Grey carbonate of 

 iron on exposure to watery infiltrations, accompanied by atmospheric oxida- 

 tion, becomes converted into the yellow hydrous sesquioxide ; and bisulphide 

 of iron, which readily decomposes under similar circumstances, becomes con- 

 verted partly into sulphate of protoxide, and partly into hydrous sesquioxide, 

 to the presence of which the yellow joint-surfaces of the London clay and 

 gault are due. 



Yellow clays have also been derived from red beds by the red anhydrous 

 sesquioxide and the lower hydrates receiving water of combination. 

 (c) Bed clays and marls, e.g. those of the Keuper, Old Eed beds, permian, coal- 

 measures, the middle wealden, the Neocomian strata of France, the plastic 

 clay of the London and Paris basins, and other tertiary strata, derive their 

 colour from the presence of anhydrous sesquioxide and the lower hydrous 

 oxides of iron which occur in variable proportions, and are generally asso- 

 ciated with small quantities of iron in other states of combination, the 

 colour of which the red oxide obscures. Eed haematite may be cited as a 

 familiar example of almost pure anhydrous sesquioxide of iron, which, when 

 finely divided, has a strong colouring power. The red Keuper marls 

 receive their colour from about 3 per cent, of this anhydrous sesquioxide, 

 whilst the red clays of the argile plastique of Paris, and of the Neocomian 

 beds of Beauvais, used in the manufacture of the celebrated Beauvais 

 pottery, contain as much as 20 per cent., equivalent to 15 or 16 per 

 cent, of metallic iron. Nearly all such red clays are variegated by lighter 

 patches from which the oxide of iron has been abstracted ; partly by a 

 segregational process, drawing together the iron into ferruginous nodules of 

 hydrous sesquioxide, and also from its dissolution by the acids of organic 



