﻿90.2 



9.8 



87.4 



12.6 



95.6 



4.4 



72.2 



27.8 



S3.2 



16.8 



72.5 



27.5 



67.0 



33.0 



422 cox. 



mentioned in this paper. The raw clays were suspended in water, pressed 

 through a 60-mesh sieve, and dried at 110°. 



Passed through 60 Retained on 60 

 Number. mesh (per cent), mesh (per cent). 



3 

 12 

 19 

 25 

 26 

 27 

 28 



L have not been able further to study these properties at this time. 



Bourry 10 has formulated a very satisfactory classification of clays, 

 and he gives the following as a differentiation of the principal kinds of 

 kaolin : 



I. Pure kaolins do not contain more than from 5 to per cent of silica and 

 2 per cent of fluxes, the proportion of each of the fluxes taken singly being not 

 more than 1 per cent. They are always brought to this degree of purity by 

 washing, and present themselves in the form of a white powder, occasionally 

 slightly tinted with yellow, gray or red, and very unctuous to the touch. After 

 firing they form a body of milky whiteness. They are used in the manufacture 

 of porcelain and fine faience. 



II. Alkaline kaolins. — The only difference between these kaolins and the pre- 

 ceding is that they contain a higher proportion of alkalies, which can mount up 

 to 5 per cent, coming from the mixture of feldspar and mica. In the meantime 

 the proportion of oxide of iron rises sometimes to 2 per cent. They have the 

 same use as the preceding, but it must be observed that a very careful washing 

 results in taking away the greatest part of the alkalies. 



III. Silicious kaolins. — In these kaolins, hydrosilicate of alumina is mixed in 

 a considerable proportion with quartz in the state of impalpable powder. This 

 content of silica depends greatly on the care bestowed in the washing; it can 

 rise to 20 or 25 per cent. These kaolins furnish a body of small plasticity, and 

 are of a light, granulous texture. They are used in the manufacture of porcelain 

 and certain kinds of faience, but for bodies which have no need to be very plastic. 



IV. Alkaline silicious kaolins contain at the same time a considerable propor- 

 tion of alkalies and of silica, and partake of the properties and uses of the two 

 preceding kinds. 



V. Ferruginous kaolins. — These kaolins contain a quantity of oxide of iron 

 which is too great to give an entirely white body after firing. When this content 

 is not too high it is utilized in the manufacture of porcelain and faience of inferior 

 quality. For a greater proportion u they can be used in the manufacture of 

 refractory productions, but on the condition that they have not too much alkali. 



The clays, the analyses of which are given above, have not been sub- 

 jected to washing. Such treatment would probably improve and change 

 the classification of some of them. As they are, numbers 1, 2, 3, 4, 5, 



lu Treatise on Ceramic Industries, London (1901), 59. 



11 Too much iron must be avoided, for it is a fluxing impurity and will lower 

 the fusing point of the clay, especially when in the ferrous condition or in the 

 presence of silicates. Fire clays should not greatly exceed 3 per cent of iron. 

 If the clay contains 5 per cent or more it is suitable only for bricks. 



