CLAY WORKING INDUSTRIES. 77 



in which all of its bases are combined with all of its fixed acids. This 

 aggregate composition is best expressed by means of the " Oxygen 

 Ratio." 



By the oxygen ratio is meant the ratio existing between the oxygen 

 of the silica and any other acid elements, against the oxygen existing in 

 the alumina, iron, lime, magnesia, and alkalies or other bases present. 



It is the peculiar quality of silica in the capacity of an acid to unite 

 with any indefinite quantity of bases, and the only present means of 

 classifying these miscellaneous compounds is by the ratio of the oxygen 

 in their bases and acids. By this means silicates can be classified as being 

 subsilicates, protosilicates, sesquisilicates, bisilicates, trisilicates, etc., 

 or nearest to one or other of these standard compounds, as the case 

 may be. 



In considering this theory of the importance of the oxygen ratio of a 

 clay it is not meant to disparage the important influences exerted by its 

 feldspathic and ferruginous impurities ; but only to point out that a given 

 amount of these fluxing impurities would probably cause clay of a certain 

 oxygen ratio to vitrify at a comparatively low heat, while another clay 

 containing the same fluxing impurities, but of quite a different ratio, 

 would still be unaffected at the same temperature. 



The ferruginous impurities of clays consist, as has been before 

 stated, of "both of the oxides of iron, anhydrous or hydrated carbonates 

 of iron, sulphide of iron, sometimes sulphate of iron, phosphates of iron, 

 titanates of iron and possibly silicates of iron. In all of this array of 

 minerals it is to be expected that the action of the iron would vary ; it is 

 known to play two important parts as free or uncombined sesquioxide of 

 iron, and free protoxide of iron, and is now believed by many chemists to 

 act in a third and still more important role as hydrated sesquioxide of iron, 

 chemically combined with the alumina and silica to form an iron clay. 

 Instances of this substitution of part of one base for another are not 

 infrequent in chemistry. They are called double-salts. For instance, 

 alum is a sulphate of alumina but an iron alum may easily be made in 

 which part of the alumina is replaced by the similar oxide of iron. This 

 iron alumina sulphate them becomes a fixed chemical compound, only 

 broken up by chemical means. This theory,based on the part that iron plays 

 in clays, goes further to explain the various and confusing reactions of 

 iron in clays than any yet brought forward. For instance, two clays of 

 the same iron content are found to be of entirely different color on burn- 

 ing, one being red in every particle and the. other of a light color with 

 here and there a dark grain. The latter will stand a large increase of 

 temperature with no further change than shrinking somewhat, and the 

 black grains becoming black blotches of a fluid cinder. The red clay, on 

 being brought up to the same heat successively grows closer grained, then 

 vitreous or glassy in fracture changing color from darker to lighter red, 

 and then through the brown colors till finally the vitreous fracture disap- 



