CHEMICAL PROPERTIES OF CLAY. 61 



will absorb moisture from the air and slake. No injury may 

 result from this if the lime is in a finely divided condition, and 

 uniformly distributed through the brick, but if, on the contrary, 

 it is present in the form of lumps, the slaking and accompanying 

 swelling of these may split the brick. 



If, however, the temperature is raised higher than is required 

 simply to drive the carbon dioxide, and if some of the mineral 

 particles soften, a chemical reaction begins between the lime, 

 iron and some of the silica and alumina of the clay, the result 

 being the formation within the clay of a new silicate compound 

 of very complex composition. The effects of this combination 

 are several. In the first place, the lime tends tO' destroy the red 

 coloring of the iron, and impart instead a buff color to the burned 

 clay. This bleaching action, if we may call it such, is most 

 marked when the percentage of lime is three times that of the 

 iron. It should be remembered, however, that all buff-burning 

 clays are not calcareous, and that a clay containing a low per- 

 centage of iron oxide may also give a buff body. Another effect 

 of lime, if present in sufficient quantity, is to cause the clay to 

 soften rapidly, thereby sometimes drawing the points of incipient 

 fusion and viscosity within 41. 6° C. (76 F.) of each other. 

 This rapid softening of calcareous clays is one of the main ob- 

 jections to their use, and on this account also, it is not usually 

 safe to attempt the manufacture of vitrified products from them, 

 but, as mentioned under magnesia, the presence of several per 

 cent, of the latter substance, will counteract this. It has also 

 been found possible to increase the interval between the points 

 of incipient fusion and viscosity by the addition of quartz and 

 feldspar. 1 



Clays with much lime carbonate 2 require only 20 to 1 24 per cent, 

 of water to convert them from a dry condition into a workable 

 paste, whereas other clays needed 28 to* 35 per cent, to develop 

 the same degree of plasticity. 3 



1 The Collected Writings of H. A. Seger, Vol. I, p. 336. 



2 The proper name for such is marly clay, but the term should not be con- 

 fused with the greensand marls of New Jersey. 



3 Loc. cit. 



