523 NEW YORK STATE MUSEUM 



or to obtain a cream colored ware. For the latter purpose semi-fire 

 clays yield the best results, but are not always obtainable; hence 

 calcareous clays must be used. It is therefore desirable to know the 

 amount of lime carbonate which is allowable. A good, but not at 

 the same time vitrified, brick can be made from a clay containing 

 20/^-25;^ of lime carbonate, provided it is evenly and finely dis- 

 tributed through the clay. 



The objection to highly calcareous clays is that the points of in- 

 cipient fusion and vitrification lie so close together that it is not 

 safe to burn them hard, because of the risk of fusing them. It has 

 been found possible to separate these points however by the addition 

 of quartz and feldspar to the clay, or by adding sand containing a 

 large proportion of them.-*- 



Aside from lowering the fusibility of a clay, and affecting its 

 color when burned, lime also exerts a powerful effect on the 

 shrinkage. 



Soger ^ found that calcareous or marly clays required usually only 

 20^-24^ of water to convert them from a dry condition into a work- 

 able paste, whereas other clays needed 28^-35^ of water to ac- 

 complish the same change. 



In burning, such clays lose not only their combined water but 

 also carbon dioxid, and consequently they are more porous than 

 other clays up to the point of sintering, and this porosity, attended 

 by diminution of shrinkage, increases with the amount of lime 

 carbonate contained in the clay. The shrinkage may indeed be- 

 come zero^ or the brick even swell. 



The small difference between the points of incipient fusion and 

 viscosity have already been mentioned. 



Gypsum, the hydrated sulfate of lime, is not uncommon in some 

 clays, specially those which originally contained carbonate of lime 

 and pyrite. The oxidation and decomposition of the latter produce 

 sulfuric acid, which attacks the lime carbonate, producing lime 



1 See " Glazed brick ", p. 652. 



2 Seger. Ges. Schrift, p. 265. 



