42 GENERAL DISCUSSION OF CLAYS. 



Both quartz and flint are infusible at very high 

 temperatures but the presence of other minerals may 

 serve -to flux them. Quartz tends to diminish the 

 shrinkage of the clay, and if wanting it has to be 

 added during the process of manufacture. Its addi- 

 tion also tends to decrease the plasticity. 



CALCITE. 



This mineral which is carbonate of lime, effervesces 

 when moistened with muriatic acid, so that its pres- 

 ence in clay may often be detected by the addition of 

 this chemical. Calcite is a soft mineral and occurs in 

 the clay, either in the form of little rhombohedral or 

 powdery particles. Clays, which contain a large 

 amount of it in finely divided condition, are 

 said to be marly, and in some clay deposits 

 certain layers may contain a larger percent- 

 age of carbonate of lime than others. The 

 carbonate of lime found in clays may be derived 

 from particles of limestone ir the clay if it is a sedi- 

 mentary one, or from the decomposition of lime-soda 

 feldspar in the case of either sedimentary or residual 

 deposits.. Percolating water may also introduce it 

 into the clay. 



GYPSUM. 



Gypsum or the sulphate of lime is found in clay in 

 the form of grains, needles, well developed crystals, 

 or lamellar masses. It is so much softer than calcite 

 that it can be scratched by the finger nail, often has a 

 pearly lustre, is transparent, and does not effervesce 

 when acid is poured on it. In hard burned brick gyp- 

 sum simply acts as a flux, but in lightly burned ones 



