GLAYS OF NEW YOKK 521 



action on the clay, by tlie formation of a doiuble silicate- of iron, 

 lime and alumina, except at biglier temperatures. 



Carbonate of lime is very abundant in clays, eitber sedimentary 

 or residual, whicb have been derived from areas underlain by cal- 

 careous rocks. It may result from the decomposition of lime-bear- 

 ing feldspars. Its presence as carbonate can be frequently deter- 

 mined by treating tbe clay witb muriatic acid, which produces 

 effervescence if more than 4^ or 5^ of lime carbonate is present. 



The effect of carbonate of lime in a clay depends on its physical 

 condition. If present in the form of lumps or pebbles, it is very 

 injurious, and is commonly removed by screening or washing, or 

 at times the clay is simply washed to break up the lumps. If 

 present in a finely divided condition, it may not only be harmless 

 but even desirable, provided there is not an. excess of it. Clays with 

 20^-25^ of carbonate of lime can be used for common or even 

 pressed bricks, also for earthenware. Calcareous clays find an ad- 

 ditional use in the manufacture of glazes. 



The effects of carbonate of lime may be briefly stated as follows. 

 In burning the lime carbonate is broken up into- carbon dioxid and 

 caustic lime. If the clay is not raised to the temperature of 

 vitrification in order to make the lime unite by fusion with other 

 ingredients, the lime will absorb moisture from the air and slake. 

 The swelling which accompanies this may, if the lime is in lumps, 

 cause a bursting or flaking of the brick. 



Lime also tends to destroy the red color produced by iron in 

 burning, giving a buff, or greenish product, depending on the in- 

 tensity of the firing. To destroy the iron coloration, it is necessary 

 for the clay to contain three times as much lime as iron. Buff 

 colors are not always due to this causes, for a sinall percentage of 

 iron in a clay may yield the same hue. 



In high grade clays large amounts of lime do not need to be con- 

 sidered, for such materials can not be used ; but in the manufacture 

 of building brick, pressed brick, or terra cotta, it is sometimes neces- 

 sary to use clays with a large amount of lime, either froan necessity, 



