CHEMICAL PROPERTIES OF CLAYS. 17 



ing silicates which are apt to be met with in most of 

 the impure clays. 



When present as a silicate, lime acts as a flux, and 

 is less liable to exert a decolorizing action on the clay 

 than carbonate of lime. Bleaching action is caused 

 by the formation of a double silicate of iron and lime, 

 when the clay reaches a temperature approaching vit- 

 rification, and the color developed is either yellow, or 

 yellowish green, according to the intensity of the 

 firing. 



Carborate of lime is an abundant constituent of 

 some clays, and its presence, if over three or four per 

 cent, can usually be detected by the effervescence 

 which is produce! when muriatic acid is poured on the 

 clay. This compound of lime is far more injurious 

 than the silicate, although, if present in the clay, in a 

 finely divided condition, it may not only be harmless 

 but even desirable, provided there is not an excess of 

 it, for clays with as much as twenty to twenty-five per 

 cent, of lime carbonate have been used for making 

 common or even pressed brick and somtimes earthen- 

 ware. It is well, however, to try and keep the amount 

 lower than this if possible. Highly calcareous clays 

 have often found a use in making of slip glazes. 



If the carbonate of lime is present in the form of 

 pebbles, a most undesirable effect is produced, for it 

 is well known that when heated to redness, the com- 

 pound is broken up into oxide of lime and carbonic 

 acid gas ; this oxide of lime, when cooled, absorbs mois- 

 ture from the atmosphere and slakes, the result being 

 a swelling of the material and a consequent splitting 

 of the brick. Now if the clay be heated to a tempera- 

 ture sufficenit to decompose the carbonate of lime, but 

 not high enough to make it unite with any free silica 

 present, the lime of course slakes on cooling. It is con- 

 sequently imporatnt either to b,urn the clay sufficently 

 or remove the lime pebbles from the clay by screening 

 or by some other method before using. 



For a high grade ware, calcareous clays are seldom 

 employed, but in the manufacture of brick and terra- 



