76 CLAYS AND CLAY INDUSTRY. 



or rocks containing any pyrite they are almost sure to contain 

 soluble salts. Those flowing from lime rocks are usually "hard" 

 on account of the lime carbonate which they contain. Still 

 another source of soluble salts in raw clay lies in some of the 

 artificial coloring materials which are sometimes used. 



Soluble sulphates are sometimes formed, in burning, through 

 the use of sulphurous fuel, that is, coal containing more or less 

 iron pyrite. When the coal is burned part of the sulphur in the 

 pyrite is expelled, and, uniting with the oxygen, forms sulphuric 

 acid gas (SO s ). This passes through the kiln, and, if it comes 

 in contact with carbonates in the clay, converts them into sul- 

 phates, because many substances, such as lime (CaO), have a 

 stronger affinity for sulphur trioxide (S0 3 ) than for carbon 

 dioxide (C0 2 ). 



It frequently happens that clay products come from the kiln 

 apparently free from any superficial discoloration or coating, 

 but develop one later on if subjected to moisture. In this case 

 the salts had been formed within the body of the ware during 

 burning, and are brought to the surface after the ware is exposed 

 to the weather. The moisture, which is then absorbed, brings 

 the salts to the surface upon drying out. 



The coatings thus far mentioned are all white in color. In 

 some instances, however, the product becomes covered with a 

 yellow or green stain, which is caused either by the growth of 

 vegetable matter on the surface of the bricks or by soluble com- 

 pounds of the rare element vanadium. Some. white coatings seen 

 on brick, however, come from the mortar. 



Quantity of soluble salts in a clay. — The amount of soluble 

 salts present in a clay is never very great, but less than o.i per 

 cent, is often sufficient to produce a white incrustation. A num- 

 ber of soluble salt determinations were made on clays from dif- 

 ferent localities in the State in order to obtain some idea of the 

 quantity to be found in the clays of different formations. In 

 making these tests a five-grain sample of clay was taken and 

 heated in distilled water for about one-half hour, after which the 

 solution was forced through a small pasteur filter connected with 

 a force pump, and the clear filtrate evaporated to dryness in a 



