682 NEW YORK STATE MUSEUM 



to moisture. This is generally due to the formation of salts during 

 burning, and tliey are specially annoying on account of their tardy 

 appearance. The salts formed during drying do not necessarily 

 arise simply from the combination of sulfur in the fire gases with 

 bases in the clay, but may also be due to iron pyrite which, during 

 burning, aids in the formation of white washing sulfates in the in- 

 terior of the bricks. The formation of white washing sulfates dur- 

 ing burning is described by Gerlach as follows: "A part of the 

 sulfur in the iron pyrite is loosely combined with the iron, and 

 oxidation of this part begins at approximately 650° F., whereas 

 the other parts burn at ordinary heat. The products of disintegra- 

 tion are oxid of iron, and aulfurous acid gas. This chemical re- 

 action is expressed as follows: 1) Ft^a -^-00= FeS + SOg, and 

 2) 2FeS+70 = Fe203 280^. The sulfurous and gas SOg when 

 heated in contact with solid porous bodies is oxidized by the super- 

 fluous oxygen of the air of combustion to sulfuric acid, or conyerts 

 existing oxids into sulfuric salts. It was for a long time erroneously 

 believed that the presence of water or watery vapor was necessary 

 for the formation of sulfates." Gerlach's conclusion is that it fol- 

 lows that white washing sulfates are formed in large quantities only 

 when sulfurous acids and carbonate of lime or other carbonates 

 o<3cur together in chemical action. Sulfurous acid has nO' injurious 

 effect on clay containing no carbonates of lime, magnesia, or alka- 

 lis; such clays accordingly can be burned with sulfurous coal with- 

 out any fear of white washing sulfates, while clay containing 

 carbonate of lime requires a fuel free from sulfur. 



Gerlach sums up the causes of efflorescences as follows. 



White effl.orescence 



Source 1 The green clay 



a Caused by the presence of sulfates in the clay 

 . h Caused by the formation of sulfates during the storage of the 

 clay 



