CLAYS OF NEW YOKK 681 



fore according to Dr Gerlach. tlie main cause of sulfates, wliicli 

 give rise to " white wash ". Sulfates may also come from the sulfur 

 contained in the water used in the tempering of the clay, such 

 waters often containing gypsum, and, as many clays often require 

 30^ or perhaps more of water to render them plastic, it is easily 

 seen that the clay may receive a large addition of lime sulfate. 

 This sulfate might be present in the mineral coloring matter added 

 to the bricks. Rapid drying causes the water to evaporate more 

 quickly and a lesser amount of the dissolved sulfates is apt to be 

 brought to the surface of the ware. 



2 Sulfates arising during water-smoking and burning. In the 



water-smoking of a kiln those bricks nearest the fireplaces will lose 

 their moisture first, and before the bricks farthest from the fireplace 

 are heated to a temperature sufiicient to convert their naoisture into 

 steam; therefore much of the watery vapor driven off from the 

 bricks which were heated first will be deposited on the surface of 

 those farthest from the fireplace, and be absorbed by them to a 

 certain extent. If it happens that these green bricks contain soluble 

 sulfates, the deposition of this condensed vapor on them will tend to 

 increase the sulfates in solution, and when their water is driven off 

 all the sulfates will be carried to the surface in solution and de- 

 posited there. This condensation of the waiter will be harmless, if 

 the clay contains no soluble sulfates or if the contained soluble 

 sulfates have been previously rendered insoluble by the addition of 

 the proper chemicals. Another source of difficulty may coaue from 

 the use of sulfurous fuel, for it is known that many coals contain 

 more or less iron pyrite. This sulfurous acid gas in passing through 

 the kiln will only too willingly attack carbonates present in the clay 

 and fomi sulfurous salts, which as the heat of the kiln increases, 

 come to the surface, and are there oxidized to sulfuric salts or sul- 

 fates, these causing efflorescence or discoloration. 



Efflorescences formed on burned ware. It not infrequently hap- 

 pens that clay products come from the kiln apparently free from 

 any superficial discoloration and later develop one when subjected 



