CLAY WORKING INDUSTRIES. 7i 



in little grains or crystals it has but small effect on the color of the clay ; 

 the more intimate the mixture or combination the greater its effect. 



The amounts of iron found in clays vary from minute fractions of 

 one per cent, to twelve or even fifteen per cent, of the whole. 



Ivime and magnesia are found in small but persistent amounts in 

 nearly all clays. These alkaline earths are probably present in many 

 cases as silicates, as they are insoluble in water and nearly so in acids — 

 but many clays contain notable amounts of carbonates of lime and mag- 

 nesia. These carbonates are sometimes present as small pebbles or grains 

 of limestone, brought into their present position by the glacial agencies of 

 recent geological time. In this condition they do very great harm to the 

 clay in manufacturing processes. When intimately mixed with the whole 

 substance of the clay their effect is less apparent. Clays of this descrip- 

 tion generally burn to a greenish color, or even cream colored, if the lime 

 is present in large amounts. The cream colored bricks which are found 

 so commonly in Canada, northern Michigan, Wisconsin and Minnesota 

 are manufactured from this class of clays. 



Titanic acid is found in nearly all clays. Its presence was viewed 

 with much interest for some time, and is attributed to the mineral menac- 

 canite or titanate of iron, but titanic acid is so nearly chemically inert 

 that it is not commonly determined in an analysis. It is weighed in an 

 ordinary analysis partly with the silica and partly with the alumina. It 

 is not known to have an}- effect on either color or fusibility. 



Organic matter is a frequent but not an important constituent of 

 clays. In many of the Ohio clays and shales large quantities of decom- 

 posing carbonaceous matter was in suspension in the water which was 

 depositing the beds of clays; and as a result the shales or clays are col- 

 ored bluish or black. When a clay is colored by this cause it can readily 

 be detected, as a comparatively low heat is sufficient to drive out the 

 organic matter, leaving the clay to be colored by its mineral constituents. 

 It is not often that organic matter is a source of any detriment to a clay; 

 it is generally so finely divided or present in such volatile organic com- 

 pounds that it leaves the clay sound and solid after the gases are expelled 

 by heat. When it occurs as small lignitic grains, as it sometimes does in 

 the clays of later geological age, it is a possible source of porosity after 

 burning. In some parts of the Huron shale, there is so much organic 

 matter that it greatly facilitates the burning of the clay, just as some 

 black band iron ores contain enough organic matter to aid in their own 

 calcination. 



Rare minerals containing barium, strontium, lithium, cobalt, copper, 

 zinc and such bases, and phosphoric, sulphuric, and hydrochloric acids 

 are liable to be occasionally found in clays, but are not frequent enough 

 to be of any real importance in practical work. 



Thus we have seen that mineralogically clay is a compound of a clay 

 base with sand, feldspar, mica, and other silicates, colored by iron oxides, 

 or organic matter. 



