CLAY WORKING INDUSTRIES. 79 



^Nearly all clays contain small amounts, probably united in the feld- 

 spathic minerals present; when the amounts of lime and magnesia are 

 larger they are most frequently present as carbonates. These carbonates 

 are not often present in the fire clays but they are a frequent and impor- 

 tant constituent of the shale beds of Ohio. The carbonates are some- 

 times finely divided and intimately mixed with every portion of the clay, 

 as in the slip clays used by potters for glazing, on account of their low 

 melting points, or are disseminated in grains and pebbles as in the clays 

 of glacial origin. When in this latter form the clay is useless, except for 

 the commonest and crudest purposes, as the particles of lime having be- 

 come caustic by heat, slack on exposure to the air again and mark the sur- 

 face of the ware with unsightly pit holes. Besides assisting materially in 

 lowering the melting point of the clay, these alkaline earths exercise an 

 important action on its color. The famous cream-colored brick clays of 

 our northwestern states contain twenty to thirty per cent, of lime and mag- 

 nesia and this is sufficient to disguise the presence of five per cent.of iron. 



In smaller quantities seven to twelve per cent, of the earths are suffi- 

 cient to make the vitrification of the clay occur at so low a heat that it still 

 retains the bright red color due to its iron and the strength that it is capa- 

 ble of giving. 



If the clays contain less than seven or eight per cent, of lime and 

 magnesia and the heat used in burning be high the clay generally becomes 

 a dirty green in color by the formation of lime and iron silicates. 



The alkalies, potash and soda are the most powerful fluxing agents 

 which clays contain. They are present, as has been explained, as feld- 

 spathic and micaceous minerals and are therefore ready to begin to melt 

 as soon as the temperature rises. Free oxide of iron is the least obnox- 

 ious state in which iron can be present in a clay, since it requires more 

 temperature to get it to unite with the minerals around it than a pre- 

 viously formed silicate of iron would require. So also the potash and 

 soda are in condition to do the most harm to the melting point of the clay, 

 being already combined as silicates. 



The actual amount of any of these active fluxes, which a clay can con- 

 tain and retain its fire qualities is impossible to say. The result of these 

 fluxes depend on the amount, on the state of division of the clay, and the 

 state of division of the fluxes, and on the character of the silica with 

 which they are associated, and also largely on the number of fluxes rep- 

 resented. It is a general rule in the theory of fluxing compounds that a 

 mixture of bases will make a more fusible compound than an equal 

 amount of any of the bases separately. 



(d) The Changes Occurring in Clay by Burning, In forming a concep- 

 tion of the qualities that a clay will develop on burning, its composition, 

 considered as to its oxygen ratio and as to the probable effect of each sep- 

 arate impurity, is seen to be of the greatest importance. 



