8 



deformation and flow will be other things equal the safest and easiest 

 clay to burn. 



We have now to consider briefly the elements which actually deter- 

 mine the fusibility of clay. Quartz is an oxid of silicon formed by the 

 association of two atoms of oxygen, with one atom of the element sili- 

 con to form the oxid silica, which is expressed by chemists by the 

 symbol SiO'2. Silicon is one of a group of elements which alw r ays form 

 acid oxids, so called because if heated with the proper amount of certain 

 alkaline or basic oxids like lime, which is an oxid of calcium (CaO), 

 they will combine to form fusible slags which are neutral, showing neither 

 an acid nor an alkaline character. Silica has a very high melting point, 

 and lime an even higher one, so high in fact that it withstands the extreme 

 temperatures of the electric arc. If, however, the two oxids be mixed 

 together the fusing point is lowered. Thus in clays the point of fusion 

 depends upon the amount of fluxing agents present, such as the basic 

 substances, iron oxid, soda, potash, lime, and magnesia; and, although 

 fineness of grain is also an element in determining fusibility, the point 

 of fusion will usually fall with the percentage of these basic substances 

 present. For this reason the common red brick clays which get their 

 color from iron oxid are, as a rule, of low fusing point. 



Color. The color of a clay before it is burned is of little importance 

 except in so far as it serves to indicate the presence of iron. .In the 

 kiln, however, except in perfectly white clays which burn white, color 

 changes invariably take place. Many black clays stained by organic 

 matter burn white, while others burn to reds, buffs and cream colors. 

 Nothing of importance has ever been accomplished technically in caus- 

 ing a dark burning clay to burn white. It is known that an increase 

 in the amount of lime will produce a paler burn if the color is due to 

 iron, but, on the other hand, such an admixture has certain deleterious 

 effects, such as lowering the fusing point unequally. 



ABSORPTIVENESS. 



The power that some days have to rapidly absorb water and other 

 substances, has already been discussed. This property is frequently 

 made use of in the arts. In addition to this action certain unctuous 

 varieties of clay which will not usually adhere to the tongue have the 

 power of removing coloring matters from solutions by actually adhering 

 to the fine particles of matter held in suspension and " sweeping " them 

 away. These clays are known as fullers earth and are much used as 

 clarifying and bleaching agents. They are usually of a grayish-green 

 color very soft and greasy to the touch and if placed on the tongue they 

 appear to melt away. 



PURIFICATION AND TREATMENT OF CLAYS. 



In ordinary brickmaking the clays are not usually given much pre- 

 liminary treatment, as it would increase the cost of manufacture too 



