CLAY WORKING INDUSTRIES. 223 



The supply of clays that are fit to make the highest grade bonds is 

 actually more scarce and difficult to obtain than good, fire resisting, flint 

 clay. 



The compound of the mixture for producing refractory brick depends 

 on the following considerations : 



1st. The source of fire resisting power is mainly in the flint clay. 



2d. The source of physical strength when ready for use depends on 

 the plastic bond; when the bricks are in position and under a high heat, 

 the flint clay becomes softened and cohesive and it then furnishes 

 the real strength of the brick, but at these high temperatures there is 

 no danger of friction from material as highly heated as the bricks them- 

 selves. 



3d. To allow of rapid change or alternations of high and low tempera- 

 tures, the bricks must be of a loose, open grained structure, so as to admit 

 of rapid absorption or radiation of heat. 



4th. To produce a sound and strong brick composed of non-plastic 

 material which has a very high shrinkage and a plastic bond, it is neces- 

 sary, first, that the bond clay should be of very low shrinkage and 

 second, that part of the flint clay should be calcined p«evious.to mix- 

 ture, so as to take out the shrinkage and allow the union of the particles 

 to remain unbroken when burned. It is impossible to make a strong and 

 good fire-brick from a mixture of dissimilar clays of high shrinkage, for 

 one kind invariably tends to separate from the other when under heat, 

 and the bond is therefore destroyed. It is to this fact that the failure 

 of all attempts to make a good refractory material out of pure quartz; and 

 pure kaolin is due. Both are infusible, but when mixed and burned 

 together, the kaolin bond shrinks away from the grains of quartz, and 

 the whole structure becomes perfectly worthless and rotten. No satis- 

 factory refractory material was made from quartz until the use of clay as 

 a bond was given up, and milk of lime in small quantities was substituted. 



On these four conditions, the mixtures for refractory clay bricks 

 depends. 



It will readily be seen that these conditions are to some extent ini- 

 mical to each other, and to secure a brick of the highest fire qualities, it 

 is necessary to sacrifice its physical strength to a large degree. 



A common mixture for this purpose consists of 45 per cent, calcined 

 flint clay, 45 per cent, raw flint clay and 10 per cent plastic bond. A 

 brick made of such a mixture is very loose and porous in structure, very 

 friable and easily worn away by friction, even when hard burned. To 

 use such a brick to advantage it must be placed where the heat is so 

 intense that the flint clays become slightly soft and cohesive and the sub- 

 stance of the brick becomes plastic by the influence of heat. Therefore 

 to use such a brick where the heat is only moderate and where it would 

 be subjected to abrasion of stock or tools, would be a waste of money, 

 for a brick of lower fire qualities would stand the heat equally well 

 and the friction very much better. 



