BRICKWORK. 923 



are rendered useless by reason of the excess or imjiroper 

 incorporation of certain substances. Good brick-eartli is 

 seldom found in a natural state fit for the jjurpose of making- 

 a high-class brick without the addition or extraction of certain 

 constituents. Even where brick-earth does exist of the 

 requii'ed balance for making sound ordinary bricks, such 

 material is not likely to produce an article suitable for facings 

 of the required finish and tone. 



A good brick-earth should contain from ^ to ^ alumina, 

 ^ to j- silica, and the rest of its bulk should consist of carbonate 

 of hme, carbonate of magnesia, and oxide of iron, besides 

 which there will necessarily be a small percentage of other 

 mineral substances, and perhajjs of organic matter. The 

 above proportions are not likely to make the bricks vitrify to 

 such an extent that they will run together, yet they contain 

 sufficient flux to fuse the various constituents at furnace heat. 



If a clay is too strong — that is, almost wholly composed 

 of silica and alumina, without a sufficient proportion of lime, 

 magnesia, soda, or other salts — then such clays can be 

 improved by the addition of sand, or loam and lime, to act as 

 a flux, or ashes to provide alkalies for the same purpose. 



If a clay is too loamy or sandy, then it is absolutely necessary 

 that great attention should be paid to providing the necessary 

 flux, or the particles will not properly combine. 



Marls are the best of all natural clays for brick-making 

 purposes, though they are often improved by the addition of 

 frequently deficient substances. 



Many mineral substances besides those already mentioned 

 frequently enter into the composition of brick-earths, some 

 improving and others depreciating the value of the material ; 

 and although chemical analysis may be of great service in 

 guiding the brick-making experimentor, it must be borne in 

 mind that nothing short of jjractically making and burning 

 a samjjle brick will be a satisfactory test, as frequently the 

 substances are in combination with one another when they 

 would be better separate, or in some other chemical state 

 which renders them of little or no service for practical 

 purposes. 



The tint of a brick may be much aftected by the different 

 processes of manufacture, but the presence of such substances 

 as lime, oxide of iron, manganese, alkalies, &c, are the greatest 

 coloring factors in brick-earths. In order that a brick building 

 may have a true artistic finish it is absolutely necessary that 

 the facings should be of a pleasing tone, yet, as I have already 



