10 FARM BUILDINGS IN SOUTH AFRICA 



The presence of carbonate of lime in considerable quantity is indi- 

 cated by effervescence on the brick earth being treated with an acid. 



Different brick earths, on being burned, produce different colours ; 

 thus a pure sandy clay burns almost white, while the presence of 

 manganese is indicated by a brownish colour. Lime makes the bricks 

 whitish. The coloration produced by the presence of iron has already 

 been discussed. The presence, in the same brick earth, of two or 

 more of the above constituents, produces modifications of the charac- 

 teristic colours peculiar to each. The coloration is affected to a less 

 degree by the intensity of the heat to which the bricks have been 

 subjected, those near the fires being generally of a darker colour (and 

 harder) than those near the outside of the clamp, on account of the 

 greater vitrifaction caused in them. 



It is seldom that a really first-class brick earth is got from one 

 pit, but often a good result can be attained by mixing the earths 

 obtained from two pits. 



A good indication, as to whether or not a brick earth contains 

 a sufficient proportion of sand, is afforded by moulding a few trial 

 bricks and placing them in a shady place to dry. If they crack while 

 drying, the brick earth requires the addition of some sand. 



Mixing. — The brick earth, having been dug from the ground and 

 cleared of rubbish and stones, is well mixed or puddled with water, 

 either by a fag-mill, or in a small pit by natives wielding shovels, 

 until it becomes quite homogeneous, soft, and plastic. 



A pug-mill consists of a vertical cylinder containing fiat agitator 

 arms fixed to, and rotating with, a vertical shaft. The surfaces of 

 these arms are set obliquely, so that while mixing they also force the 

 brick earth downwards, causing it to stream slowly through a hole 

 in the side of the cylinder near its lower end. A small pug-mill is 

 usually driven by a horse, or mule, walking in a circular path. 



The admixture, during the process of puddling, of a small quantity 

 of coal dust, or fine screened cinders, will assist the subsequent 

 burning. The mixture, after having received its first puddling, is 

 allowed to stand for a few days, and is then re-puddled. 



Moulding. — This operation may be performed by hand in wooden 

 moulds, such as that illustrated by Fig. 2. On account of the shrink- 

 age which occurs, the dimensions of the mould must be considerably 

 greater than those which it is desired the finished brick should have. 



