

BUILDING MATERIALS. 251 



Frost is one of the most destructive agents on stone, and especially 

 with stone which imbibes moisture. 



4. Bricks. These are formed by submitting common clay to a heat 

 sufficient to convert it into a solid mass like stone ; and when bricks are 

 carefully made, they have great strength and durability, almost equal to 

 stone. 



The best clay for the making of bricks, tiles, &c., is that composed of 

 pure clay and sharp sand, and should be perfectly free from small stones 

 of any kind. Clay is not always found existing with a proper admixture 

 of sand for brick-making; and when there is a deficiency of sand, it 

 should be added in proper quantity, to ascertain which, experiments 

 should be tried with the clay, mixed with different proportions of sand. 



Machinery is now coming into use for pressing and moulding the clay. 

 This causes a great saving of labour ; and from the density given to bricks 

 by the pressing, they are much superior in quality. Great attention is 

 required in the drying, burning, and cooling of the bricks, to get them of 

 good quality. First-class bricks give a clear ringing sound when struck, 

 and are of a red and brown colour, and exhibit a fine texture when 

 broken. Bricks which are of a pale colour are not of good quality. 

 Thin bricks are usually the best, as they are burned more equally ; but 

 generally they are made about nine inches long, four and a half inches 

 wide, and two and a quarter inches thick. 



" In estimating the quantity of brickwork, ascertain by calculation the 

 number of cubic feet in the wall ; reduce the number of these cubic feet 

 so ascertained to the standard of brick and half, by multiplying them 

 by eight and dividing by nine ; or find the area of the face of wall in 

 feet, multiply the number of feet thus found by the number of half-bricks 

 in the thickness of the wall ; divide the result by three, which will give 

 the superficial feet ; divide the amount by two hundred and seventy-two, 

 and the result is the number of ' rods ' of the standard thickness." * 



Where bricks have to be used for hydraulic purposes, they are found 

 to be much improved in durability by having been sunk in water for 

 some days, and then burned again. 



Firebrick is that which is used in building fireplaces. It is made of 

 a kind of clay which remains uninjured by strong heat. 



One rod of a brick-and-half wall will take four thousand five hundred 

 bricks of the sizes which I have stated as being in general use ; one thou- 

 sand of the same kind of bricks will weigh two and a quarter tons. 



5. Slates. Quarries of roofing-slate are found in England, Scotland, 

 and Ireland. The best are procured from Caernarvonshire in Wales, and 

 good slates are also found near Valentia in Ireland, and Ballahulish and 



* ' Book of Farm-Buildings.' 



