OUTER WALLS OF STABLES. 219 



In some districts the brick earth contains clay and sand in 

 proportions which are suited to good brick making, and is 

 practically free from undesirable constituents. " All brick 

 clays contain iron, and the colour of a burnt brick almost 

 entirely depends on the amount of it which is present. Thus, 

 clay containing less than I or I j per cent, of iron, changes in 

 the kiln to various shades of cream colour and buff, whilst 

 those containing more than 2 per cent, range in colour from 

 yellowish-fawn to dark red. Blue bricks are made from the 

 same clay as the red, by controlling in a peculiar way the 

 supply of air in firing, and by carrying the heat somewhat 

 further. It is asserted by some that the red is changed to 

 the black oxide of iron in the process" (Chambers' Ency- 

 clopedia), The dark blue or purple Staffordshire bricks are 

 a well-known example of good " blue bricks." If on breaking 

 a blue brick, we find that the internal surfaces of the fractured 

 parts are of comparatively light colour, we may conclude that 

 the brick is of inferior quality. 



Tiles, earthen drainage pipes, and the various varieties of 

 paving bricks are all made in the same way. Although the 

 respective powers of absorption for water of different kinds 

 of bricks and tiles vary within wide limits, no kind of 

 unglazed brick or tile is absolutely waterproof. 



The quality of the bricks used, and the amount of exposure 

 (as at the seaside) to damp and rain which the building 

 may have to endure, will naturally have to be taken into 

 consideration in determining the thickness and mode of 

 construction of the walls. The best arrangement for a brick 

 wall is to have it double ; the exterior wall being 9 in. thick 

 and the interior wall 4^ in., with a 2 in. hollow space 

 between them. This is commonly called a "14 in. wall." 

 An ordinary brick is 9 in. long, 4^ in. wide, and 3 in. 

 thick. Or an " 1 1 inch wall " may be constructed with the 

 exterior and interior walls each 4^ in. thick and a 2 in. 



