BUILDING CONSTRUCTION 123 



and most durable of all bricks made, and are consequently used for 

 engineering purposes such as the piers of railway bridges, the lining 

 of railway tunnels, and other works which have to sustain heavy 

 stresses. 



They are sometimes designated as " Staffordshire Blue Metallic 

 Bricks," and are recognisable by their deep blue-black colour and 

 smooth metal-like face. They are useful for the jambs of gate- 

 ways, stable doors, dadoes of courtyards and other situations likely 

 to receive rough usage. 



Staffordshire blue bricks specially made for paving stables are 

 much in use. They are practically non-porous, extremely hard and 

 very durable, but, unfortunately, they are slippery. To overcome 

 that they are usually grooved in process of manufacture, although 

 the grooving gives rise to another fault, viz., a lodgment for dirt 

 and moisture. When these bricks are used the groove should be 

 not less than inch wide, not more than inch deep, and should 

 run with the long axis of the brick. 



Dutch Clinkers. Dutch clinkers are only used for paving. They 

 are very hard, well-burned bricks, vitrified throughout, and 

 occasionally warped in the kiln. Their size is about 6 inches by 

 3 inches by 1| inches thick, and the colour bright buff, although 

 the addition of oxide of iron will yield a black paving brick. They 

 are chamfered on the edges, thus forming a grooved floor to resist 

 slipping (see section on Flooring). 



Perforated Bricks. Perforated bricks are ordinary building 

 bricks made with a number of perforations running from bed to bed. 

 They are frequently used in the south of England and London for 

 lightening the work, but are rarely used in Scotland. 



Hard pressed ventilating bricks or air bricks are sometimes used 

 instead of gratings. They may be had in a great variety of patterns 

 (see Ventilation). 



Firebricks. Firebricks are made to resist heat in furnaces with- 

 out melting or spaling, therefore they do not contain iron and very 

 little lime or salt, any of which would act as a flux under great 

 heat. The best fireclay for firebricks contains about 97 per cent, 

 of silica or free sand, to which 1 per cent, of lime is added to bind 

 the material in moulding. Ordinary bricks contain about 50 per 

 cent, of sand, the remaining components being alumina, lime and 

 iron. 



Figure 49 illustrates the various shapes in which bricks are 

 obtainable. 



TILES. Ordinary roofing tiles, usually red in colour, are made 

 from a good plastic clay. The flat plain tiles, much used in England 



