CLAY. 371 



and forms a paste which is not plastic. A variety consists 

 of silica 44*0, alumina 23*1, lime 4*1, magnesia 2*0, pro- 

 toxyd of iron 2-0. Gr = 2'45. 



Lithomarge is a compact clay of a fine smooth texture, 

 and very sectile. Its colors are white, grayish, bluish-white, 

 reddish- white, or ocher-yellow, with a shining streak. Gr= 

 2*4 — 2*5. The tuesite of Thomson, a white lithomarge from 

 the banks of the Tweed, is said to make good slate pencils. 



Clay for bricks is the most ordinary kind ; it should have 

 slight plasticity when moistened, and a fine even character 

 without pebbles. It ordinarily contains some hydrated oxyd 

 of iron, which when heated turns red by the escape of the 

 water in its composition, which reduces it to the red oxyd of 

 iron, and gives the usual red color to the brick. It also fre- 

 quently contains lime ; but much lime is injurious, as it 

 renders the brick fusible. A clay is extensively employed 

 at Milwaukie, in Michigan, which contains no iron, and 

 produces a very handsome cream-colored brick. About 

 9,000,000 of this kind of brick were made at that place in 

 1847. 



In making bricks, the clay is first well worked by the tread- 

 ing of cattle or by machinery : after this, it is moulded in 

 moulds of the requisite size, (9f- inches, by 4$ and 2|,) and 

 then taken out and laid on the ground. A good workman 

 will make by hand 5000 in a day, and the best 10,000. 

 After drying till stiff enough to bear handling, the bricks are 

 trimmed off with a knife when requiring it, and piled up in 

 long walls for farther drying. They are then made into a 

 kiln by piling them in an open manner, (so that the flame 

 and heated draft may have passage among them,) and leav- 

 ing places beneath for the fires. The heat is continued 48 

 hours or more. 



The best brick are pressed in moulds. They have a 

 smooth, hard surface. Near Baltimore, Md., bricks are thus 

 made by a machine, worked by a single horse, which will 

 mould 30,000 bricks in 12 hours ; the bricks are dry enough 

 when first taken from the mould for immediate burning. 



Burnt bricks were not used in England before the elev- 

 enth century, when they were employed in the construction 

 of the abbey of St. Albans. But they date historically as 

 far back as the city of Babylon. Unburnt bricks have also 

 been used in all ages. Those of Egyptian and Babylonish 

 times were made of worked clav mixed with chopped straw. 



