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Brick earth is originally a wind-blown loess dust deposited under extremely cold, dry conditions that can be used for making house bricks. The Brickearth is normally represeneted on 1:50,000 solid and drift edition geological maps. In the Thames Valley area, where the Brickearth overlies certain River Terrace Gravels, this has been reclassified on more recent maps as the "Langley Silt Complex".
It is a superficial deposit of homeogenous structureless loam or silt. It requires little or no admixture of other materials to render them suitable for the manufacture of 'stock bricks'. Brickearth typically occurs in discontinuous spreads, about 2m to 4m thick, overlying chalk, Thanet Beds or London Clay. There are extensive brickearth deposits in Kent, particularly on the North Downs dip slope and on the Hoo peninsula, sections of the Medway and Stour valleys. The mineral content of brickearth is critical for brickmaking and precise proportions of chalk, clay, and iron.
Following are the constituents of good brick earth
1 . Alumina: It is the chief constituent of every kind of clay. A good brick earth should contain about 20% ti 30% of alumina. This constituent imparts plasiticity to the earth so that it can be moulded. If alumina is present in excess, with inadequate quantity of sand, the raw bricks shrink and warp drying and burning and become to0o hard when burnt
2. Silica: It exists in clay either as free or combined. As free sand, it is mechanically mixed with clay and in combined form, it exists in chemical composition with alumina. A good brick earth should contain about 50% to 60% of silica. The presence of this constituent prevents cracking, shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks. The durability of bricks depends on the proper proportion of silica in brick earth. The excess of silica destroys the cohension between particles and the bricks become brittle.
2. Lime: A small quantity of lime not exceeding 5 % is desirable in good brick earth. It should be present in a very finely powered state because even small particles of the size of a pin head cause flaking on the bricks. The lime prevents shrikage of raw bricks. The sand alone is infusible. But it slightly fuses at kiln temperature in presence if lime. Such fused sand works as a hard cementing material for brick particles. The excess of lime causes the brick to melt and hence it shape is lost. The lumps of lime are converted into quick lime after burning and this quick lime slakes and expands in presence of moisture. Such an action results in splitting of bricks into pieces.
4. Oxice of iron: A small quantity of oxide of iron to the extent of about 5 to 6 [prcent is desirable in good brick earth. It helps as lime to duse sand. It also imparts red color to the bricks. The excess of oxicde of iron makes the bricks dark blue or blackish. If, on the other hand, the quantity of iron oxide is comparatively lee, the bricks will be yellowish in color.
5. Magnesia: A small quantity of magnesia in brick earth imparts yellow tint to the bricks and decreases shrinkage. But excess of magnesis leads to the decay of bricks.