The bricks taken from a clamp will be found very unequal in quality. Those from near the eyes are often fused together, or misshapen, forming burrs. Those near the outside are underburnt and soft, and are called Place bricks.
Again, much depends upon the proportion of breeze used in the clamp. Too much will cause the bricks to be weak and porous.
1 In some clamps a third layer of breeze some 2 or 3 inches thick is introduced between o and the mass of raw bricks above it.
Illustrations of Parts of a Clamp.
Fig. 6. Section of part of Clamp on line C D.
Fig. 7. Broken Section on line A B. Burnt Brick shown thus.
Mixed with the clay - 1/2 chaldron; in the clamp - 1/4 chaldron. Besides about 2 to 3 cwts. of coal in each fire-hole, that is about 1/2 cwt. per 1000 bricks.
There are several descriptions of kilns used for burning bricks, but it will only be necessary to refer to those that are likely to be used by the engineer or builder in establishing a temporary brickfield to supply bricks for special works in progress.
Several forms of kiln, used chiefly in permanent brickmaking yards, may be excluded, or very lightly touched upon, as being interesting to the brick-manufacturer rather than to the engineer.
Scotch Kiln.1 - The form of kiln most commonly used in the
Fig. 8. Elevation.
Fig. 9. Ground-plan.
United Kingdom for making a moderate supply of bricks is known as the Scotch kiln.
The Scotch kiln is a rough rectangular building, open at the top, and having wide doorways at the ends. The side walls are built of old bricks set in clay, and in them are several openings called fire-holes, or "eyes," (e e, Fig. 9), built in firebricks and fireclay, opposite one another.
The dried raw bricks are arranged in the kilns so as to form flues connecting the fire-holes or eyes, and they are packed so as to leave spaces between the bricks from bottom to top, through which the fire can find its way to and around every brick.
Fig. 10. Cross Section.
1 Sometimes called the Dutch Kiln.
After the dried bricks are "crowded" i.e. filled into the kiln, the ends are built up, and plastered over with clay.
At first the fires are kept low, simply to drive off the moisture.
After about three days the steam ceases to rise; the fires are allowed to burn up briskly; the draught is regulated by partially stopping the fire-holes with clay, and by covering the top of the kiln with old bricks, boards, or earth, so as to keep in the heat.
In from 48 to 60 hours the bricks will be sufficiently burnt, and they will be found to have settled down.
The fire-holes are then completely stopped with clay, all air excluded, and the kiln is allowed to cool very gradually.