This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
169. When either the hand-made, soft-mud, or stiff-mud processes are used, the bricks, after drying, are built in a large mass, or kiln, containing from 100,000 to 300,000 bricks. Eyes, or flues, are left at the bottom as receptacles for fuel. The bricks are laid loosely together in order to allow the heat to pass in and around them. When ready, the fire is started, slowly at first, but afterwards increased to an intense heat; and after burning for a period determined partly by the fuel used, but mainly by experience, the fires are allowed to die out gradually.
On opening a brick kiln after burning, the quality of the brick therein contained may be divided into four classes: First, the extreme outside brick, which are burnt so little that they are almost worthless. Second, a layer inside the above, in which the brick are underburned and soft; these are called pale or salmon brick, and are unfit for foundation or face work, but are used for filling in between stud partitions, and sometimes between harder brick in the inside of walls, although their use for this purpose is not recommended. In the third layer of the mass forming the kiln, is found a class of brick well burned, hard, well shaped, and of a good red color. This kind of brick is good for any purpose. The brick in the fourth or inner layer of the kiln, just above the flues, are overburnt, very hard, very brittle, and usually distorted, cracked, and even vitrified; they should not be used in any structure subject to shock, but are often used for paving brick.