Bricks made by the soft mud process always have to be dried before placing in the kiln; those made by the stiff mud process are generally, although not always, stacked in a dry house from twelve to twenty-four hours. The drying of the bricks is an important process, and where bricks are manufactured on a large scale the drying is generally accomplished by artificial means.
After being sufficiently dried the bricks are stacked in a kiln and burned.
Three styles of kilns are used for burning bricks, viz.: Up-draft down-draft and continuous.
Up-draft Kiln. - This is the style of kiln that was almost universally used in this country for burning bricks previous to 1870, and is still used more than either of the other kilns, especially in small yards where the bricks are manufactured by hand.
The old-fashioned up-draft kiln is nothing but the bricks themselves built into a pile about 20 to 30 feet wide and 30 to 40 feet long, and perhaps 12 or 15 feet high. The sides and ends of the piles are plastered with mud to keep in the heat, and the top is generally covered with dirt and sometimes protected with a shed roof.
The bricks are piled in such a way as to form a row of arched openings extending entirely across the kiln, and in these arches the fire is built. The dried bricks are loosely piled above these arches, and as the kiln is burnt those nearest the fire are so intensely heated as to become vitrified, while those at the top of the kiln are but slightly burned, with a gradual gradation of hardness between them. It is from this difference in the burning that the terms "arch brick," "red brick" and "salmon brick" originated. As there is nothing but the natural tendency of heated air to rise to produce a draft, its direction is of course upward, hence the name.
The modern up-draft kiln has permanent sides made of a 12 or 16-inch brick wall laid in mortar, and heat is generated in ovens with iron grates built outside of the permanent walls, and only flames and heat enter the kiln through fire passages in the walls connecting the furnaces with the kiln proper. The top of the kiln is also paved with smooth, hard bricks, laid so as to form a close cover that can be opened or closed as desired. The bricks are piled in the same way as described above, the arches being left opposite the furnaces. With these improvements the bricks can be much more evenly burned and with a less consumption of fuel. The burning of a kiln of brick requires about a week. After the fires have been burning a sufficient length of time they are permitted to go out, and all the outside openings tightly closed to keep out the cold air, and thus allow the bricks to cool gradually. It requires much skill and practice to burn a kiln of bricks successfully.