BURNING.
The high price of wood in and near large cities makes it as a rule entirely out of the question to use it generally for burning brick in such localities.
The process of burning by coal is the one that we shall de scribe ; the principle is the same as for wood, the agents em ployed to produce the heat being the only difference.
Brick-kilns requiring wood for fuel are flat in the bottom and paved with brick ;• coal-kilns have part of this pavement cut away under the portion which is to form the arch of the kiln and the place filled with grates, and under each of the grates there is a trench dug all the way through the kiln, called the " ash-pit." A space at each side of the kiln, is dug out to the depth of the ash-pits, the top covered with a slanting shed, and the space is called the " kiln shelter," and serves as shelter for the laborers and fuel while the kiln of brick is being burned. Before fire is placed in the coal-kiln the ashes made in burning the previous kiln of brick are drawn out of the pits into the kiln shelter, thrown into wheelbarrows and carried out of the way, and after fire is started in the kiln the ashes are drawn each day.
The roof over the kiln is next examined to see that it is not leaky, and then every alternate brick which was laid flat, and called the " burnt platting," is stood upon its end, this being done in order to allow the steam, or as it is called in burning, the " water smoke," to escape as rapidly as possible.
The platting having been raised, the next step is to start a small fire in the mouth of each arch, using light splintered wood, and building it up with lumps of coal ; the fire should be started on the side of the kiln that will allow the smoke to be blown by any wind entirely through the arches of the kiln.
After the fires have been started in the mouth of all the arches on the windward side of the kiln, they are next made in the mouth of the arches on the opposite side.
The fires are built up gradually from each side until they meet in the centre of the kiln, and this is called " crossing the fires."
The fires should be " crossed" much more slowly for dry or damp clay machine-made brick than for hand-made brick.
When brick produced by the hand process are well dried, and there is no dampness in the bottom of the kiln or in the ash-pits, the fires can be crossed in forty-eight hours ; but for machine-made brick they should never be crossed inside of seventy-two hours.
It should be noticed that the steam, or " water smoke," is freely coming out of the top of the kiln from the time that fire is put into it.
The fires are gradually increased until the fifth day, or say, in other words, the one hundred and twentieth hour after set ting fire ; by this time the "water smoke" or steam from the top of the kiln should have changed from a white, watery, into a bluish black smoke, and the fire should in the night-time be seen plainly coming through the top.
At this period the kiln is said to be " hot," and the brick are now ready to shrink, or as it is termed in burning, to " settle," and all the platting is put down and tightened. Care must, to this point, have been observed to increase by degrees the heat, the firing having been gradually reduced from four hours to about two hours between fires at this stage.
The fires that the brick are now to receive are the most in tense and the heaviest that will be applied to them ; the oxide of iron is now to be converted into peroxide, or, as the men around the kiln would call it, "the brick are to be painted red." Before these fires are given, a long iron rod, a little longer than one-half the width of the kiln, having a flat, nearly circular piece at the one end, open in the centre, and having an iron handle at the other end, as shown in Fig. 5, is run on top of the grates and under the fires to loosen them.
The instrument is called a " moon," and its object is to enliven the fires and to get rid of the ashes, as well as to break up the clinkers.