This section is from the book "The American Garden Vol. XI", by L. H. Bailey. Also available from Amazon: American Horticultural Society A to Z Encyclopedia of Garden Plants.
The next most important point is the size of the coal. In the writer's experience two sizes of coal are best, one of the usual "furnace" size and one of "nut" size or "small stove" size. In starting the fire see that the ash pit is clean. It never should be otherwise. All ashes should be removed as fast as they gather. Many a grate has been ruined by allowing ashes to bank up under the bars. Start the wood fire with light kindling and when well started add hard wood, and on this, when well started, place enough furnace coal to just cover it. Give it all the draft until the coal is about half lighted. Then add a little more (say one good shovelful). When this is burning briskly with blue flames on top, add one shovelful of the fine coal. Keep up the draft until this is beginning to burn and then add more fine coal, enough to completely cover the fire. Keep on the full draft till this coal is fully on fire with yellow flames only. . Now close the draft completely and this mass of coal will burn steadily with a powerful heat for from four to ten hours without further attention, according to the weather.
Such a method of starting a furnace fire takes an hour or more, but once done the fire keeps in good effective condition for many hours. The exact time depends wholly on the state of the clouds and the force and direction of the wind. With a clear sky and no wind it will burn from eight to twenty-four hours without attention and give a moderate amount of heat for the whole time. With a high wind, particularly a north wind, it will burn out in four hours or even less time. The economic use of the coal in the furnace, therefore, depends wholly on matters out of doors and quite beyond our control. The stoker's art consists in governing the draft according to the condition of the weather. With a cloudy sky by day or night without wind the fire requires more draft than with a clear sky without wind. If on a cloudy day the sky clears, the fire will at once burn faster and to save coal the draft must be reduced. A wind always quickens a fire and the higher the wind the greater the care needed in reducing the draft. A warm southerly wind does not urge the fire so much as a dry northerly or westerly wind. Rain without wind requires plenty of draft.
Rain with high winds means less draft but not so much as high winds without rain. . The point is just here : So many pounds of coal are in the furnace ; it will burn fast or slow according to these conditions of the weather. Economy in the consumption of this coal is governed entirely by the amount of air supplied for combustion. This means the proper regulation of the ash pit door and the damper in the smoke pipe. In the writer's experience it is far better to control the fire by controlling the amount of air supplied to the fire than to retard or reduce the volume of the products of combustion in the chimney. Burning is a chemical process depending on the supply of oxygen. Cut off the air (or oxygen) and the process is slow and the fuel is burned with economy It is plain that stoking is an art, and the true economy of fuel in greenhouses is in the skill, good judgment and common sense of the stoker.
After a fire has been started in the manner described it will burn from four to eight hours without attention.
It will burn very much longer, but if the fire is started in the morning it needs attention again in the evening in order to keep up a good fire all night. The writer's plan is to open the drafts about nine o'clock in the evening and let the fire burn up brightly for from 15 to 30 minutes (according to the weather), and then to add a little fine coal. When this is fairly lighted the grate is well raked out to clear away the ashes. This must not be done until the first coal is lighted, because by this time the old coal is so nearly burned out that it is easily put oat if disturbed. As soon as the fresh coal is burning briskly more fine coal is added and then all the sifted coal that has accumulated during the day is put on the fire, covering it over quite dark. In a moment blue flames appear and in from 15 to 30 minutes, according to the weather, all the coal will be a glowing mass of yellow fire. If it is red or white, the draft has been on too long. A little experience will show how long to leave the full draft, but the fire should never be allowed to reach the white hot stage. Such a fire will last from ten to thirty-six hours with a slow draft in quiet weather and be sufficiently alive to start a fresh fire at the end of that time. The average time is, however, about ten hours.
The treatment of the fire next morning is very simple. Give full draft till the fire burns up brightly; add a little fine coal. When alight, rake out ashes and add large coal. When this is started fill up with small coal. About once in ten days the fire should be allowed to go out in order to give the furnace a complete cleaning out. The writer's experience has been confined to hard coal. Soft coal burns faster than anthracite, and while it differs in this respect, the actual work of stoking bituminous coals is just as much of an art, and requires the same skill, judgment and observation in governing the fire according to the conditions of the sky and the direction and force of the wind.
 
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