This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
I have received many valuable ideas from the pages of the Gardener's Monthly during the past few years, and by way of return give an idea that I have put in practice and found to work well in my own case, but have not noticed elsewhere, though it may not be new to others. It is to use ordinary roofing slate for the bottom of greenhouse flues. They can be had anywhere, nine or ten inches wide by eighteen or twenty inches in length, and are cheaper and better than brick to use in long horizontal fines, occupying less space; and the slate I use only requires to be supported twenty inches apart instead of the eight inches that brick require. As these flues are always better on, or partly buried in the earth, for a short distance from the furnace, to modify the excess of heat by lessening the radiating surface, no slates are needed so close to a furnace that there is any danger of their cracking by the heat when they are covered with a coating of mortar in which the side bricks are imbedded. From the nature of things, the top of a flue is always hotter than the bottom, and I have never thought it safe to try to use them on top, where they are more liable to breakage from other causes; but would have no hesitation in covering the flue also at a distance of forty to fifty feet from the furnace.
They will absorb and radiate nicely, but we know that a blaze or extreme heat will cause them to fly to pieces. The slates I use each have a surface equal to five, and a part of them of six bricks. These are supported on stone at their junctures, and leave a clear space under the flue for free circulation of air, and this is an additional gain.
In choosing slate for this use, I do not pick out very thin ones, but with a little care in first laying the flue there is no weight to break them, the brick being nearly self-supporting when mortar is dry. With this foundation it is easy to build these horizontal chimneys five inches in inside diameter, and thus insure a good draft.
There are special tiles made for this use, but they have no advantage that I know of over slate, and are not to be had every where.