Although slate is not strictly a building stone, yet it is largely used for covering the roofs of buildings, for blackboards, sanitary purposes, etc., and the architect should be familiar with its qualities and characteristics.
The ordinary slate used for roofing and other purposes is a compact and more or less metamorphosed siliceous clay. Slate stones originated as deposits of fine silt on ancient sea bottoms, which in the course of time became covered with thousands of feet of other materials and finally turned into stone.
"The valuable constituents in slate are the silicates of iron and alumina, while the injurious constituents are sulphur and the carbonates of lime and magnesia."
One of the most valuable characteristics of slate is its decided tendency to split into thin sheets, whose surfaces are so smooth that they lie close together, thus forming a light and impervious roof covering. These planes of cleverage are caused 6y intense lateral pressure, and are generally at very considerable though varying angles with the ancient bedding.
Strength and Hardness. - From various tests that have been made on the quality of slate, it appears that, in general, the strongest specimens are the heaviest and softest, as also the least porous and corrodible. "The tests for strength and corrodibility are probably those of greatest importance in forming an opinion regarding the value of the slate under actual conditions of service." *
Mr. Mansfield Merriman suggests that specifications should require roofing slates to have a modulus of rupture for transverse strength greater than 7,000 pounds per square inch.
If the slate is too soft, however, the nail holes will become enlarged and the slate will get loose. If it is too brittle the slate will fly to pieces in the process of squaring and holing, and will be easily broken on the roof. "A good slate should give out a sharp metallic ring when struck with the knuckles; should not splinter under the slater's axe; should be easily 'holed' without danger of fracture, and should not be tender or friable at the edges."
The surface when freshly split should have a bright metallic lustre and be free from all loose flakes or dull surfaces.
* Mansfield Merriman in Stone, April, 1895.
Color. - The color of slates varies from dark blue, bluish-black and purple to gray and green. There are also a few quarries of red slate. The color of the slate does not appear to indicate the quality. The red and dark colors are generally considered the most effective, and the greens are generally used only on factories, storehouses and buildings where the appearance is not of so much importance.
Some slates are marked with bands or patches of a different color, and the dark purple slates often have large spots of light green upon them. These spots do not as a rule affect the durability of the slate, but they greatly detract from its appearance.
As a rule the dark color of slate, particularly that of the slates of Maine and Pennsylvania, appears to be due to particles of carbonaceous matter contained in the slate.
Absorption. - A good slate should not absorb water to any perceptible extent, and if a slate is immersed in water half its height the water should not rise in the upper half; if it does it shows that the slate is not of good quality.
"If, upon breathing upon a slate, a clayey odor be strongly emitted, it may be inferred that the slate will not weather."
Grain. - A good slate should have a very fine grain, and the slates should be cut lengthways of the grain, so that if a slate breaks on the roof it will not become detached, but will divide into two slates, each held by a nail.
Market Qualities. - The market qualities of slate are classed according to their straightness, smoothness of surface, fair, even thickness, and according to the presence or absence of discoloration.
Uses. - The principal use of slate is for roofing purposes, but it is also used for billiard tables, mantels, floor tiles, steps, flagging, fittings for toilet rooms, and for school blackboards.