Slate, a very dense but soft and easily scratched material with low porosity, can be used effectively outdoors as well as indoors. Higher absorbency varieties are not suitable for exterior areas in freezing climates. Slate is a fine-grained metamorphic rock formed by the metamorphism of shale and clay under lower pressures and temperatures which tends to split into sheet-like (cleavage planes) structures formed in response to differential stress. It comes in a multitude of colors and has two lines of breakability (cleavage grain).
What is Slate Made of?
Mainly grains of mica and quartz, plus smaller amounts of chlorite, hematite, and other minerals. Most slate is gray to black, but the rock may be red or purple, depending on its mineral content. The surface of slate is generally uneven and cleft planes can spall, due to the cleaving of the stone along its layers. Has low to medium absorption of oils and other liquids. Should be sealed with an oil-repellant penetrating sealer to prevent staining and reduce soiling. Because of the cleft planes and the strong possibility of spalling, it should never be cleaned at pressures above 800 PSI and then only if the grout is in good condition.
Vermont type slate is typically smoother, denser, one color (red, green, black, blue). Indian and Chinese slates are dense and soft, of all colors and variegated colors. Brazilian slate is typically dense and hard, interesting colors and patterns of color.
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