A very dense, but soft and easily scratched material with low porosity, slate can be used effectively outdoors as well as indoors. Slate is a foliated metamorphic rock which is derived from shale and consists mainly of grains of mica and quartz. Excellent exterior paving stone. Higher absorbency varieties are not suitable for exterior areas in freezing climates. 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 strong possibility of spalling, it should never be cleaned at pressures above 800 PSI and then only if the grout is in good condition.
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