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).
Composition: 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 strong possiblility 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.
Some Indian and Chinese slates will flake and powder from foot traffic. This is not rust or other minerals coming to the surface. These are spots of softer material becoming loose particles. Some slates may be quite hard, but have a soft spot that can powder out. Some black and brown slates (if unsealed) can lose color from aggressive cleaning.
StoneTech™ Professional BulletProof™ is recommended for any slate that tests for sufficient absorbency. (Test slate with water drops. This is a water based sealer that must all penetrate. Therefore, the slate must be able to absorb a water drop in 3 seconds. If the slate is not sufficiently absorbent, use Impregnator Pro™.
Slate is very stain resistant. However, some foods can cause stains. EnhancerPro™ gives slate a beautiful 'wet' look that does not cause a glossy built up finish. Many people make the mistake of applying heavy waxes or floor finishes that rob the slate of its natural beauty and slip resistance. High Gloss Stone Sealer can also be used where a glossy finish is desired (interior only).