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In page Slate:

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Slate tiles are often used for interior and exterior flooring,[11] stairs,[12] walkways[13] and wall cladding.[14] Tiles are installed and set on mortar and grouted along the edges. Chemical sealants are often used on tiles to improve durability and appearance,[15] increase stain resistance,[11] reduce efflorescence, and increase or reduce surface smoothness. Tiles are often sold gauged, meaning that the back surface is ground for ease of installation.[15] Slate flooring can be slippery when used in external locations subject to rain.[citation needed]