Finishes of Slate


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The color of slate is determined by its chemical and mineralogical composition. Gray and bluish gray color are due chiefly to the presence of carbonaceous material; many other colors are due to iron compounds. Slates containing large proportions of finely divided carbonaceous matter are black. Other colors that are found are blue-black, red, green, purple, mottled, yellow, brown, and buff.

Permanence of color has considerable importance, for although some slates maintain their original color for may years, others change to new shades within a comparatively  short time.

Some slates tend to fade under the influences of the elements. Such changes may be due to the presence of small quantities of iron lime-magnesia carbonates, which decompose readily and form a yellow hydrous iron oxide, limonite. Therefore, slates are of two types, "unfading" and "fading." Unfading color is not a quality verifiable by any current ASTM or other test method.

 

Reference: Dimensional Stone Design Manual, Marble Institute of America

 

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