The shales from which slates originate were deposited previously as clay beds. Subsequent earth movements tilted these beds of shale, at first horizontal and the intense metamorphic that converted these into slates folded and contracted them. Slate, then, belongs to the metamorphic group of rocks and can be defined as a fine grain rock derived from clays and shales and possessing a cleavage that permits it to be split into thin sheets.
Interior use only slates are designated with an "I" or "interior" on specification sheets.
Exterior use is labeled as either "E" or "exterior" on specification sheets.
Variation
1. Limited color variation: Relatively uniform in background/field colors, veining and or movement.
2. Moderate color variation: Background color has some variation and veining/movement may appear. Inspection/blending prior to installation is suggested.
3. Extreme color variation: Background color has significant variations and contrasts. Veining variations and movement may be inconsistent. Inspection/blending prior to installation is strongly suggested.
Slate: ASTM C629.
a. Water Absorption: 0.40%.
1) Classification I (Exterior): 0.25%
2) Classification II (Interior): 0.45%.
b. Modulus of Rupture:
1) Across grain: 9000 psi (62.1).
2) Along grain: 7200 psi (49.6).
c. Abrasive Hardness: 8.0
d. Acid Resistance:
1) Classification I (Exterior): 0.015 inch (0.38 mm).
2) Classification II (Interior): 0.025 inch (0.64 mm).