ASTM C 217 - Weather Resistance

The standard defines the depth of softening as an expression of weather resistance of a slate. This depth is determined by a shear/scratch tester or a hand scraping tool.
The idea is that slates contain pyrite, calcite and carbon and that during weathering calcite turns to gypsum, leading to a bulking. This bulking results in a softening of a slate and thus, the depth of softening correlates somehow with the weather resistance.

At least three specimens has to be prepared, each 2 by 4 in. (50 by 100 mm) to determine the depth of softening in a 1 % solution of sulfuric acid. The surfaces of the specimens has to be polished and finished with No. 80 abrasive.

On the fresh specimens the depth of a groove is measured. The groove is made by a shear/sratch tester. After the same specimens soaked 7 days in 1 % sulfuric acid a new groove is made and the depth measured. The deeper the groove the lesser weather resistant is a slate. In other words, the depth of the resulting groove is an expression of the intensity of softening and allows statements about the weather resistance.

All procedures are exactly defined in the standard so that the results are reconstructable and comparable with other slates.

The classification of the roofing slates is as follows: