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Technical Data


ASTM is the American Standard for Testing and Materials, later changed to ASTM International (2001).


This society was originally formed to answer frequent safety concerns in the American Railway industry during the 19th century.


It became the organization responsible for engineering and material regulations in many spheres through the twentieth century.


Overall the testing for slate is notated ASTM C406. This encompasses three areas, as follows:


  •  C121 -Water Absorption

  • C217-Weather Resistance

  • C120-Flexure (Modulus of Rupture)


For water absorption, the slate is weighed, submerged in water for 48 hours and weighed again. The amount of water absorbed is expressed as a percentage of the total weight.

Weather resistance is tested by measuring the thickness of the slate, scraping the surface and washing the slate in water. The amount of slate removed is calculated, measured in 1000ths of an inch.


Flexure or modulus of rupture is calculated to estimate the amount of light foot-traffic the slate might support, or pounds per square inch it takes to break the slate.


  • ASTM  C121  Water absorption cannot exceed 0.25%

  • ASTM  C217 Weather resistance no more than 0.002

  • ASTM  C120  Flexure should be at least 575lb


ASTM S-1………..75-100yrs (minimum)
ASTM S-2.……….40-75 yrs
ASTM S-3.……….20-40 yrs

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