Hardness
The scale-forming and lather-inhibiting qualities which water, high in calcium and magnesium ions, possesses.
the measure of a material's resistance to deformation by surface indentation or by abrasion.
Measured in degrees and based on the penetration into the rubber of a definined indentor under a set load. Three scales are commonly used: IRHD (International Rubber Hardness Degrees), Shore A and Shore D for hard materials over 90o Shore A. IRHD is preferred for for most specifications but Shore A is also in widespread use. See ASTM D2240, D1415