ETYM AS. heardness.
The measure of a materials resistance to deformation by surface indentation or by abrasion.
1. The property of being rigid and resistant to pressure; not easily scratched.
2. The quality of being difficult to do.
Physical property of materials that governs their use. Methods of heat treatment can increase the hardness of metals. A scale of hardness was devised by German–Austrian mineralogist Friedrich Mohs in the 1800s, based upon the hardness of certain minerals from soft talc (Mohs' hardness 1) to diamond (10), the hardest of all materials.
See also Brinell hardness test. The hardness of water refers to the presence of dissolved minerals in it that prevent soap lathering, particularly compounds of calcium and magnesium. Treatment with a water softener may remove or neutralize them.
Der Gehalt an gelösten Calcium- u. Magnesiumverbindungen. Nach den Deutschen Härtegraden gilt: 0 – 4 d sehr weich; 4 – 8 d weich; 8 – 18 d mittelhart; 18 – 30 d hart; < 30 d sehr hart. Hartes Wasser ist unerwünscht, weil Seifen mit den Calciumverbindungen unlösl. Salze bilden u. weil sich in Dampfkesseln Kesselstein absetzt.