The hardness of a substance is not always obvious.
A a number of tests determine the hardnesses of minerals and metals. Testers plot the results through a number of common hardness scales associated with these tests. Attributes tested can include such properties as how the material reacts to scratching, what it takes to cause a dent and how a surface rebounds.
Scratch
The Mohs Hardness Scale is a scale of rating the hardness of minerals. It doesn't have a separate measuring standard for rating minerals (as with specific gravity comparing the weight of things to a pure cubic centimeter of water), instead it uses a relative scale. The scale is set up so that the harder minerals can scratch the softer ones. On this scale diamond is 10, corundum is 9, topaz is 8, quartz is 7, feldspar is 6, apatite is 5, fluorite is 4, calcite is 3, gypsum is 2 and talc is 1.
The Turner sclerometer measure is a more accurate means of measuring hardness related to scratching by giving a specific number. The sclerometer is used to see how wide a scratch is made when there is a set amount of pressure on a diamond while it is moved across a sample of material.
Indentation
Metal indentation testing for harness consists of the application of pressure with a pointed object against a surface to create an indentation that remains. The area of the resulting dent is then measured. Hardness, how the material resisted being dented, can be rated using the Brinell, Knoop, Vickers and Rockwell scales.
A separate scale is used for wood. The Janka hardness test measures wood hardness.
Bounce
The height of the bounce of an article, with a diamond tip, dropped from a set height, can be measured to give a bounce-back hardness evaluation. The scleroscope is an instrument used to obtain the total distance of the rebound. The common hardness scale showing the rebounding for this dynamic hardness test is the Bennett Scale.
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