Friday, 26 October 2012

How to Measure Tile Hardness

Tile hardness is a key issue in determining whether a tile is suited to a particular application. High traffic areas require a harder, more resistant surface.

Hardness is measured using the Mohs scale of mineral hardness, generally called the Mohs hardness scale, is used by geologists all over the world as an identifying tool for minerals. Quite naturally it has also been adopted by the tile industry as a scale to measure the hardness of tiles. Tiles after all, are really just artificial stone. The pressing and firing simulate the process by which metamorphic rocks are formed deep in the earth.

The basic principle of the Mohs scale (no apostrophe) is the scratch resistance of a softer mineral being scratched a harder mineral.

Developed in 1822 by Friedrich Mohs a German Geologist and mineralogist, it is one of several methods used to describe the hardness of minerals and materials in science, however it is the one most commonly used. Although it has been 200 years since the first implementation of the Mohs hardness scale, the method of comparing one mineral to another to ascertain its hardness is much older. The technique was mentioned in 300BCE by Theophrastus in his treatise On Stones.

The Mohs scale is strictly a relative scale, but that's all that anyone needs. In terms of absolute hardness, diamond (hardness 10) actually is 4 times harder than corundum (hardness 9) and 6 times harder than topaz (hardness 8). Because it isn't made for that kind of precision, the Mohs scale uses half-numbers for in-between hardnesses. For instance, dolomite, which scratches calcite but not fluorite, has a Mohs hardness of 3½ or 3.5.

The hardest known substance to man at the time of the scales creation was diamond which is at the top of the scale*.

The Mohs Scale.

TALC- Mohs hardness 1- Absolute hardness 1

GYPSUM- Mohs hardness 2- Absolute hardness 3

CALCITE- Mohs hardness 3- Absolute hardness 9

FLOURITE- Mohs hardness 4- Absolute hardness 21

APATITE- Mohs hardness 5- Absolute hardness 48 (B2b CERAMIC >5)

ORTHOCLASE FELDSPAR- Mohs hardness 6- Absolute hardness 72 (B1a PORCELAIN >6)

QUARTZ- Mohs hardness 7- Absolute hardness 100

TOPAZ- Mohs hardness 8- Absolute hardness 200

CORUNDUM- Mohs hardness 9- Absolute hardness 400

DIAMOND- Mohs hardness 10- Absolute hardness 1600

You can buy a set of “Mohs mineral pencils” a set of pencils each mounted on the tip with a small piece of each of the minerals above, these for geologists are often more show pieces, and in the field it is important to know the hardness of ordinary materials to make mineral identification easier. A steel knife (very good for many applications in the field) has a hardness of about 6-6.5 whist a fingernail has a hardness of between 2 and 2.5. Just these two tools can make mineral identification so much easier!

Hardness of some other items.
  • 2.5 Fingernail
  • 2.5–3 Gold, Silver
  • 3 Copper coin
  • 4-4.5 Platinum
  • 4-5 Iron
  • 5.5 Steel knife blade
  • 6-7 Glass
  • 6.5 Iron pyrite
  • 7+ Hardened steel file
In recent years man-made nanomaterials have been produced that are harder than diamond. Currently two rarely occurring natural minerals have topped the scale. Only small amounts of wurtzite boron nitride and lonsdaleite exist naturally or have been made in the lab so it has been difficult to test them accurately however they are calculated to measure 18% and 58% stronger than diamond respectively.

How to perform a Mohs test yourself.
Difficulty: Easy
Time Required: Seconds
  1. Find a clean surface on the specimen to be tested. 
  2. Try to scratch this surface with the point of an object of known hardness, by pressing it firmly into and across your test specimen. For example, you could try to scratch the surface with the point on a crystal of quartz (hardness of 9), the tip of a steel file (hardness about 7), the point of a piece of glass (about 6), the edge of a penny (3), or a fingernail (2.5). If your 'point' is harder than the test specimen, you should feel it bite into the sample.
  3. Examine the sample. Is there an etched line? Use your fingernail to feel for a scratch, since sometimes a soft material will leave a mark that looks like a scratch. If the sample is scratched, then it is softer than or equal in hardness to your test material. If the unknown was not scratched, it is harder than your tester.
  4. If you are unsure of the results of the test, repeat it, using a sharp surface of the known material and a fresh surface of the unknown. 
  5. Most people don't carry around examples of all ten levels of the Mohs hardness scale, but you probably have a couple of 'points' in your possession. If you can, test your specimen against other points to get a good idea of its hardness. For example, if you can't scratch it with a copper coin, you know its hardness is between 3 and 6. If you scratch your specimen with a piece of glass, you know its hardness is equal to or less than 6. Real porcelain can be scratched by quartz but not glass.