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A proposal for a simple scale representing the perceived frailty of various soft

Last Updated: 2nd Nov 2016

By Alysson Rowan

Alysson Rowan, Devon, 2016

When describing the relative hardness or softness of rock in the field, it is often necessary to express some form of hardness, particularly in the softer rock deposits.

Moh's scale of hardness is generally inappropriate, since thie relates to the individual minerals comprising the rock. Nor does Moh's scale adequately express the nature of a fragile rock that will crumble easily, as the scale refers to resistance to scratching, indenting etc.

When dealing with some rock types, the hardness of the rock may well be significantly below the hardness of talc (Value 1 on Moh's Scale of Hardness.)

In order to express the competence of a rock, a different scale is required, and one which describes, clearly, the manner in which that rock will react to handling, that is to say, its friability.

Rocks vary in competence, that is to say, how resistant they are to desctruction when force is applied to them. Most often, the less competent rock types are sedimentary in nature, although some rock-forming mineral masses of igneous, metamorphic or hydrothermal origin will fall into this category.

Setting the zero-point as a fully-incompetent material makes sense, in this case, the zero will represent such materials as loose sand, dust, etc.

The upper limit of the scale is arbitrarily set at 10, and will represent a material that requires at least a strong hammer-blow to damage.

Thus, the proposed scale is as follows:

Table 1: Outline of the three major rock hardness groups

0POWDERMaterial that is un-bound and will flow under gravity.
1 - 3INCOMPETENTRocks that are easily broken down when pressure or friction is applied using the bare hand.
4 - 7SOFTRocks that may be broken down using stronger pressure or basic mechanical aids, such as a piece of wood.
8 - 10RESILIENTRocks that are generally resistant to damage without resorting to metallic tools.

Table 2: Full proposed scale of hardness scale for softer rocks

0PowderPowderFlows under gravity, completely unbound grains.
1IncompetentUnboundA gentle touch will cause destruction of the material.
2 Barely BoundHandling will liberate copious surface material and will loosen chunks of material. May be destroyed by finger pressure.
3 Loosely BoundHandling will liberate copious surface material. Readily marks the skin. Small amounts may be crushed between the fingers.
4SoftModerately CompetentHandling will liberate a moderate amount of surface material May mark the skin. Small amounts may be easily pinched off using the fingernails.
5 CompetentHandling will liberate a small amount of surface material
6 Fairly CompetentSurface material is may be removed using a finger-nail
7 Fully CompetentSurface material may be removed using a wooden scraper and moderate pressure
8ResilientModerately ResilientGentle pressure with a metal tool will remove surface material
9 Quite ResilientModerate pressure with a metal tool will mark the material
10 Extremely ResilientStrong pressure with a metal tool will barely mark the material
HardHard Strong hammer blows may mark the surface

As ever, comments and constructive criticism are welcome.

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