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A material's strength is its resistance to breaking under compression. Strength varies with degree of coherence and, for most clay-rich materials, with moisture content. The scales presented below are not standards, but may serve for systematic comparisons.

The hammer tests should be made with a 2lb hammer on pieces about 4-inch thick placed on a solid hard surface and tests with the hands should be made on piece about 1.5-inch thick. The pieces must not have incipient fractures, and therefore several should be tested.

The sound tests, which are only accessory, should be made on solid outcrops or on fragments thicker than 1ft.

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1. Loose.

Sediment flows when dry and thus cannot be sampled in aggregate.

2. Very friable.

Sediment crumbles so easily that pieces are difficult to collect intact.

3. Friable.

Sediment crumbles under light pressure in the hands.

4. Somewhat friable.

Rock breaks in the hands under moderate pressure.

5. Firm, slightly friable.

Rock breaks with difficulty in the hands but readily by hammer blows, sounding a dull "whop".

6. Weak, non friable.

Rock cannot be broken in the hands but breaks under light hammer blows, sounding "whop".

7. Moderately strong.

Rock breaks under hard hammer blows, sounding "whack" with a ring

8. Strong.

Rock breaks under hard hammer blows, sounding "whack" with a ring.

9. Very Strong.

Rock difficult to break with hammer, sounding "boink"

10. Unusually strong

Rock impossible to break with hammer, sounding like steel.

 

This information originally contained in Geology in the Field by Robert R. Compton, P. 368.

 

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For more information, link to Mohs Hardness Scale.

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