Hardness can be approximated with a rebound test, which works best on anvil/hammer surfaces. Drop a hard ball 10" onto an anvil and note the rebound. Use a ball bearing (somewhere around 1/2") or better yet, a tungsten carbide ball. (Go to SMALL PARTS, INC and search for -tungsten carbide-). Cut an acrylic tube 10" long and mark the inches on paper taped to the outside. Place tube on anvil and drop ball, read the rebound at bottom of ball. Remember that results are specific to your test ball. If the ball rebounds 7", that figure can be used or converted into a percentage of the total drop distance (7/10) or 70%. Anvil rebound data.

We've recommended a 10" acrylic tube, but in fact, you can use a tube of any length if you divide the scale into tenth parts. A 70% rebound is a 70% rebound, from whatever height.

What does this tell you? It will allow you to make comparisons between different metal surfaces. Use the same test ball in all tests. The rebound test combines results due to hardness and elasticity. Liquid Metal, for example, is an alloy with very high elasticity and moderate hardness. A ball will bounce on it's surface for over two minutes. For a Quick-Check rebound device.

The hardness of knives is usually expressed as a number on the Rockwell Hardness C scale (HRC) and most of the time runs between 55 and 63 HRC. File tests for hardness tend to be somewhat subjective. They depend on the hardness of the file and the pressure used. The hardness of a common metal file is usually around HRC 60-61 and like the rebound test, file testing is more useful in making comparisons rather than generating a specific number. Hardness Testing File Sets have files of graduated hardness to use as referents.

Gain size can be estimated, again by comparison, by breaking the blade and comparing to a broken file and/or other broken metal. The look and feel of broken cast iron (rough, sparkley) is indication of very large grain size. Small grain size is good, it indicates greater strength and hardness.

Every knife steel has an optimal hardness and one of the easiest tests for this can be made while sharpening. Soft steels create a burr when sharpened --during the process of sharpening the steel at the edge of the blade BENDS and forms a burr. If the steel were progressively hardened, at some point burr formation will cease. This is right at the point where the steel BREAKS rather than bends. This BREAK/BEND point is the optimal hardness for any knife steel. A burr means too soft, no burr means too hard. It's right at the hint of a burr that things are just right--hardness/strength is optimal.

A SIMPLE RULE Brinell Hardness divided by two, times 1000, equals approximate Tensile Strength in pounds per square inch. (200 Brinell /2 X 1000 = approx. 100,000 Tensile Strength, p.s.i.).