[jyl]

Found at http://www.knife-expert.com/rockw.txt

"THE C SCALE

The scale used for testing the hardness of knife blades and
other hardened steel items is the Rockwell C scale. C scale tests
use the diamond Brale and the full 150 kgf major load. The
theoretical maximum hardness is infinity, which would be 100 on
the Rockwell C scale. Infinite hardness is of course impossible,
and few substances test very much over RC 70. The range of
hardness for functional knife blades is mainly between about RC
50 and RC 63. Most good sport knives test around RC 58 to RC 62.
Blades toward the high end of this range tend to be good at edge
holding, but very difficult to resharpen. Blades toward the lower
end are easier to sharpen, but may not stay sharp as long,
especially under demanding use.

Because Rockwell hardness testing yields a number, it seems
absolute and precise. However the test has important limitations
often overlooked by both the marketer and the consumer. First of
all, a single test is only valid at the point of testing. Any
blade, especially one that has been hardened by hand and eye,
should be tested at several points.

Second, the Rockwell test is a surface hardness test. It
cannot reveal anything about the hardness of the interior of a
steel item.

Third, to quote from Wilson Instruments, maker of Rockwell
hardness testers, "... the Rockwell test is a measure of the
resistance of a material to permanent indentation. Indentation
hardness is not a fundamental property of a material. However,
reliable relationships have been established between the various
tests and important properties of materials -- for example,
tensile strength and machinability. Furthermore, indentation
hardness has become one of the more reliable controls of the heat
treatment and quality of manufactured parts."