From googling and looking around.. I saw a guy post this about it..
The Rockwell "C" scale is probably the one that most folk on this forum are familiar with/aware of at some level. The measuring principle involves placing the workpiece on an anvil, which is basically the bottom of a big C-clamp, then pressing a precisely-cut diamond indenter into the workpiece at 2 different loads and measuring the difference in the movement of the diamond between the 2 loads. The tester itself is a big, heavy lump because it needs to have negligible deflection under the greatest, 150 kgf, load used. They are usually benchtop machines. It leaves the impression from the diamond indenter in the workpiece.
Impact methods of hardness testing are basically just more refined versions of the "drop a ball-bearing on your anvil and see how high it bounces" test for face hardness: it's fine for an anvil, but if you harden a piece of, for example, 1/8" thick O1 steel, put it on the anvil and repeat the test on the thin bit, the reading will almost certainly not accurately reflect the hardness of the O1. The problem is that some of the impact energy is lost at the interface between the O1 and the anvil, meaning that it's not available to drive the impactor back the way it came from. The amount of energy lost is variable with surface finish, straightness, cleanliness and doubtless many other things.
The only way to easily eliminate all these variables with an impact test is to restrict the test to only those workpieces that are big enough to effectively be the “anvil” themselves. That is why the minimum thickness required is 1" which means it isn't going to work for most knives.
