Thread: What actually is Hardness in natural stones, and how it affects sharpening

I have followed the Shoobie Doobie Asagi thread with great interest for weeks now, very nice stones shown here and there throughout and that thread as developed some great off topics. The topic of Hardness in Natual stones is universal and I do have some opinions about how it affects sharpeing that I would like to explore.

Sharpening is the act of removing steel, it is a grinding process that is used to create a geometrical profile to a blade. If a stone is not grinding it is not sharpening, period.

The hardness of a stone is the action at the stones very surface and it is all about the stones ability to shed particles from the stones surface during the sharpening process, period. All stones are hard, but their relative softness might be important also as an attribute.

I think it is overly complicated to merge synthetic stones and natural stone together in the discussion of natural stones, but to not recognize that some synthetic stones are hard or not is helpful in part because we here on this forum do use some of the same synthetics as others here do, so their names and numbers can be a jumping off point. But that is about as far as it goes in my opinion.

I know that some members would like to know if all Nakayama stones are harder than Ohira. This is not really a question with a logical answer as many will agree. Each natural stone is unique in itself, and even within one mine, any given strata will provide a wide varity of stone characteristics and hardness is one component that can vary slightly. And I further suggest that within any one mines history, the hardness of the stone mined can vary for particular reasons from one decade to another.

The hardness of a stone and a stones cutting abilities are related.

On the hardness side, if a stone does not shed dulled used grit particles, but instead retains those as bound up dulled grit particles on the surface, this is when the stones cutting speed begins to decrease and eventually halts. When it halts the cutting action, the stone action will only be that of polishing the blade, a burnishing effect, it will not continue to grind or remove steel. This is fine if you want a polished blade that looks shiny. As the glazing of the stone progresses I will suggest that the stone will continue to cut steel but with ever decreasing efficency and there will be a point in time when the cutting action actually stops. If you continue to rub the blade on the stone after this point other factors come into play as far as the metallurgy is concerned.

On the stones cutting abilities side, it must be recognized that not all stones are created equal, and as measured per weight some stones have a greater amount (per pound) of grit particles verses binder clays, while some stones on the other hand have less grit and a greater amount per pound of binder clays. The stone with more grit per pound will always have more cutting power if all other things are equal like surface hardness, grit size, etc.

If you an scratch a stone with a tool this is a crude type of proof of the hardness, the stone has in fact shedded grit particles. Incorporating a given force upon a tool with given and known characteristics, this simple test on a stone could be useful in rating stones as having a certain hardness. The closest entity we have on this forum to conduct this test are the various descriptions of hand pressure, and various qualities of tool steel in the from or shaving razors. This is a start. Alx