I am continually amazed at how many of these gray stones now new on the market are stamped Ozuku. Brand new stamps, brand new paper labels on what look like brand new stones all attributed to a mine that was closed in about 1920. Someone has an enormous stockpile of stone in storage and brand new generic stamps (Ozuku never stamped their stones) and I suspect that not all the stones are from the Ozuku. When these fresh to the market stones are simply gray with no skin on the back it is hard to tell which mine they are actually from. I would like to explain something about hard gray stones.
Think compaction. All of those famous mines, now closed, began digging and extraction at the very upper elevations at the mountain tops where the stone was easily found as the strata lay bare and not below the topsoil. As the shaft was dug and the vein depleated the miners were forced to move down the mountain side (actually steep hills) in order to locate the vein again.
These mountains are not made of solid stone good for sharpening, the sharpening media is a very thin and irregular layer that spreads sporadically up and over the mountain sides like a hundred blankets upon a bed, but with the added complicated feature that the blankets are torn into strips and tumbled about and scattered. Because of this randomness, the miners in the old days had to continually relocate the vein/layer by trial and error, digging test holes all by hand as each of the upper tunnels payed out. Over the centuries with each depleated and abandoned tunnerl or shaft, the next was dug at a lower elevation. The first shafts were dug 800 years ago at the top of the mountain while each suceeding tunnel was dug lower and below. By the years 1900-1920 most of the mines had closed because they had located and pulled out all the the sharpening media that was good and useable. In the end, after centuries of production for each of the mines the last few tunnels were dug at the foot or bottom of the mountain slope leaving dozens of empty tunnels and shafts above. It is at these lower elevations that compaction plays an important role in the character of the fresh stone.
In the upper reaches of the mountainous area of Umegahata where the majority of the famous Yamashiro mines including Ozuku, Ozaki, Soubu and a whole slew of obscure mines, the valley is very narrow and steep. As each of these mines closed down the last shafts they dug and opened up were near the valley floor. The miners found that there were limits as to how deep they could dig. Not only was it expensive and dangerous to go below a certain level in the mountain because of the sheer pressures from above, but any new shaft would be located below the water table so any new shaft would quickly flood. As a result few upper valley mines chose to dig below the watertable for various reasons including amongst others the lack of access to enough electricity (are rare commidity in 1895) for pumps. To cut their losses and for safetly reasons as each mine was abandoned usually the lowest shafts were bulldozed or dynamited as well as some of the upper elevation shafts and tunnels.
The honyama Nakayama Mine (naka means middle) was between Umegahata and Narutaki. The Umegahata Valley is populated by a string of little villages all set up above a small river that flows towards Kyoto, Nakayama is on a wider flat area down stream and Narutaki district and the actual Narutaki mine site is even farther down from Nakayama and away from Umegahata.
While the upper valley mines reached their lowest limits with the water table issue in relatively steep rugged territory the Naurtaki Mine hit the same problem where valley floor is much wider and not so steep. This location allowed the owners easy access to dig below the watertable using modern machinery including steam shovels and heavy duty pumps. There was production for a few years or so as an open pit mine, and reportedly a lot of thick dense stone came from Naurtaki at this stage, the majority of it was very hard and did not suit the refined tastes for the market of handsharpeners in the building and craft trades and the sword polishers who demanded only the very best quality stones. So consequently some of the stone simply ended up as crushed material for the expanding abrasives market that was rapidly developing after the first world war. A hospital now sits atop a portion of the original Narutaki mine site.
In the 1960s the Nakayama mine hit the valley floor with their lowest tunnel and the owners also found the intrusive water a problem. Not only did they find water but the stone quality was of a harder consistancy due to the untold eons of pressure from the mountain above compressing the deeper layers below. The owners of the mine had always promoted their stone as the finest, decisions, decisions. As documated, Kato-san the principle owner did develop one last shaft at this lowest possible elevation, and there is a photo taken taken with a flash of him in this shaft looking like a seaman with his rain gear on including a large rain hat and rubber boots digging away. As it turns out the conditions were not worth the effort and soon after he boarded up the mine shaft. The stone that was marketed from that shaft contained a unique ink stamp that pictured a block and tackle pulley to symbolize the effort it took to raise all of the stone up and out of the shaft. A few years later Kato-san hired Ishihara-san the owner of the Ohira mine to use a bulldozer and seal up this last shaft. Ishihara-san was compensated with a quanity of Nakayama raw stock for his efforts and skills as a heavy machine operator.
Now I have been using the terms shaft and tunnel interchangeably, but historically all of the holes dug preceeding the 20th century at these mine sites were more like tunnels dug horizonally into the mountain sides rather than down like a coal shaft might be. The tunnels intentionally were dug at a slight upward incline in order to facilitate rubble removal using hand carts or sleds. As the tunnel was hollowed out retriving stone, the cavern floor was dug down yealding the good stone and thus the tunnel became taller by digging out the floor. Alternately a shaft by definition is a vertical opening and requires the rubble to be hauled up and out of the shaft, a much more intensive and complicated situtation. A side note is that when removing stone from a mountainside location the downhill perspective provides a ready chute for evacuating both the good stone and the waste which usually was just jetisoned down the hillside below the mine.
There is one theme in all of this. As each mine ended production, the stone dug at the lower elevations was markedly harder in composition, a result of simple geological compaction. The force of a whole mountain upon a fragile sediment type deposit like the various minerals that compose these stones alters not only their character but all their abilities to function. An attribute of the Honyama type stones is their friability, that function that allows the stone to recharge the cutting particles by sheding the old in order to introduce some new sharper faster cutting grit while allowing the older grit to continue to break down into ever finer but still abrasive material all held for use in a slurry formation.
A stone that does not shed used grit particles has a limited defined cutting ability curve that evolves from a fast action to slower action as the voids between the individual grit particles begin to fill up and clogged with compacted swarf made up of ground metal and less effective cast off grit particles. The access to the actual sharp grit itself becomes less likely as the surface of the stone becomes encased in a layer of glazed glass like steel.
The good, the bad and the ugly of all this is: what is marketed to us, what is attractive to us visually, and what really works as sharpening media. Sharpening a tool, beit a razor, chisel or scissors or whatever is a task undertaken by the craftsperson, it is he/she who decides what stone to use and this is as much an expression of aesthetics as it is efficency. A little history can play a roll in what we have at hand and how we use it for the roll of abrasive.