|What is hard in a Japanese natural sharpening stone,
And what does it mean to the sharpener?
Common accepted thought is that not all natural Japanese sharpening stones from the Yamashiro district were created equal.
Although they were all created at the same time, roughly about 225 million years ago, the mechanics of their creation and
transition to the island of Japan has left us with geological variations within the geographic system known as the Tamba
Terrane or the Ultra Tamba Terrane. (further reading here). The historical aspects are that the Terrane traveled piggyback style
on the back of the Pacific Tectonic Plate under the oceans surface for 150+ millions years until it finally was scraped off due to
subduction onto the edge of the Okhotsk Plate where it came to rest in what is now known as central Honshu, Japan.
The first stage, stage one of the creation of the Tamba Terrane occurred during a volcanic event that belched forth and
dispersed the mineral dust that the awasedo/tennen toishi is composed of and surrounded by. The event was either that of an
active underwater Volcanic Vent spewed the minerals into the surrounding waters where the waterborne dust was carried to
where it finally settled into layers dictated by the specific gravity of the dust particles, or as an airborne event where the dust was
carried by winds only to settle in pools as sediment. In theory the larger heavier particles settled first while the lighter smaller
particles drifted further away from the vent/source. It has been suggested that the rate of accumulation was about 1cm per
1,000 years. Taking this accumulation rate account, the average thickness of the Tamba Terrane is 15 meters, at this pace if
the sediment rate were to remain constant, the total accumulation sequence would figure out to take around 1,500.000 years
from beginning to end.
Considering either the water dispersal or the atmospheric dispersal methods, any particle distribution has variables that in
theory could include; the liquid nature and stability of the currents within the dispersal and settling area, the composition
(saline, carbonic and other acids, gases) nature of the fluid, introduction of foreign material through cross pollination or
subsequent events, change in wind or water current speeds, major additional subsequent geological events, introduction of
alternate organic life forms material etc.. Any of these and any other timely events could and should affect the purity and
regularity of the individual strata layers of the terrane and their granular composition. It is a fact there are many variables
displayed as well as similarities within the Tamba Terrane that can be noted as specific characteristics identified to certain
locations and generally considered unique to that location. A short list would include from east to west within the terrane: silica
particle size, radiolaria concentration, specific minerals included in the clays fillers, compaction ratios, alternate minerals within
the chert Gokume waste layers found between the awasedo sharpening media layers, the absence of gokume waste layers, to
name a few.
For the second stage of its existence the Terrane resided deep below the surface of the ocean as it was transported upon the
surface-skin of the Pacific Plate from a location on the globe that is now near Hawaii to the current location of central Honshu,
Japan. It was during this migratory journey that the first pressures great enough to compress the sediment material into a rock
like structure came to bear, from the enormous weight of the oceans above. This pressure allowed the minerals to begin a
crystallization process and to formalize their layering structures.
Phase three of the epic is the most recent, and occurred as the Terrane was scraped off the Pacific Plate as an Accretionary
Wedge whilst the Plate subducted under the Okhotsk Plate. The additional physical changes to the Terrane structure at this
juncture are related to the mechanics of the actions of the physical transfer of the Terrane and by the resulting formation of the
mountains where the Terrane resides. Coupled with the gravitational and tectonic forces expressed, the Terrane was also
exposed to inner earth's heat as the two tectonic plates exposed flaws and fissures in the mantle allowing the Terrane to
"toast" as it slowly slid by these features which further allowed crystalization of mineral components.
The relative change in location from the seabed to an above sea level environment, allowed the introduction of totally new and
unique exposure factors to the materials of the Terrane which altered its make-up to some degree. These included the
introduction of; oxygen and other gases, fresh (non saline) water intrusion, water borne soluble minerals from outside sources,
erosion, temperature variations, lateral and vibratory pressures from earthquakes, variations resulting from the eventual incline
positioning of the Terrane, topsoil and vegetation variables, etc..
Although simply speaking the basic mineral formula of the Tamba Terrane and the resulting tennen toishi (natural sharpening
stone) material was determined millions of years ago, the present location of the deposit, and its most recent state of evolution
is pertinent to the users of the stones as sharpening media. Just as the previous 200 million years have shaped, tempered,
softened and jumbled the Terrane, surely the more recent 25 million years have also affected the quality and nature of the stone
in both bold and subtle ways that are distinct and bespeak to the unique character of the stone, as well as to their current
geological location from which they are extracted and mined from at the various locations within the Tamba region.
Almost any form of the mineral silica dioxide (SiO2) will abrade steel, and much of the recognized sediment formed sharpening
media found naturally in the world share this common abrasive material. The stone found in the Tamba Terrane has a
concentration of a chert type of silica in addition to ancient radiolaria and diatoms. These hard minerals are mixed with various
clays to form the unique sedimentary rock found in the mountainous region just outside of Kyotos city limits. This mixture as
compressed stone appears at various locations with a radius of about 50km east west by 20km north south. Mt. Atago is
considered as the historical east to west center post or landmark. Within this oval shaped area are various minor peaks and
elevated valleys from which the famous historic stone mines populate this semi rugged area that contains few modern roads
but thousands of footpaths.
I am going to make an important point here that is not common knowledge and has not been written about that I am aware of in
any language outside of Japan in regards to awasedo tennen toishi. This factor has a direct relationship to the hardness and
sharpening characteristics of the various examples of the Honyama (original mountain) stones of the Yamashiro Umegahata
valley district. And it will seem simple at first, so please indulge me and allow me to explain.
The Tamba Terrane was created at its origin in stages, with each stage representing an Ita or layer of a particular mineral
content and the layers lay horizontal to the earth like a series of blankets. Each blanket of awasedo sharpening stone material
is distinct in the minerals they contained from one layer to the next, and the blankets are separated from each other in the Hon
Kuchi (guchi) Naori formations in the Yamashiro area by a coarser mineral component called Gokume that is mostly made of a
coarse chert, and each layer of awasedo meets one of those gokume layers and often you will see those awasedo stones that
have a residue of the gokume attached like a skin called Kawa, Each layer or Ita (layer) has an upper and lower skin
separating it from the next gokume layer. This whole geological complex traveled in its entirety under the ocean only to be
scraped up onto the dry land where over time those blankets were eventually forced to lay vertical up and over and around the
steep mountain sides as the mountains continued to well-up and be formed from beneath deep within the earths crust. These
blankets layers now found laying intact and folded up an over the mountain peaks represent a sort of chronological geologic
curtain. The miners have over the past 800 years dug tunnels horizontally into this curtain made up of blankets representing
stone layers from the side, not down into from above as pits.
All of the mines over all of these past 800 years were all dug by hand in a more or less horizontal direction into the side of the
mountains as tunnels, and not dug down vertically into the ground as mining shafts. The fact that the mines are dug up on the
mountain sides horizontally into the interior is important here in order to understand the last step in the variation of the
sedimentary rock into a sharpening stone as we know them.
It was up to each miner to discover and uncover the mineral seam representing access to the desired stone. Most of this was
by trail an error digging exploratory tunnels. At the top of the mountain digging was easier because of the lack of topsoil
covering and often the stone was awasedo seam was exposed or just below the surface. As these early shallow diggings at
the top were depleted the miner moved down the mountainside to a lower elevation to search with test tunnels for the awasedo
which were there hidden behind a curtain of topsoil and vegetation. At the top of the mountain some open pit clear cutting
mining was done, but down below all of the work was done by hand as tunnels. This lower elevation stone was residing under
the weight of the mountain above and was also prone to ground water seepage. Many of the mountains or steep hills that were
dug for awasedo rise about 200 to 400 meters above the Umegahata valley floor, as each mine tunnel was abandoned, the
stone depleted, the miner moved down closer to the valley floor. At these lower elevations and closer to the valley floor the
natural ground water table begins to infringe on the layers of awasedo, this water coupled with the millions of tons of pressure
exerted from the above mountain mass creates a harder more compact stone character. The physics or science of this
compaction is beyond my base abilities, the miners had to equate the changes with empirical observation coupled with the
demands of the wholesale desirability of harder compacted stones.
The resulting depletion of the accessible stone that matched the wholesalers requests and orders at some point forced each
and every awasedo stone mine to eventually close. One by one up and down the valley the mines closed as their operations
reached the valley floor or to the water table levels. The last mine to close in the valley was the Nakayama mine managed by
Kato-san of Umegahata. Kato-san's last tunnel was more of a vertical pit that was dug within the ground water table, he was
forced to use water pumps to continue working and I have seen a photo of him in the mine wearing a full set of rain gear
including a rain hat. Some stones that were dug from this mine can be found with an ink stamp displaying a block & tackle
pulley. These were all ultra hard gray stones, too hard for craftspersons.
Below is a sketch of the progression from 1190 until the 20th century and how the tunnels were structured.