Hello M. *
I think it’s fantastic that you have inherited an authentic Japanese tennen toishi from your father. You might be the only one here who can stake that claim. The tomae strata stone that you have is from the Yamashiro area, a little valley just west of Kyoto and most likely from the town of Umegahata, and it could very possibly be from either the Nakayama mine or the Shobu mine. Each of these mines and others nearby share the Hon Kuchi Naori, a section of the Tamba Terrane that in some spots displays a rolled or ball form in the stratas configuration. If I could examine the back of the stone I could most likely pin it down.
Here is my example of a tomae with a kan pattern.
The strata is tomae, the ring pattern is kan or wood grain like as in tree rings. This is most assuredly a very hard stone and with out question it would benefit by the use of a nagura stone. To raise a slurry you have 3 options: the tomonagura you have, a Gifu nagura, or a diamond nagura. Each has benefits and drawbacks. I suggest that the tomonagura you have will do the trick, but you should also try raising a slurry with a diamond nagura like a fine DMT or an Atoma 600-1200. The diamond DN nagura has the unique feature of creating a slurry that is an exact match in composition to the host stone because it is made from the host stone. The quality and fineness of your host stone I am sure will provide you with all the features you are looking for in a slurry.
Here is another example of the kan pattern, this stone is very, very, very hard and from the Narutaki mine.
Unless you have created a perfect bevel on each side of your razor with a middle stone like a 5k, 8k synthetic, using your final very hard tomae stone will be an uphill challenge. So set your bevels carefully, examine them with a microscope if you can and make sure that they join at the edge and then you can go on to using your beautiful Jnat. You should be able to shave some hair after setting your bevels with the synthetics thus proving the meeting of the bevels at the edge.
The synthetics are very aggressive and in their cutting action they do not usually relinquish a slurry on their own without some coxing. The bevels they make however are very flat and true which is what we want them to create. You can try to shave off these flat bevels but they can be harsh. This is in part because of the razors really flat bevels abilities to lay so very close to the skin. I think you will find that if you incorporate a slurry into your regime while honing, the bevels will become ever so slightly rounded, and this will provide you with a slightly rounded bevel that rides a hairs breath above the skin surface and will thereby create a more comfortable shave.
Here are some photos taken at 350 power of several stages in honing up a razor. First is the bevel setting using a 5,000 grit stone on a blade that was not too dull to begin with. The 5k scratches in Photos #1 lead all the way to the edge of the blades edge but are “foiled” or prevented from affecting the extreme edge because the steel at that area is so flexible and thin that is folds out of the way rather than being abraded by the 5k grit. As the blade is turned over to set the other bevel this foil edge folds again in the opposite direction and our of the grits reach. This foil is a false edge of unsupported steel that will crumble immediately under any stress.
The next photo #2 represents the speed of the stone with a slurry incorporated, practically all of the deep long and distinct 5k scratches are removed with just 20 one way strokes. A stone can be hard or soft but the cutting speed is determined by the grit; its sharpness and friability, the grit content per weight of the stone, and the hardness of the grit particles in relation to the steel being cut.
The next photo #3 shows some progress after 20 more 5 inch strokes with not weight on the blades edge, just the guiding hand on the back of the spine. The long deep synthetic scratches are replaced by the short shallow scratches left by the tennen toishi, the bevel is beginning to round a bit and you can see that the microscope cannot stay in focus following a longer scratch because of the convex bevel and the short depth of field (focus) of the lens.
The #4 photo shows the results of the simplest form of stropping, the palm of my hand. The results are: the false edge has worn off and the true edge is reveled as much more linear form, and the grit impregnated in my palm (leather) strop has abraded with very shallow scratches the edge and flattened some of those closer scratches. This photo also displays the final results of the speed of the stone, the removal of 6k scratches comfortably within 40 short strokes.
Photo #5 begins to show the effects of using just plain water with again the moderate hand pressure. Not much change here as the grit particles are not encouraged to cut but to instead polish the blade. May users will spend several hundreds of strokes polishing their blades.
Photo #6 shows only a slight modification to the edge and the blades bevel, but, if you go back into the first 3 photos you will notice focus sharpness of the long scratches and now in this photo the inability of the microscope lens to follow these long scratches. This is an indication of the bevel curving into a convex form.
Just to pull this all into perspective I have added a photo under the same 350 power magnification of an adult human hair taped flat against the blade. It is out of focus because of its thickness and the thickness should be about 100 microns.
A slurry made up of grit as fine as the host stone also accelerates the sharpening process because the very nature of a slurry is that it is self generating, the loose grit frees up bound fresh grit during the sharpening process so it can provide fresh sharp particles. The grit of Japanese naturals is a friable material meaning it cleaves and chips into smaller pieces, and because of this the slurry in use is continually refining itself, becoming finer and finer as the grit particles crumble. They do not crumble because they are soft, but instead because they fracture along their crystalline inclusions, something that synthetics do not do readily.
Many razor users begin with a slurry when using their Jnats and continue as the sharpening session progresses to begin to dilute the slurry in steps that eventually end up with a clear water solution on the stone that acts as a lubricant. What is happening to the razors bevel during this stage is that the cutting or abrading action is slowing down while a polishing effect begins to occur. The normal biting action of the grit is diminished under just clear water as the grit is now being engaged while still bound up tight in the stone (not free and rolling around in a slurry), and the friability nature of the stone has stagnated to a stage where the surface grit is becoming rounded in profile. Rounded grit is less aggressive, and bonded grit just gets duller and duller coupled with the fact that if any swarf of steel particles are removed or ground off from the blade it may become imbedded in any voids of the porous surface of the stone.
This last clear water honing stage is popular because it gives the bevels a look and feel that is similar to the profile provided by those made with synthetic stones and the edge is actually sharper. But at what cost.
An edge composed of two converging perfectly flat bevels is an easily understood geometric shape, two sides of a triangle. A cutting tools edge is at that intersection where two planes meet, and it can be relatively sharp or dull. But the 3rd dimension of a blade is the material or meat behind the edge found as those two planes recede and it is made up of steel. This steel can be strong or weak, or weakened or strengthened. Weakened by loss in temper, rust, micro fracturing etc., or strengthened by cryogenics or again, geometry, as in the form of a secondary bevel or micro-bevel. The thicker the steel is and the closer to the edge that the thicker the steel is, the stronger that edge will behave.
Another form of a secondary or micro bevel is a slightly convex bevel plane, this can increase the thickness of the steel directly behind the edge considerably and especially if both the bevels are slightly convex. The beauty of natural stones is that the slurry generated during the sharpening process or created from a nagura is that this thickness to the bevel is increased in direct relation to the fineness of the grit particles in the slurry which are acting as miniature blacksmiths. An amazingly subtle curve can be accomplished with no directional hand movement from the person holding the tool.
This engineered graceful convex bevel can help to create a longer lasting more comfortable shave. Another way to form this convex bevel is by stropping a blade on leather or linen. With or without added abrasive dust or sprays, almost any strop will harbor and contain various environmental contaminates that can act as abrasives. The very nature of stropping encourages a somewhat curved edge bevel as the somewhat cushion surface of the leather gives way under even the slightest pressure of the blade and this is why in part (besides the removal of a wire or burr edge), that a stropped blade will feel more comfortable than one fresh off the hone. The foreign grit found in a used strop may not necessarily be finer than the stones grit and therefore might not improve the actual edge, but it will affect the geometry of the bevel.
To sum up the above I would suggest that honing a razor is a preceedure that at best follows observable steps that lead to a result close to that which was achieved previously. If you build a repeatable system you are on the road to better edges. If you are unable to to observe closely as in the above photos, them you must replace a visual with a tactile system based on some form of empirical evidence. good luck, Alx