Project #9 - Making finger-stones

Since my own knives are made with stock removal method there is no possibility to make a kasumi finish. But since most traditionally made Japanese kitchen knives have San-Mai (or similar) construction where the cutting core steel (jigane) is clad from outside (hagane) with iron (or soft stainless steel, or some sort of damascus steel) it is possible to bring a very nice contrast between the hagane and jigane because each of them reacts differently to fine abrasives. While the hard jigane can be easily brought to near mirrir finish, the hagane will tend to be 'cloudy' or 'milky'. This can be achieved through use of certain sharpening stones (synthetic and/or natural).

The final step in making kasumi finish is often made with finger-stones (see excellent information on the topic by Maksim Enevoldsen who of course is an expert on the topic)

The main point of fingers stones is that they allow you to smoothen the finish after the previous steps. If you (like me in the example at the bottom of the article) have used sanding paper to refinish the blade - it will be solely the finger stones that will bring the contrast between hagane and jigane - sanding paper as fine as #2500 will make them both look semi-polished.

Here I only wanted to show that it really is not hard to make finger stones. In fact this was my first attempt to make and use finger stones and so I am everything but expert on the topic.

Tools

I have used the following tools & materials

• Tomo Nagura Extra from Maxim (as of 2016 - according to Maxim it is either Takashima or Ohira, relatively soft, around lv 2.0) as a stone material for the finger stones.
• hammer
• home-made chisel (and old chisel of size about 15 - 25 mm would work too)
• steel saw (not a mist have, but improves the yield)
• pressed-cotton cloth (standard material would be rice paper)
• G/flex epoxy (any glue that remains certain flexibility after hardening would work)
• JKI flattening diamond stone
• JNS 300
• Gesshin Synthetic Natural Stone (effectively a 3k-5k stone)
• Hakka natural stone (not really necessary)

Some of the tool and first attempt to produce some piece usable for finger stones.

Material

Basically any soft (muddy) natural sharpening stones can be used to make finger stones. I have used Tomo Nagura Extra from Maxim (JNS) which, according to his words is either Takashima or Ohira. It is indeed a soft stone that creates mud very quickly. It also means that each finger stone will only last a few minutes of blade polishing.

Tomo Nagura Extra - a large soft stone excellent for finger stones.

For backing I have decided to use a press-made cotton tissue that I happened to have. Otherwise rice paper is recommended.

The process

My main concern was that I had a bad feeling about just hitting the stone with a hammer and producing few usable pieces. To improve my chances I have used a piece of about 80 x 20 x 4 mm of  unhardened carbon steel (left over after cutting out a blank) and I quickly ground the short edge to create a primitive chisel. Since the stone is soft and has a pronounced layer structure this chisel was plenty strong for the job.

One last step before trying to chip some finger stones off the stone I have used steel saw to cut a few mm deep into the surface of the stone - so that it would be easier to get stone pieces of suitable size and also to increase the probability that the chips would have more homogeneous thickness.

Once this was done I could proceed to try to use to chisel to create stone pieces of 2-3 mm thickness. This worked relatively well and I was able to produce several thin-ish pieces.

After cutting the stone with a steel saw to gain more control in the subsequent
splitting with a chisel.

And this is what you get after carefully splitting the stone.
Try to follow the natural layers for best efficiency.

Once I had the pieces I have used JKI diamond plate (grit ca. 150) to flatten both sides of the chips and thin them down to about 1.5 mm (give or take). Since the stone is soft this was really easy and fast to do.

Flattening the produced stone chips from both side and trying to get the ground surfaces parallel.
I  aimed for a thickness of about 1 - 2 mm

Flattened stone pieces - time for gluing.

The next step was gluing the stones to the cotton tissue. It is recommended to use glue that remains a little flexible. I have decided to test a new (to me) G/flex epoxy that should not get glass hard.

Gluing the stones to the cotton tissue with an epoxy.

I have used a piece of plastic bag as backing to avoid gluing the stones to my working surface and placed the cotton tissue on it. Then I have applied glue to one side of each of the finger-stones and placed them on the cotton tissue and allowed the glue to cure until the next day.

After glue cured the pieces are cut out. Obviously - I should have used more glue.

Note: Make sure you use enough glue so that it can soak the tissue you are using. and provide a good bond with the stone surface.

Once the glue was cured I have smoothed and thinned (to less than 1 mm) the finger stones with JNS 300 and Gesshin Synthetic Natural (ca 3-5k finish) and finally smoothed with a Hakka natural stone, though the last step is not really necessary as the stones create slurry which smoothness their surface. Please note that the nagura I have used was very soft and would abrade quickly.

Final thinning of the finger stones on JNS 300.

Finger stone after final thinning. The stone should be thinner than 1 mm.

The last step is to gently break each finger-stone such that the pieces remain attached to the tissue. This allows then the finger-stone to be used on slightly curved surfaces. I found that pressing the prepared finger stones against spoon worked perfectly. If some pieces break off from the tissue surface remove them - they could cause scratches on the blade.

Braking the finger stones so they work well also on curved surfaces.

Ready to use finger stone.

Also - if the produced finger-stones are too large to use comfortably, I would cut them in smaller pieces. Anything considerably larger than your thumb would not be comfortable to use.

Note - before you start to use a finger stone make sure its surface is clean and there is no stone debris from the previous steps on as it could scratch the blade.

If you want to produce a more refined finish than you may want to get a different stone that is a little harder. However make sure you are getting a good quality stone - if it has inclusions that are coarser or harder it will just scratch your blade instead of leaving a nice finish.

Tip (from Jon) - try to save the stone power (or finger stone mud) that get's created in the process - you can use it (with a piece of cloth, leather, cork or even a cotton pad) to quickly re-finish a blade surface or remove patine.

Tip 2 (from Jon) - if part of the surface you plan to finish is flat (sometimes wide bevels are ground flat) and not curved, you can make the finger stones (or just some of them) a little thicker and so they will last you longer as you do not need to break them.

Testing

Of course - the stones were made for a purpose. Here is just a quick test-run on a Hide gyuto which I was thinning and re-finishing (Project #11).

Knife finished up to #2500 sand paper grit (that was a lot of work)

First few strokes with the finger stone.

After just a couple of minutes. The result is not perfect because I have some scratches
from lower grits that were not removed properly, but the finger stones seem to work.

Finger stone after final thinning. The stone should be thinner than 1 mm.

Still some more work to be done, but this is looking good! :)