Inclusions in Jnats and other natural whetstones

[bLee]

Hi,

Since I bought an Ohira Renge Suita at the weekend, I informed myself about inclusion in Jnats and whetstones.

Ohira Renge Suita

What are inclusions?

There are two types of inclusions: lines (suji = tendon or fiber) and spots (hari = needle). These are accumulations of a mineral or different minerals, which differ in their hardness and/or their grain size from the host rock.

How do inclusions form?

To understand how inclusions form, you have to know the formation of the rock (I'll leave that out, because half of you would be bored, if someone is interested, I'll explain it).

What influence do these inclusions have on honing?

A distinction is made between "toxic" and "non-toxic" inclusions. Where the word "toxic" refers only to the effect on the edge and not to the health of the user. Accordingly, toxic inclusions are poison for the honing process and should not come in contact with the razor during honing. They differ in hardness or grain size from the host rock and cause damage to the edge (notches and scratches). Non-toxic inclusions do not have a major impact on the honing process because their hardness is less than or equal to the host rock.

There is a second distinction between "living" inclusions, those which can damage the blade of the knife during honing and are to be avoided, and "dead" inclusions, which do not touch the blade during honing, since they only occur at the edge of the whetstone.

Whether toxic or non-toxic. If a hairline inclusion runs along the entire length of the rock, it can cause the whetstone to split along the line. This happen, because the minerals within the inclusion continuously decompose and losing there binding force. In addition, they are very porous and absorb water. Temperature differences cause the water to expand and split the rock.

What are inclusions?

Generally, toxic inclusions mostly consist of SiO2 in different variations as well as other silicate minerals. In addition, hematite and other alteration minerals may occur. The main reason for their toxic property is probably their hardness of about 7 (Mohs hardness scale) and their coarser grain size, which can lead to scratching of the metal. Non-toxic inclusions usually consist of calcite and other minerals (manganese, phosphorus, etc.), which usually have a hardness of less than 4 and so have no influence on the honing process.

Pyrite (hardness 6) represent a special case, since it can crystallize independently in the rock and forms small spots all the way to clouds and veins. These are often considered aesthetically pleasing, but have no influence on the honing process itself. (Pyrit is a very interesting mineral in the context of Jnats, but more on in a another thread)

Small pyrite inclusions (non-toxic)

Pyrite inclusions close-up

Ohira Renge with pyrite (non-toxic)

How to tell if inclusions are toxic or non-toxic?

Well that's not always easy to say. Often non-toxic inclusions are not palpable with the fingers. However, if the whetstone has been freshly lapped, a haptic discrimination is impossible. The following rule applies: The proof is in the pudding. Are you unsure if a suspicious discoloration in the rock or a thin line is toxic or non-toxic, you should do the first honing trials with a cheap razor or knife. It can be seen quite quickly if the face of a razor scratched when you purposefully hone over a particular spot.

You could also test for calcite with weak hydrochloric acid. But if the whetstone itself is carbonated, the result is not clear. The simplest way remains the simple trial and error.

Japanese terms:

Harike - toxic reflective needle, point or dot pattern. It can also be just a single large crystal, which has a larger grain size than the host rock.
Atarazuno hari - non-toxic needle, point or dot pattern

Goma - small black spots that look like sesame seeds. They can be toxic and non-toxic

Goma in close-up


Kanesuji - toxic hairline cracks often run over the full length of the whetstone.
Kesuji - non-toxic hairline cracks / fissures

Iwatani Karasu with Kesuji and Atarazuno hari

Kanesuji

Kanesuji

Kanesuji

I have not been dealing with Jnats for a long time and certainly there are some of you who have a lot more practice experience with inclusions than me. I'm interested in how you test inclusions and how you handle them. I've seen some ways to remove them but also videos where whetstones were simply crushed into Tomo Naguras. I look forward to your experience.
Of course, the contribution contains and should not be seen as being complete and may be supplemented and corrected.

- Jan

 

[Gamma]

There are more than two types of inclusions - there are two basic types of Suji which are, basicaly, hairlines - some are safe, some are not. There are several types of spots and the toxic needles are another thing entirely. Sometimes, you just run into a big pebble that wipes out your edge and then there's rust that runs though a stone that can be invasive or not. Hard crystal formations - vugs - and other stuff too. The list is pretty long actually.

I take a pretty simple position on this stuff; if there is something in the stone that is impeding one's efforts, it can be lapped out, scratched out, avoided by honing around it, or avoided by using another stone.

 

[bLee]

I already thought there are more terms related to inclusions in the Japanese language. Unfortunately, I only found this few.
Anyway, you're absolutely right there are a lot of different inclusions that differ in their shape and size. From a geological point of view, these are the authigenic minerals: quartz, sulfides and carbonates and their weathering products.

 

[Pokeystotle]

I’ve come across a Jnat that’s description includes “blue” renge that can cause rust on carbon steel. It is also said to cause a “tight feeling.” What exactly does anyone think this “tight feeling,” refers to? The context was not clear enough to determine the intended meaning and I assume the writer’s first language is not English.