Do swords "develop" post polish?

[Pincheck]

I have a sword of unknown origin. For background, it's mumei, o suriage but still quite long with a 686mm nagasa, it has a noticeably narrow shinogi-ji, little hira niku. No idea what it is or even approximate age, but I like it a lot. Anyway, when I first got it it was in poor shape. It was in gunto mounts of better than average quality with a defaced silver mon. Well out of polish with quite a bit of damage, but something about caught my eye. Whoever the polisher was back in the day, he did a marvellous job as the lines were so crisp you could shave with them. I thought it worth the effort to save so I sent it away for polishing.

 

Prior to the polish, there wasn't much you could say about it other than it had no discernible hada and had a laser straight suguha, but the steel was dark grey, almost black.

 

Immediately post polish, it had no discernible hada and a laser straight suguha but the steel was light straw colored, almost yellow.

 

Fast forward a few years and the steel has gone dark grey again, almost black and now it has a pronounced ko-itame hada, which reminds me of sand, with a few patches of coarser hada and some that even looks like massame hada. It also developed some shirakke utsuri and jie nie. The change is quite remarkable and I was wondering if that is normal? I don't do anything to it, other than the occasional wipe with clove oil (not oil of cloves, the nihonto specific clove scented mineral oil).

 

Pre-polish

https://i.ibb.co/G3sBXXRJ/Prea-IMG-0703.jpg
https://i.ibb.co/9mnsjgzP/Prea-PICT0013.jpg
https://i.ibb.co/yFqh5FGQ/Pre-IMG-0700.jpg

 

 

Immediately post-polish
https://i.ibb.co/NnxW1K6v/Post-IMG-0730.jpg
https://i.ibb.co/4wBhSwVx/Post-PICT0036.jpg
 

3 years later

https://i.ibb.co/1f2tjMc3/Full-Length2.jpg
https://i.ibb.co/Ng7WJ3fW/IMG-1579.jpg

https://i.ibb.co/nMJw1wmS/Devimage14.jpg
https://i.ibb.co/0VMX42qN/Devimage20.jpg[
https://i.ibb.co/cKRwtGTF/Dev-IMG-1564c.jpg
https://i.ibb.co/39TXw6XM/Dev-IMG-1566.jpg

 

Edit: Unfortunately, it looks like imgbb has compressed all the detail out of the photos which rather ruins this post. Not sure if there's a way to fix that.

Edited Tuesday at 12:07 AM by Pincheck

[Rawa]

Was it exposed to the sun?

Stored in shirasaya?

[Rivkin]

Very many polishers believe they do. Reasons provided vary - "water trapped inside steel during polishing", "micro-erosion of weaker particles which still held out during polish".

Some polishers will request a specific oiling schedule, a few older polishers would recommend continuous uchiko application.

[eternal_newbie]

The straw colouring post-polish makes me wonder if it was actually the residue of previously applied oil (tsubaki / camellia oil is used by several polishers and dealers and apparently tends to solidify if left unattended too long).

 

As you applied and wiped off choji oil over the years it would have eventually taken off the old residue with it, which would have slowly allowed the obscured hataraki and hada to be exposed again.

[Pincheck]

Who knows then. I doubt it’s water ingress, I would expect that to flash off in days not years and it would probably provoke isolated rust spots which this has never shown. Micro-abrasion is possible but I doubt it could account for the profound change I’m seeing. It’s literally developed like a photograph, you honestly wouldn’t think it’s the same blade. I doubt it’s dried on camellia oil as I picked it up from the polisher only a few days after he said it was finished, which was a couple of weeks after he said he’d started it. Plus it would have to be like paint to obscure this hada. No, it hasn’t been lying in the sun. 

 

There are a few things I can think of that might cause the sort of change I’m seeing. High levels of phosphorus can lead to a layer of iron hydrogen phosphate hydrate forming on the surface of the steel which can be quite dark. That seems unlikely though as tamehagane is famously low in phosphorus.
 

Hematite is formed during the forging process which unavoidably gets folded into the steel producing fine bands of impurities between the welds. Hematite can be anything from grey to black although I do not know if that colour changes in contact with air or moisture or if it will be straw coloured when freshly polished, although I rather think not. 

 

Probably the most likely is that high carbon steel is known to change colour on contact with air. A very thin layer of FeO, Fe3O4 and Fe2O3 forms on the surface of steel which causes it to go through several color changes due to the way it reflects light from the top and underside of the layer, in the same way an anti-reflective coating on a glass lens works. At 50nm thick it looks pale yellow, 70nm it’s straw,  at 90nm it starts to look more brown, by 100nm it’s a blue gray, finally looking black by the time it gets to 150nm thick. My guess is there is a very thin layer of this stuff at varying thicknesses depending on the grain boundaries in the steel which is picking out and highlight the hada as it grows. 

[Robert S]

20 minutes ago, Pincheck said:

Probably the most likely is that high carbon steel is known to change colour on contact with air. A very thin layer of FeO, Fe3O4 and Fe2O3 forms on the surface of steel which causes it to go through several color changes due to the way it reflects light from the top and underside of the layer, in the same way an anti-reflective coating on a glass lens works. At 50nm thick it looks pale yellow, 70nm it’s straw,  at 90nm it starts to look more brown, by 100nm it’s a blue gray, finally looking black by the time it gets to 150nm thick. My guess is there is a very thin layer of this stuff at varying thicknesses depending on the grain boundaries in the steel which is picking out and highlight the hada as it grows. 

I agree with this theory, with the caveat that not every blade seems to go through the same sequence or degree of change, which suggests to me that there is a role being played by non Fe components of the metal in how this process happens.

[Pincheck]

11 minutes ago, Robert S said:

I agree with this theory, with the caveat that not every blade seems to go through the same sequence or degree of change, which suggests to me that there is a role being played by non Fe components of the metal in how this process happens.

I reckon so. The rate at which this oxide layer grows depends on the carbon level of the steel and you can bet that any other impurities or additives will also make a difference. It has to be said that nugui oil usually contains magnetite and other things designed to allow the polisher to darken the steel. Magnetite is just iron oxide and a thin enough layer will behave to same way. So some polishers may add this effect, thus disguising it’s natural development. 
 

Since this mechanism could (theoretically) pick out grains or veins in steel if they varied in composition compared to the neighbouring steel, I wonder if that is (one of) the ways in which chikei and maybe jie nie can appear. 

[Jacques]

Who is the polisher ?

[ROKUJURO]

20 hours ago, Pincheck said:

......Hematite is formed during the forging process which unavoidably gets folded into the steel producing fine bands of impurities between the welds. Hematite can be anything from grey to black although I do not know if that colour changes in contact with air or moisture or if it will be straw coloured when freshly polished, although I rather think not. 

 

Probably the most likely is that high carbon steel is known to change colour on contact with air. A very thin layer of FeO, Fe3O4 and Fe2O3 forms on the surface of steel which causes it to go through several color changes due to the way it reflects light from the top and underside of the layer, in the same way an anti-reflective coating on a glass lens works. At 50nm thick it looks pale yellow, 70nm it’s straw,  at 90nm it starts to look more brown, by 100nm it’s a blue gray, finally looking black by the time it gets to 150nm thick. My guess is there is a very thin layer of this stuff at varying thicknesses depending on the grain boundaries in the steel which is picking out and highlight the hada as it grows. 

This is basically correct, BUT the oxide layer on iron forms slowly in normal atmospheric condition. It is called "flash rust" when it gets more intense. It can be accelerated with higher temperature, so the above described colour changes can be seen when heat-treating carbon steel. In the West, these annealing colours are traditionally a guiding factor in tempering (= YAKIMODOSHI). 

As described above, the colours forming superficially are indeed super-thin layers of iron oxide, BUT they are not very stable and can be polished away. A straw colour would then correspond to an annealing temperature of about 200°C.

In this case I don't see how such a colour would remain on a blade after the TOGI process, but a KANAHADA NUGUI (made with finest powdered magnetite (FeO x Fe2O3), suspended in oil), could darken the steel to an extent. If this magnetite is not pure and contains some hematite, the blade can even show a brownish hue.

In the course of time, and with regular UCHIKO treatment, this colour will fade, resulting in a clean metallic surface.

This is what I come up with when trying to explain the above described colour changes to myself.  

[Katsujinken]

12 hours ago, ROKUJURO said:

a KANAHADA NUGUI (made with finest powdered magnetite (FeO x Fe2O3), suspended in oil), could darken the steel to an extent. If this magnetite is not pure and contains some hematite, the blade can even show a brownish hue.

 

I was going to say exactly this. Here's a video of this process (link to specific timestamp at 46:08)! 

 

[eternal_newbie]

31 minutes ago, Katsujinken said:

Here's a video of this process

 

Lovely video, by the way. No unnecessary dramatics or music, just a trained professional absorbed in his work.