Question regarding preservation.

[French nihonto]

Hello, gentlemen. A few days ago, I was performing maintenance on one of my katanas, a Nanbokuchō-period piece I’ve owned for nearly ten years now. To my surprise, I found a few small rust spots on the mune near the kissaki, a first for any of my blades. I’ve encountered this sort of issue before with blades I’ve received after long shipping periods, and I’ve always managed to resolve it, but it had never happened to a blade while in my care. I’m not overly worried; I’ve started a treatment protocol involving regular oiling. I usually leave only a very thin layer of oil on my blades, but I’m going to apply a thicker coat to help stabilize the situation. Mostly, I’ve been trying to figure out the cause. It is true that we’ve had a particularly hot summer, but the storage room maintained a humidity level of between 50% and 65%, not perfect, but not too bad either. I suspect the shirasaya. The blade is stored in an old shirasaya likely pre-war, so it is probably contaminated and past its useful life. The ideal solution would be to send the blade to Japan for a new shirasaya, but the shipping is long, and I’m uneasy about such a lengthy trip through humid environments with a contaminated mount, especially since the blade’s polish is still in excellent condition. I’ve considered another option, which brings me to my question: storing the bare blade under a Plexiglas dome (see photo), museum-style, and making it as airtight as possible. I found a company that sells the dehumidification units used in museums; since the blade would be bare unmounted , lower humidity would be better ( 30% target ) However, even in Japan, museums rotate their collections to preserve the blades, despite having optimal display conditions. So, what are the risks associated with this type of storage in the short, medium, and long term? I’d be interested to hear from anyone with experience in museum exhibitions. I’m also aware of the issue regarding UV exposure—what about that?
Happy to hear your thoughts
Max

[Robert S]

I think my first question would be whether you could just run a dehumidifier in the storage room, and thus reduce the risk for all of the blades.  As for keeping them out without shirasaya, I actually think that's the safest option, as long as they're not at risk of physical damage, in a humidity controlled space, and not touching anything which could host humidity build-up.  With any saya there's some risk of contact or humidity build-up.

[John C]

I use these type of reuseable dessicant packets in a tansu drawer lined with cedar (basically a cedar chest with drawers). Cheap and effective. You can use as many or as few as you want. Until you get your high-speed museum set-up, that is.

https://www.amazon.com/Wisesorb-Rechargeable-Dehumidifier-Reactivated-Desiccant/dp/B0CYZN82JD/ref=sxin_19_pa_sp_search_thematic_sspa?

 

John C.

 

[Takezo]

60% humidity is the top. Anything above that will invite rust. If your room gets above 60% then there is your answer. Occasionally what happens when you oil a blade and it goes into the shirasaya is the blade may have light contact with the wood, either entering/exiting or sitting. This can rub off oil, or it’s possible you did not have it completely oiled when you thought you did. I have a case made by Keisuke Nakamura and he has personally told me that no oil is required on bare displayed blades as long as humidity is below 60% and air is not blowing directly on the blades. In fact, he displays his blades this way himself and told me Japanese museums use the same standard, which he studied before making his cases. The room my case is in has a constant 48-55% humidity and I have not seen any problems. I actually keep light oil despite Keisuke’s instructions, as being in the United States makes it much more difficult to acquire a quick polish if it were to rust. 
 

Also yeah, if your saya is dirty and is holding any particulate with water content, could be that too. 

[Takezo]

Also before you try to store your sword in any type of airtight container, I would spend some time recording hygrometer readings in it. Remember airtight containers also trap moisture, so it might seem counterintuitive, but breathing can be good.

[French nihonto]

Thank you for your reply. I hadn’t had any problems before. I already have dehumidification systems near the blades. Before sealing the blade in an airtight enclosure, I will of course run tests and take humidity readings. The goal would be 30–40%, but above all, ensuring there is absolutely no fluctuation.

[Lewis B]

Control of temperature fluctuations are key, especially sudden changes. Keep the variation to a minimum and above the dew point (dependent on temperature and humidity) as possible because sudden swings can encourage air bound water molecules to condense on metallic surfaces.

 

"Condensation is the phase transition of water vapor into liquid. On metals, this occurs in four molecular stages: adsorption, nucleation, coalescence, and droplet growth. As the metal surface drops below the dew point, kinetic energy drops, forcing water molecules to bind to the metal via hydrogen bonds and electrostatic forces"

[French nihonto]

13 minutes ago, Lewis B said:

Control of temperature fluctuations are key, especially sudden changes. Keep the variation to a minimum and above the dew point (dependent on temperature and humidity) as possible because sudden swings can encourage air bound water molecules to condense on metallic surfaces.

 

"Condensation is the phase transition of water vapor into liquid. On metals, this occurs in four molecular stages: adsorption, nucleation, coalescence, and droplet growth. As the metal surface drops below the dew point, kinetic energy drops, forcing water molecules to bind to the metal via hydrogen bonds and electrostatic forces"

The lower the humidity, the lower the dew point at the base of the object, I believe?

[Schneeds]

I display the bare blade in basically a sealed display safe and don't have issues despite hot humidsummers.  I've considered using a long clear sleeve of plastic and heat sealing it with a dessicant packet inside.  That would maintain the same humidity basically indefinitely.

 

Shirasayas look nice, but all wood will absorb moisture to some degree.  

[French nihonto]

To wrap up this topic: I used Uchiko (which I don't usually use on my blades) to remove tiny rust spots, and they came off very quickly. I placed the bare blade inside a glass display dome with a dehumidifier packet. It has been in there for four days now. The internal humidity fluctuates between 27% and 30%; it has been at 30% for a full day, which is more than sufficient—and this also proves that the dome is virtually airtight.

[eternal_newbie]

Fingers crossed that this does the trick.

[Schneeds]

30%RH is plenty low.  Don't be alarmed if you see it rise to the upper 30's; desiccant packets have a floor at which they can't absorb below, and its usually 35-40.  

[Alex A]

You using choji oil?

 

Aoi recommends you dont.

 

Just in case you are.

[French nihonto]

Il y a 4 heures, Alex A a dit :

Utilisez-vous de l'huile de choji ?

 

Aoi vous recommande de ne pas le faire.

 

Juste au cas où vous le seriez.

Yes, I applied a very thin layer of oil to the blade—though I did hesitate. I believe Aoi advises against it because the oil dries relatively quickly; once the blade is exposed to higher humidity, that dried oil layer tends to absorb moisture. However, I think this is easily remedied by simply wiping the blade down with 99% alcohol immediately after taking it out.

[French nihonto]

The only minor issue I see with this is that, in an airtight or nearly airtight space, the oxygen will eventually be consumed—by microorganisms, etc.—and turn into carbon dioxide. In the presence of moisture, carbon dioxide converts into carbonic acid. However, I think it would take a very long time for the oxygen to be used up, and in a worst-case scenario, since the humidity is very low, the chemical reaction that creates acid wouldn't occur. To verify this, I’m going to buy a small oxygen indicator.

[John C]

Great. One more thing to worry about - micro-burps. 

 

John C.

[reinhard]

Whatever you put on a blade.

Some of it will stay in the saya.

 

reinhard