Japanese books say to leave the nakago (the tang of the sword) to age, because the condition of the nakago indicates how old it is.
I think this is good advice for the mid-1600s. But we are past the Edo period now. Swords are historical treasures. If we continue to let nakago “age gracefully” then there is a future where they turn into dust.
Not now, not next century, but not so far past that.
The black oxidated state of nakago are fairly stable, but not perfectly stable. Otherwise, there would be no “graceful aging” at all. Logically, we cannot have it both ways, that the nakago is safe from eroding away and that different period nakago will show different aging conditions. If these nakago were not slowly rotting away on a centuries timescale, then they would hit a stable point and then never change.
In examining the NBTHK Juyo Token it is very frequent that signatures have become partially or completely obscured on old blades just from rusting away. The older the blade, the more likely the signature has wasted away.
We see this as well on Shinto blades that have not been cared for properly. The lack of care has accelerated the rotting away of the nakago in these cases. But even in cases of well maintained blades, when we get to the Nanbokucho and even the Muromachi period, it’s very frequent that parts of signatures are missing.
The major disaster here is that we are losing irreplaceable information. As well as irreplaceable examples. We are left in some cases now to guess the maker as important parts of the signature that would settle the question of who made the blade are missing.
At this point we should be simply looking to lock everything in at the status quo and halt all slow erosion of the nakago as well as fast setting rust of the upper part of the blade, by making sure the nakago gets oiled as well as the upper.
After oiling the upper part of the blade, you should take a fingertip of oil and use this to wipe onto the nakago. You may not have to oil the nakago as frequently as the upper, but this should be part of standard care practices at this time.
Similarly, uchiko is a disaster that is accelerating the number of polishes being requested and executed, and thereby reducing the lifespan of existing swords.
These processes have not been questioned, it is just something that people do because it’s how it’s always been done.
Museums now will not let people take photos of paintings using flash photography because the photons from a million flashes will over time degrade the pigments in the paint. This is the same attitude we need to take to swords.
That means, oil the nakago and don’t use uchiko ever.
You wouldn’t rub the Mona Lisa with sandpaper. So you don’t do this with swords either.
Kinpunmei and shumei
There is a special case with lacquer inscriptions. These are put in place by appraisers. Putting a sword in and out of a tsuka causes the nakago surface to rub and eventually removes these appraisal signatures.
This is a major problem, because old Honami appraisals are also valuable parts of the history of the blade.
There are innumerable examples of Juyo blades now with partially or fully illegible appraisals on them. Simply from the storage methods we use.
This is a travesty and should not happen. It is a reflection of inadequate care procedures.
I have had examples that passed through my hands with just traces of shumei left on them and when I saw the blade again 10 years later they were now completely gone.
I don’t have a simple solution for this problem.
Other than a loose fitting tsuka or a channel carved in the tsuka to specifically clear the mei. If you are having a shirasaya made for a blade with kinpunmei or shumei on it, you should consider asking for this. A channel carved over the attribution will protect it from wear. It will loosen the tsuka somewhat but unless you’re intending on cutting anyone down with the sword, this is I think a tradeoff you can live with.
As well, do not oil over one of these lacquer inscriptions. Lacquer should not react with oil, but you should play it safe and furthermore, rubbing the mei with your fingers all the time will do harm over the long run.
Anyway it’s something that bears some discussion in the community, because currently there is almost no action in terms of preserving these things and it means that centuries from now, they will be gone.