You made an interesting point with your ‘echo chamber’ analogy, but
perhaps not exactly the one you intended.
But getting back to mokume. Mokume was developed as material from
which tsubas ( sword guards ) were made. The function of a tsuba
is to stop advancement of the blade. If we consider the fact that
average katana can slice a gun barrel as easily as bread, it
should definitely give us a pause and consider how likely it is
that mokume is just a sandwich of non-ferrous metals.
There’s a program called ‘Mythbusters’ in the states that takes
internet myths, and other tall tales, and puts them to the test. One
of the myths they chose to test was the idea that katana could chop
through gun barrels.
It turns out that after some pretty serious testing, no, they can’t.
They also couldn’t find any record of any such thing actually
happening, or the remnants of any of the cloven barrels in question.
(even the Japanese sword societies can’t prove the stories, much
though they’ve tried.)
If you stop and think about it, the human body simply doesn’t have
enough power to drive a 1cm wedge through a bar of steel 1/2 to 1.5"
in diameter in one shot, regardless of how sharp the cutting edge
may be. (I have several real katana in my collection, and have
examined many more. They’re often surprisingly thick. The junker
Shingunto that I have by the desk here measures out at.318" just
above the habaki, and it’s pretty thin as katana go.) The issue isn’t
the cutting edge, it’s the power required to wedge the cut open to
get the thickness of the rest of the blade body through the cut.
Having forged katana of my own, I made some ‘practice blades’ to
beat up on, and see what they could do. Against soft or fleshy
targets, they’re terrifying. When they start running into metal
armor, or other blades, they’re subject to the laws of physics just
like any other blade. Just specifically to test the notion of
nonferrous tsuba being effective, I took a full-on 90 degree whack at
the edge of a 1/4" thick disk of brass held in a vise. Hitting
tangentially, I took a crescent out of the side of it, about.750
long, and perhaps.200 thick at the thickest. Sort of like a curl of
cheese. That wasn’t what I was trying to do. (I also cracked the edge
of the test sword.) Doing a full ‘over the head’ chopping move
straight down onto the disk, I managed to get the blade to cut
straight in about.300" or so. Cracked the edge of the blade again
too. Took a chunk out of it, in fact. (The bit that was buried in the
brass disk cracked out between the two closest ashi.)
It should also be pointed out that the earliest tsuba were iron.
Those were the tsuba that were in use during the medieval warring
states period. The rise of the Tokugawa Shogunate put an end to
massed battles of katana wielding samurai, in about 1600 AD. After
that, katana became largely ceremonial and decorative. (still
beautiful and lethal, but less strictly functional.) Mokume wasn’t
invented until about 1700 or so, and doesn’t start appearing in
sword fittings until that time, so it was never intended to stand up
under battlefield conditions. It was always decorative. If you
understand the forge-welding process the swordsmiths used to put the
katana together, extending the same techniques to non-ferris metals
via the traditional techniques of mokume-gane make sense as a very
logical extension of an existing technique.
If you’d like, could recommend several very good books on the
history of katana in general, tsuba, and traditional mokume-gane.