On 28 Sep, 21:44, "Spamlet" wrote:
I did wonder if soaking the lens in xylene might gradually swell the dried
balsam back into a more see through condition. *If not, how does one
separate the elements and what are they stuck together with these days?

I think balsam was still used into the '80s, but the quality did vary
between makers (late WW2 German ersatz stuff is supposed to be
particularly iffy) and its fairly easy to work with.

You strip them down with heat, I would suggest using a waterbath on a
hotplate at a bit below boiling point. Go easy on speed of raising the
temperature though. LET THEM FALL APART AND DO NOT PRY AT THEM! Clean
up once apart with acetone (or I guess xylene too).

Re-assemble with canada balsam, which is available and easy to work
with. Your big problem here is centring the lenses, and the difficulty
of that depends on how accurately their edges were ground, relative to
the optical centre. If that's accurate, you can align them
mechanically with a few accurate blocks on a surface plate (and
remember that optical benches are stable, but far from flat). Best
alignment is done with some sort of laser interferometer and doing it
optically, but its years since I dealt with that level of geekery.

If they're synthetically bonded rather than with balsam, it's really
not worth bothering. However if you do, it's a long soak (and I mean
days upwards) is a solvent like xylene or most likely MEK. I've never
done this myself, and I think the success rate isn't encouraging.