Re: filmscanners: cleaning neg's, sharpening

["Alan Tyson" <AlanTyson@bknowl.freeserve.co.uk>]

As usual, the physics & chemistry is more complicated than
one would expect.

If we're after something that has as little effect as
possible on the chemical balance of the emulsion, pure water
probably isn't the best thing. It has zero concentration of
almost everything except water (it's a 55 Molar solution of
that), so almost everything will diffuse into it from
anything that has non-zero concentration. So the carefully
designed (or maybe accidental) mix of chemicals present in
the emulsion will be disrupted by prolonged soaking in pure
water. If you want to avoid changes in whatever it is that
you're soaking, you need a 'buffer solution' that roughly
matches in solute content the thing you're soaking.

How you find out the right concentrations of just what
solutes is anyone's guess. A *very* rough analogy is how
it's a bad idea to transfuse yourself with pure water if
you're dehydrated; an 'isotonic solution' that matches the
properties of your blood is required.

I'd guess that distilled, deionised, or (filtered) rainwater
would have indistinguishable practical effects on a
photographic emulsion, because they're all very different
from what's in the emulsion, and they're all water, more or
less.

Alan T
(retired research chemist, who sticks his head above the
chemical parapet occasionally in 'filmscanners').

----- Original Message -----
From: Lynn Allen <lalle@email.com>
To: <filmscanners@halftone.co.uk>
Sent: Monday, March 19, 2001 9:01 PM
Subject: Re: filmscanners: cleaning neg's, sharpening


> > Bear in mind that if you use anything but 'unexposed'
distilled water as a
> cleaning agent, you are in fact using carbonic
acid..!........

>...............don't use the "Pure Spring
> Water" drinkable stuff to wash negs--it's got little
disolved rocks in it,