[pure-silver] Re: expiration dates on film boxes
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 14 Oct 2005 14:49:49 -0700
----- Original Message -----
From: "Koch, Gerald" <gkoch02@xxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, October 14, 2005 8:10 AM
Subject: [pure-silver] Re: expiration dates on film boxes
Hi Richard,
I thought about the case for color film. Then I got to
thinking that if
the sensitivity and contrast of professional color film can
change and
are considered critical why can't the situation be the same
for
professional B&W. Maybe I am over intellectualizing the
problem. ???
Jerry
Its the amount of change that is tollerable. Color film
requires that the three (or more) emulsions match rather
closely in sensitivity, contrast, and curve shape.
Differential changes affect both color balance and tracking.
Balance can be fixed by varying the exposure of the three
components but tracking is due to a change in slope of the
curves and is not so easy to fix, at least photographically
(easy using a digital image editor). Even fairly large
changes in these properties make relatively little
difference in B&W.
Emulsion properties change for several reasons. One of
the main ones is the continuation of the process called
ripening after the emulsion is coated. Emulsions contain
substances which suppress the ripening and also various
other preservatives, etc., but, to some degree, the changes
continue even though they may be very slow. The main effect
of age on B&W seems to be emulsion fog caused by the
ripening mentioned above. Fast films can also accumulate
some fog from cosmic rays but I think this is small compared
to fog from chemical changes. Stability of the emulsion is
an important consideration in film manufacture. For one
thing, film is made in batches. The longer it will last the
less risk the manufacturer takes that the batch will spoil
before its sold. Stability is also demanded by customers for
similar reasons. Refrigeration also slows down the chemical
changes.
Emulsions also contain substances to stabilize the latent
image. The latent image is formed by electrical charges in
the silver halide crystals causing changes in the crystal
structure. Eventually, these charges can leak off destroying
the latent image. There are substances that tend to prevent
this. Of course, this is very oversimplified but is accurate
as far as it goes.
Stabilizers have been added to film and paper emulsions
for many decades and improvements have been made
continuously. Most film and paper emulsion, at least until
very recently, has been very stable. Recently, some of the
substances used as stabilizers have become environmental
hazards so have been removed. Other substances or techniques
have been substituted but I don't know how effective they
are compared with the older ones.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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