[pure-silver] Re: High B+F with DS-10
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 3 Feb 2005 18:37:45 -0800
----- Original Message -----
From: "John Black" <jblack@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, February 03, 2005 3:05 PM
Subject: [pure-silver] Re: High B+F with DS-10
>
>>
>> > Fresh, relatively high sulfite fixer and the use of a
>> > hypo clear will
> reduce
>> > fog some too.
>>
>> What's your definition of fog?
>>
>
> For photographic purposes, my definition of fog is a bit
> broader than just
> the conversion of non-light exposed silver halide to
> metallic silver. I also
> include any general (not image specific) density in a
> negative. This
> definition also includes dye remnants and stains.
>
> I once scrubbed the emulsion off of several 35mm films in
> order to measure
> the base density so I could determine the true emulsion
> fog density by
> difference. I was amazed at the presence of gray and
> lightly colored
> density I found in the plastic of the base. Tri-X had a
> gray base density
> of about 0.20 and a total fb+f of about 0.30 giving an
> emulsion fog level of
> 0.10. I have found that my fixer, even though it still
> fixes fine, will
> contribute to fog if it gets too much age on it. This
> seems to be related
> to sulfite exhaustion because it is reversible by adding
> more sulfite. These
> older fixers also plate metallic silver on the inside of
> my storage bottle
> so I suspect a high silver content may contribute to the
> fog.
>
> JB
>
Most 35mm negative black and white film has a pigment in
the support to prevent the conduction of light laterally
through the material. The density is around 0.2. This
pigment is permanent and is not affected by processing
chemicals. It has no effect on the film other than to
increase printing exposure a little. Back coated
anti-halation layers do not affect conducted light. 35mm
films are used in a way where the end of the film may be
exposed to strong light. Without the pigment or other
precaution the light will be piped along the support in
enough quantity to fog some of the film.
Color films usually have an anti-halation layer under the
emulsion. This serves the double purpose of also preventing
exposure by conducted light. It is remove during processing.
In general fixing and washing have no effect on the fog
level. One _can_ have turbidity of the emulsion caused by
insufficient fixing and another kind of fog called dichroic
fog can be caused by exhausted fixing baths. Dichroic fog is
a thin deposit of very finely devided metallic silver on the
surface of the film. It is called dichroic fog because it
looks yellow by reflected light and cyan by transmitted
light. It can also have a rainbow effect.
Fog level in film is partly inherent in the emulsion,
partly due to changes due to aging, and partly to to the
effects of excessive heat on stored film.
Manufacturers put substanced into the film to stabilize
the emulsion so that it doesn't continue to ripen or
otherwise develop fog with age, and also anti-fog agents.
Inherent fog varies with speed being generally higher as
the film speed increases. Of course fog can also be produced
by exposure to radiation of various sorts or being just
plain light struck.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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