Re: More thoughts on reversal

From: Liam Lawless <liam.lawless@blueyonder.co.uk>
Subject: Re: More thoughts on reversal
Date: Fri, 26 Mar 2004 00:37:40 +0000

> Marek's hit on something I'd forgotten about. It doesn't happen
> straight away, but after a few sheets of film have been through the
> bleach it starts to become cloudy and puts down a white precipitate of
> silver chloride while film is bleaching. Tiny particles that you're
> unlikely to notice at first; they can be wiped from the surface of the
> film during the following wash, as Marek says, but clearing and
> redevelopment somehow consolidates them and makes them much harder to
> remove (and turns them black!) I wouldn't expect the resulting black
> specks to look like grain, exactly, but to be distributed across the
> film according to how bleach has flowed over it during bleaching, and
> the way it has run off when draining. Try fresh bleach if you suspect
> this could be the cause of your troubles.

This is actually a well known problem. I don't have it at home but
Crabtree and other people published a paper describing it.

In short, you can replenish your bleach by adding sulfuric acid and
keep reusing until the bleaching is noticeably slow. Also, continual
agitation, not heating the solution (keeping the temperature low), or
both helps.

One reason this happens is that, when the silver-dichromate reaction
is rapid but diffusion (agitation) isn't fast enough, silver ion can
be super saturated within the gelatin layer and precipitates out.

Another reason is that, when the pH of the bleach solution is high,
the above phenomenon gets worse. Dichromate bleach's pH rises as you
use it, so the problem gets worse as you use the bath.

Oh, some dichromate bleach contains rather strong acetic acid or other
acid buffer to alleviate this problem.

--
Ryuji Suzuki
"All the truth in the world adds up to one big lie." (Bob Dylan 2000)