From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 07/20/01-08:19:14 PM Z
At 05:19 PM 07/20/2001 -0400, you wrote:
>>Sandy King wrote:
>
>>Also, when developing film on motor bases like the >Beseler or Unicolor
bases you should remove the tube >from the motor every couple of minutes
and give it >about 10-15 seconds of up and down agitation, >otherwise the
constant agitation fore and aft will >cause bromide drag.
>
>Sandy,
>
>Perhaps this has been discussed earlier. Can you explain what bromide
drag is? I use a Jobo CPP-2 which rotates in one direction about 1 and a
half turns, then stops and rotates in the other direction about 1 and a
half turns and repeats the cycle. Would this action help to eliminate
bromide drag? I don't think I have ever had a problem with a negative that
would have been
>related to something like bromide drag. But I would like to understand
the issue better.
>
>Jim
This is a problem that occurs because there is not very much developer
used in a drum and because agitation, especially motorized agitation, goes
back and forth only. Bromide ions come from the silver bromide in the
emulsion. Its a restrainer. The bromide, and other reaction products of
development, diffuse out of the emulsion but may be trapped at the surface
unless rather vigorous agitation is applied to move them away as a faster
rate than simple diffusion into the rest of the developer.
The back and forth motion of a fairly laminar layer of developer does not
move the bromide laden developer away quickly enough. So, it forms a line
around any high density area on the film and tends to restrain development
in areas adjacent to it in the direction of the agitation. If the drum is
agitated sideways occasionally it breaks up this pattern and eliminates the
low density streaks which otherwise frequently appear.
In an system like a roll film tank, the agitation is not so uniform and
also tends to have turbulant flow, especially in invertible tanks.
This problem used to plague motion picture processing machines where the
bromide would be dragged along in the direction the film was moving leaving
streaks from high density areas. It raises hob with sound tracks. In these
machines the problem was cured by using spray jets rather than depending on
the motion of the film alone for agitation.
Something similar can happen in tray development of a single sheet if the
tray is rocked back and forth in the same direction rather than being
rocked in both directions to get some randomness.
I think the problem is not appearent in print devlopment because prints
are developed as far as they will go so the effects of reaction products
tend to be masked.
Evidently Jobo has a method of eliminating the problem. I find I do get
bromide streaks when developing film in Unicolor or Bessler drums (I have
both) unless I manually agitate sideways periodically.
---- Richard Knoppow Los Angeles,Ca. dickburk@ix.netcom.com
This archive was generated by hypermail 2b30 : 08/02/01-11:56:46 AM Z CST