Re: [Leica] Re: Re: Pre Wash??????

[RBedw51767@aol.com]

George:

Thank you for sharing your knowledge.  It is wonderful to start the day
learning something new.  I have printed your note and it goes into my
permanent "Darkroom" library.

The best to you this day,

Bob Bedwell
Greensboro, NC


<< >Very interesting George.
 >
 >I am sure you are correct as I have not tried pre-washing yet.  I would have
 >assumed that pre-washing would have done just the reverse of what you state,
 >allowing the developer to act faster vs. slower.  What has been your
 >experience in the development time-reduction, say at 72 degrees(%)?
 >
 It's quite a common belief that prewashing requires a decrease in
 development time.  Quite the contrary.  
    As a starting point, try a 10% increase in development time with
 prewash.  With TPX (120) film, this is what I determined was needed with
 prewashing.  I no longer follow this prewashing regimen with B&W films, for
 reasons explained below.  You have to do your own testing, find what works
 best for you, then standardize your results and remain consistent.  Be
 careful about relying on anyone else's recommendations.  There are so many
 things that will affect your results that you have to establish what works
 best for you.  For example, a wide range of things like your metering and
 the accuracy of your meters, the quality of your local water supply,  the
 accuracy of your thermometers, minor variations in shutter accuracy and
 lens speeds, your agitation technique, development tank size, and so on,
 will all have an affect.  These variables can offset each other, or else
 they can act together to throw your results off in one direction.
 Statistically, the overall effect of these kinds of variations is described
 using the root mean square law.
    For this reason, I have stopped prewashing entirely.  When I try using
 new film/developer combos that are not part of my normal repertoire, I am
 never sure if the manufacturer's starting recommendations are reliable with
 a prewash.  It is psychologically disconcerting too to see the water come
 out coloured after the prewash, especially with films like APX25 and
 TPX120.  The colours of the prewash water vary from pale orange to very
 deep blue, in most cases due to removing anti halation material from the
 film.  The effect that these chemical byproducts have during the
 development process would be the basis of some interesting research.  An
 strong argument for using a prewash would be to remove these materials
 prior to developing, so they do not act synergistically with the developing
 agents.  It would seem that if these agents were detrimental to film
 development then prewashing would be a standard recommendation.
 
 
  >>