[Leica] Ups and downs of stop baths?

[rhaightjr at yahoo.com (Bob Haight)]

I just use plain water which seems to work fine.

--- Steven Keirstead <keirst@fas.harvard.edu> wrote:

> >From what I have read, a long time ago Kodak and 
> other manufacturers had different gelatin and 
> film bases that were not as cohesive as modern 
> films and emulsions. The gelatin was softer, 
> weaker and could tear more easily. Sometimes when 
> the developing film went from a carbonate 
> buffered developer into an acid stop bath, CO2 
> was formed by acid-base reaction from carbonate 
> and the effervescence in the would tear gelatin 
> specs off the film base, leaving a clear spot in 
> the negative. This would be more likely to happen 
> if the temperature was above 68?F (20?C) for the 
> process solutions. Modern films have pre-hardened 
> gelatin, so this is less likely to happen now, 
> unless you are working in tropical temperature 
> conditions.
> 
> If you are making your own developer solutions, 
> you can switch to a Borax or Borate buffering 
> system, which will not effervesce. There may be 
> commercial developers with Borax buffer, but I 
> don't know of them.
> 
> While there may be some developers that are 
> acidic or continue to have some activity in acid, 
> most are still carbonate buffered, and the pH 
> change and developer dilution of putting film 
> into a stop bath ensure a precise end to 
> development, while water alone may allow more 
> continued development. I usually use a 1.5% 
> Citric acid stop bath. A 15% (or 30%) Citric acid 
> stock solution seems to be quite stable, does 
> appear to grow microbes, does not smell like 
> acetic acid and is quickly diluted 1:9 from 15% 
> (or 1:20 from 30%) for working stop bath.
> 
> 
> >At 04:05 PM 11/22/2005, Henning Wulff wrote:
> >
> >>The main problem is that pinholes can develop. 
> >>If you do use any 'stop bath', dilute it a lot.
> >>...
> 
> Richard <richard-lists@imagecraft.com> wrote
> 
> >What exactly are pinholes? Actually holes in the
> negatives or the emulsion?
> >
> 
> -- 
>
______________________________________________________________________________
> Steve Keirstead
> Research Assistant II, Biology Teaching Labs,
> Harvard Science Center, Room 409, 1 Oxford Street,
> Cambridge, Ma 02138.
> Work Phone: (617) 495-2683 , FAX: (617) 496-9105
> 
> 
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> 



        
                
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