Re: lith film and pinholes

[Richard Knoppow <dickburk@ix.netcom.com>]

At 08:31 PM 05/09/2002 -0500, you wrote:
>I seem to remember something about air bubbles 
>forming in the presence of acids in gelatine. 
>The pinholes could possibly be forming in an acid 
>hardening fixer or a stop bath. I use plain water 
>to slow the developing and then place directly in 
>the fix. I have noticed that the more contrast in 
>the film the more likely that there will be pinholes. 
>Presoaking the film in a water bath before developing 
>could prevent the air bubbles from forming under the 
>surface of the emulsion. When submerging silver photo 
>paper in developer I have seen bubbles form. Where 
>they come from I do not understand. Yet they stream 
>to the top.  
>
>The hypothesis of an exhausted developer sounds viable 
>too. I don't get air bubbles with very diluted developer 
>but, then, exhausted developer has had its chemical 
>composition changed through use. humm... 
>
>These are just clues, for what it is worth.
>
>Mark A. Morrill
>Gummist since 1988
>
  Pin holes _can_ be caused by the evolution of gas when a film developed
in a carbonate containing developer is put into the stop bath. Both Acetic
acid and Citric acid will evolve carbon dioxide, the same reaction which
makes Alka-Seltzer bubble. Sometimes you can hear the stop bath sizzle when
prints or films are put in it. 
   Since most commercially packaged pictorial film developers use some
alkali other than carbonate or hydroxide, this is not usually a problem. 
   Print developers mostly do contain carbonate as do high contrast film
developers, so, at least in principle, the bubbles could form. 
  Whether or not the emulsion is actually disrupted depends on several
factors including the hardness of the gelatin and the amount of gas evolved. 
  It would seem that actual disruptions of the emulsion of pictorial films
is pretty rare.
  Lith film has a thinner emulsion than pictorial films, which might make
it more vulnerable to developing pin holes, and lith developers are usually
much more alkaline than pictorial developers leading to stronger reactions
with the stop bath or acid fixer.  
  The cure, where a carbonate or hydroxide containing developer must be
used, is to use a water bath to wash out as much of the developer as
possible before using either a stop bath or acid fixing bath. This should
be more than a quick dip. A short wash is more like it. 
 There can be other causes of disruptions of the emulsion and other causes
for spots than actual pin holes.
----
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com