Re: [APML] TP Stop Bath

["Andrew Schott" <aschott@cfl.rr.com>]

Only guessing but from my experience, contaminating the fix with developer
weakens it.  In the single shot mode, chemicals used for only one roll, it
probably doesn't make much difference.  But when you set up three or four
rolls of film and reuse the chemicals.  Most guys discard the developer
after each roll as it's designed to exhaust itself during development.  Fix,
however, is reused until it takes too long to clear the film.  The addition
of an acetic stop bath greatly lengthens the useful capacity of the fixer
and prevents any problems of cross contamination.  The acidic stop will
begin to harden the gelatin on the film but not anything like the fix will.

Many people spend way too much time in the stop bath.  They read in Kodak's
little yellow book that stop baths can last between 30 sec. to 2 minutes.
In my experience, that's waaay too long.  Stop bath is primarily designed to
make it easier to more accurately time your development.  With the Patterson
tanks I use, I can dump and refill a 2 reeler in about 4-5 seconds.  That
is, if I'm not waiting for every drop to trickle out as I would have to do
if I wasn't using a stop bath.  As long as the film is still exposed to
developer, it's still developing.  If it's not fully immersed, it's going to
be developing unevenly.  Once you start to drain a tank, you're no longer
developing in a controlled, uniform fashion so you want to get the
draining/refilling over with as quickly as possible.  Here's where stop
comes in.  When you get within seconds of the required development time, you
dump the tank.  In a second or two, the outflow will go from
charging/chugging/gushing flow to a rapidly reducing stream as the tank
empties, down to a trickle as the developer drips off the spools inside.  As
soon as the charging stream slows from pouring to a trickle, upright the
tank and dump in the stop.  (Notice I said *dump*, not pour, or fill.  You
should be able to swap contents of your developing tanks quickly, in less
than 2 seconds, or you're better off using open tanks in the dark.)  The
object is not to wait until every last drop of developer has drained from
the tank.  Don't worry about contaminating the stop with developer, that's
what it's for.  Any developer left in the tank will be quickly neutralized
and diluted in the much larger acetic stop solution.  Continually agitate
(read vigorous shake-swirl-slosh) for 1-3 seconds then dump the stop out.
You can let it drain a little more now as the film is in a suspended state
and is no longer processing.  After 2-3 seconds of draining, I fill the tank
with fix and agitate for two minutes.

As far as the importance of making the emulsion acidic...  I can only thing
that this must be a reference dating back to the old days of much thicker
emulsions or different chemistry than we use today.  I remember there was a
time when some photographers used a two step fix, one hypo, the other
hardener but that's limited nowadays to print processing where toning is
used.

----- Original Message -----
From: "Jim Janusz" <jjanusz@dc.rr.com>
To: <astro-photo@seds.org>
Sent: Monday, April 08, 2002 5:12 PM
Subject: [APML] TP Stop Bath


> Another tidbit from Jobo that I did not realize.
>
> Stop-Bath
> The stop-bath quickly and evenly ceases development. The emulsion is made
> acidic for the fixer.
>
> Does anyone have an idea of how important it is to make the emulsion
acidic?
> I've been using water as a stop bath.
>
> Jim
>
> Astronomy  Web Page at:
> http://www.astroimager.net/
>
>
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