[pure-silver] Re: baking soda as wash aid

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <pure-silver@xxxxxxxxxxxxx>
  • Date: Thu, 17 Feb 2005 12:52:56 -0800

----- Original Message ----- 
From: "Gene Johnson" <genej2@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, February 17, 2005 7:40 AM
Subject: [pure-silver] baking soda as wash aid


>I saw on the Summitek site a suggestion for using Baking 
>soda in the print wash water as a wash aid.  Any thoughts 
>on this? Economical and low tox if it works well.
>

    It may be alkaline enough to work the way a carbonate 
bath does but does not have the ion exchange properties of 
sulfite. I don't have the Kodak paper on wash aid readily 
available at the moment but I think sodium bicarbonate was 
among the salts they tested.
    The idea of the Kodak wash aid is that it is alkaline 
enough to obtain optimum washing conditions but not so 
alkaline as to remove the hardening from alum plus the 
sulfite is effective in displacing thiosulfate and fixer 
reaction ions.
    A simple alkaline bath, such as the carbonate bath 
recommended by Agfa, will adjust the pH of the gelatin to a 
value where it no longer binds thiosulfate and where the 
"mordanting" action of alum hardener is eliminated, however, 
it undoes the hardening and has no ion exchange effect.
    Kodak Hypo Clearing Agent is essentially a 2% solution 
of Sodium Sulfite buffered to neutral with Sodium bisulfite. 
It also includes two sequestering agents to prevent a sludge 
forming from minerals in the water when it is re-used.
    Many years before the wash aid was developed Kodak Labs 
did a test on various alkalies as wash aides. The one they 
found most effective was Ammonium carbonate, perhaps because 
it washes out fast itself. This is mainly of historical 
interest because of the later research showing that buffered 
sulfite was much superior to any alkali bath.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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