[pure-silver] Re: Wash aid


----- Original Message ----- From: "Koch, Gerald" <gkoch02@xxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, February 28, 2006 6:49 AM
Subject: [pure-silver] Wash aid


I did some searching on the web and found the protocol for using sodium
carbonate as a washing aid.

1. Wash the prints for 10 minutes.
2. Soak the prints for 10 minutes with frequent agitation in a 1%
solution of sodium carbonate.
3. Wash the prints for an additional 5 minutes.

Jerry

I don't think this will provide an adequate wash.
The purpose of the carbonate bath (Borax or Metaborate will work as well) is to change the pH of the emulsion. Sodium sulfite will do the same thing.
Gelatin has the property of taking on the pH of the last bath its in but has an inherent pH called the isoelectric point. The pH at the isoelectric point depends on how the gelatin was made. Most photographic gelatin is slightly on the acid side of neutral. When the gelatin is on the acid side of its isoelectric point the electric charges in it tend to attract thiosulfate ions and bind them. When its on the alkaline side the charges tend to repel the Thiosulfate ions. In addition, white alum hardener has a mordanting effect on thiosulfate. This effect takes place within a window of pH. When the emulsion is brought above a certain value (I've forgotten what it is exactly) the mordanting is eliminated, however, the hardening is lost. This effect, of couse, does not occur where there is no hardener in the fixing bath.
If the fixer is neutral, as it can be if no hardener is used. The carbonate bath becomes unnecessary because at neutral pH the emulsion is already above the isoelectric point. The effect of a carbonate bath on washing rate has been known since at least the mid 1930's.
A carbonate bath will reduce wash times about 50% from those recommended when a hardening fixer is used. Probably a brief rinse before use is indicated but a ten minute wash is not necessary. Wash times after the carbonate, for fiber paper should be around 1/2 hour for single weight paper and about 1 hour for double weight paper. It should not be used for film or RC paper. I think the times given on the internet are confusions with the Ilford process or something else.
A plain Sodium sulfite bath of about 2% will give nearly the same effect as Kodak wash aid. Kodak's product is buffered to neutral. This has the advantage of eliminating the mordanting effect of alum hardener without destroying all the hardening and of leaving the emulsion close to its minimum swelling. The sequestering agents are probably necessary only if the solution is used more than once, but may also have some additional effect on wash rate, I am not sure of this.
In any case, I suggest the use of Sulfite rather than carbonate. The wash times should be similar to those recommended by Kodak for KHCA. Kodak recommends a brief rinse to increase the capacity of the wash aid (of no relevance where its used one-shot) A treatment time of between 1 and 3 minutes in the wash aid depending on the material, and a final wash of 5 minutes for film, 10 minutes for single weight paper, and 20 minutes for double weight paper. These times were evidently worked out with knowledge of the protective effect of a very small residue of thiosulfate. Toning is a very much more effective means of image protection. If the longest life, and greatest protection from oxidation and sulfiding is to had some change in the appearance of the print will have to be tollerated. Kodak Brown Toner and similar polysulfide toners, will provide considerable protection when with only partial toning. Kodak selenium toner will not provide adequate protection when used at high dilutions (1:19) but will when toning is carried out with stronger solutions to the point where there is a definite color change. (My source for this is Dr. Nishimura at IPI). Gold toner also provides excellent image protection and its bluish color shift may be more acceptable for some images, but its expensive.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
Sodium Sulfite has another effect, which is not duplicated by Carbonate, namely, it is an ion exchanger and actually displaces Thisosulfate ions from the emulsion, and to a lesser extent, from the fiber base.

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