I have found in my archives a copy of an article that used to be on Ryuji Suzuki's web site. In it he says, among other things, that the carbonate bath was more relevant in the days of handening fixers, and that sulphite can assist in washing out a complex formed in fixing. I have not edited the article except to remove some heading links.
Sorry about the formatting of the table. Extra spaces are eliminated by the system.
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From Silvergrain Labs
Washing aid is used most commonly in archival processing of fiber based paper, after fixing to reduce washing time and save washing water. It is not necessary for film or RC paper.
Although washing aid is used in archival processing, there are more requirements than proper washing process to make archival images. The use of washing aid does not mean that the resulting material is archival. See archival process for guideline.
Both washing aid and hypo clearing agent refer to the same chemical process described in this article. However, hypo eliminator is a very different chemical process although they are both designed to accelerate washing of thiosulfate from fixed materials. The use of the latter, hypo eliminator, is discouraged. For hypo eliminator, see the section at the end of this article.
About the methods of testing for residual thiosulfate (hypo), refer to residual thiosulfate tests. About the washing process, refer to wash.
How it's used
If the print is not toned, follow the procedure after fixing. If the print is to be toned, fixed print should be washed for 2 minutes (RC) or 5 minutes (baryta) in running water. The washed print should be toned and rinsed, followed by the procedure below.
Fixed print is rinsed in water, and immersed in washing aid with intermittent agitation. The treated print is washed in running water.
Washing time (with non-hardening fixer)
material_____________ water rinse_________wash aid________ final wash
film ___________________ - _____________ - _________ 5 min
RC print __________________- ______________ - ______________ 5 min
Baryta (single weight) 1 min ____________ 2 min ________ 10 min
Baryta (double weight) 1 min ____________ 3 min_________ 20 min
The temperature of the final washing water can be between 13C to 30C, or however warm, as long as the emulsion can sustain.
Without using a washing aid, the washing time required would be at least twice as long, and the washing water would have to be at least 18C. That is, if the temperature of the washing water is low, use wash aid.
A standard washing aid is:
sodium sulfite 15g
sodium metabisulfite 5g
water to make 1.0 l
target pH 6.5 to 8
When scum is formed on the solution or processed material, addition of about 0.5g/L of disodium or tetrasodium EDTA with about 1.0g/L of sodium citate is suggested. Stock solution including the EDTA and citrate tends to keep longer.
Kodak Hypo Clearing Agent contains EDTA and sodium citrate to reduce the calcium scum. With hard water, sulfite in the wash aid forms insoluble calcium scum unless EDTA is added. These are also useful in preventing precipitation of aluminium hardener that may be carried over from the fixer (if acid hardening fixer is used).
Similar product, FUJI QW is sold by Fujifilm. The MSDS indicates that the product is similar to the above: 60-80% sodium sulfite, 16-26% sodium metabisulfite, 1-5% sodium citrate, 2.3% metaboric acid, 1-5% tetrasodium EDTA, 1-5% water.
(As an environmentally safer alternative, there is Silvergrain Clearwash sold from Digitaltruth. This product is free of EDTA and borates, but it uses other safer agents that are equally effective in producing high quality results without scum formation.)
Negatives that already have white stain that cannot be removed by water wash may be treated in a solution containing disodium EDTA and sodium citrate for 5 minues and washed in water.
A simplified version of washing aid is simply 2% solution of sodium sulfite, which has a bit too high on the pH, but works quite effectively, if prepared fresh every day. If the scum problem occurs, it is best to use the above formula with EDTA and citrate added.
How it works
Washing aid is usually a 2% sulfite bath, preferrably with the pH buffered to 7 to 8 region. The reason why thiosulfate from fixer takes a long time to wash out is because thiosulfate is tightly adsorbed to silver surface, gelatin matrix, alum hardener (if used), sizing material and the cellulose fiber. The sulfite anion in the washing aid can displace the adsorbed thiosulfate, and the wash water can more effectively wash out adsorbed sulfite as well as desorbed thiosulfate.
Without washing aid treatment, desorption of thiosulfate is a very slow process, and it also requires certain energy. That is, washing is very inefficient at temperature lower than 18C. However, once treated in washing aid, cold washing water can now wash out sulfite and desorbed thiosulfate as effectively.
Washing aid has quite large processing capacity. The limiting factor is dilution by water carryover, and contamination by thiosulfate. A quick rinse of prints before washing aid treatment is very helpful in expanding the working capacity.
Washing aid has shelf life similar to fine grain developers. Sulfite in the solution can be oxidized by air. The oxidized washing aid is better than no treatment, but it is not very effective.
Washing property of fixer
Without washing aid, the kind of fixer that is fastest to wash out is:
* ammonium thiosulfate fixer
* absence of acetic acid
* contains sulfite
* preferrably pH buffered to 6.5 to 8.5 range
Besides sulfite, sulfate, bicarbonate were commonly seen in literature reporting on the subject. Bicarbonate is a reasonably effective washing aid. A 2% solution of sodium bicarbonate (baking soda) may be used, although, sulfite bath is preferred whenever possible. Sulfite is superior in that sulfite can desorb not only unreacted thiosulfate but also sparingly soluble and adsorbed argentomonothiosulfate complex.
Sea water or solution of common salt is occasionally said to have the same effect as washing aid. For practical use, it is advised to ignore this information. While sodium chloride may have very small enhancement in washing compared to distilled water, it is vastly inferior to sulfites. Also, residual chloride salts can be harmful to the image; a very thorough water washing process is required following the sea water bath, negating the purpose of using a washing aid bath.
There are several studies that investigated the washing accelerating properties of alkaline bath. For example, Crabtree compared sodium hydroxide, ammonium hydroxide, sodium carbonate, Kodalk (sodium metaborate), and distilled water. This result was probably valid at that time, but the factors involved in washing property of fixed film and paper are somewhat different nowadays, because today's emulsions are highly hardened during manufacturing and use of hardener is becoming uncommon in manual processing. Therefore, it's probably not very meaningful to discuss those methods in detail. If there is anything worthy of noting, it is probably that Crabtree found that ammonium hydroxide was most effective of those.
This may be related to ammonium ion's affinity to silver ion. However, ammonia vapor is quite unplesant to use as a washing aid in practice. It may be interesting to investigate efficacy of organic amine compounds as an additive to the standard sulfite wash aid to see whether such an additive can farther expedite the washing process. However, the magnitude of the efficacy is somewhat questionable when rapid fixer is used, because ammonium ion is carried over from rapid fixer, which is known to wash faster than non-rapid fixer.
Benefits of residual thiosulfate
It is known that a trace amount of residual thiosulfate may indeed protect the silver image. When this issue is discussed, the level being talked about is about 10-20 mg per square meter, right about the borderline for archival standard. However, this should not be taken to mean that importance of thorough washing is now dismissed. Excess thiosulfate is known to cause image fading, staining or browning problem. It is best to treat the image in archival toner, and remove residual thiosulfate to the archival standard.
However, untoned RC prints are easy to overwash if washing aid is used. This author recommends that RC paper should be fixed in non-hardening fixer and washed for 5 minutes in running water, without using washing aid.
Hypo eliminator usually refers to a solution containing hydrogen peroxide or peroxide-releasing compound such as percarbonate, perborate or persulfate. These solutions may also contain ammonia, other alkaline agents, bromide, iodide, and other additives.
The idea behind hypo eliminator is to oxidize thiosulfate to harmless compounds that are not adsorbed by silver surface. However, it was later found that peroxide solutions damage and undermine image-forming metallic silver. Today, the use of hypo eliminators are discouraged.
Indeed, peroxide is a leading environmental oxidant that attacks image, and because it's a very common cause of image degradation, its fume is used for accelerated testing of image permanence. It's called peroxide fuming test.
Last edited by john_s; 03-18-2013 at 04:10 PM. Click to view previous post history.