Re: Richard circular print washer
----- Original Message ----- From: "Judy Seigel" <jseigel@panix.com> To: <alt-photo-process-l@usask.ca> Sent: Sunday, August 10, 2008 5:02 PM Subject: RE: Richard circular print washer
Vestal was just repeating the popular wisdom of the time. Its wrong and he corrected it in later books. Removing hypo from film or paper requires a rapid change of water at the surfaces. The emulsion of both washes out by a diffusion process so the rate of exchange depends on the difference in concentration of hypo in the emulsion vs the water. The greater the difference the faster the hypo diffuses. This also causes the washing rate to slow even when a very good supply of fresh water is supplied because the concentration in the emulsion becomes less as washing progresses. It is an exponential rate. When a stagnant soak is used the hypo diffuses out into a cloud around the surface. This increases the concentration in the wash and the diffusion rate becomes very slow. Of course, the diffusion is a continuous process so the hypo continues to diffuse into the body of the water until something approaching equilibrium is reached at which point the process essentially stops. Fred Picker's idea that hypo is heavier than water and will sink is wrong because hypo is miscible. There might be some slight convection at the surface but its probably not significant. The same condition will be true for washing out any substance which comes out by diffusion. Long soaks have been demonstrated to cause damage to the support and are to be avoided. The paper support of "fiber" prints do not wash by a strictly diffusion process. Ilford showed that the hypo becomes bound up in the fiber structure of the paper and comes out by frictional forces, again requiring flowing water. This process is much slower than diffusion from gelatin partly accounting for the long wash times required for fiber support. Again, because stagnant washing does not produce the mechanical effects needed to remove the hypo it doesn't work very well. Also, the heroic washing recommended by Vestal, Adams, and others, was based on the old idea that absolutely no hypo must remain in the emulsion or support if long image life is wanted. This was proved untrue by research at Kodak confirmed by independent research at Fuji c.1961. Sulfite has a specific ion exchange property for thiosulfate and silver-thiosulfate complexes thus very much accelerating the wash rate. This effect is specific to thiosulfate and its complexes. For conventional silver-gelatin film and paper, and probably for other media using silver as the image bearing material and fixed with hypo the use of a sulfite wash aid is probably appropriate. There is a simple test for residual hypo. A solution of silver nitrate usually preserved with some acetic acid. This test leaves a stain where there is an excess if hypo. Kodak has instructions for mixing and using the solution in several of its handbooks. Because the test spot will eventually darken with time its necessary to make a judgment a minute or two after applying the test. A more elaborate version was also published by Kodak in which the stain if fixed and stabilized in a sodium chloride solution. This test can be used for comparison to standards using a densitometer in order to obtain a quantitative measurement of residual hypo. Note that for conventional silver based emulsions the quality of the fixing bath is extremely important because incompletely fixed silver is not completely soluble so can remain in the emulsion eventually damaging the image. Sulfite wash aid will debond some of this material but using a two bath fixer with attention to maintaining low dissolved silver content is extremely important for image permanence. A note on the Richard washer: These appear to have been popular in the 1950s and 1960s. It is a large tub where the waster enters at the center and swirls around to leave at the periphery. I think they were made mostly to wash quantities of relatively small prints. Its very important in washing that a free flow of water is available to both sides of the print for reasons elaborated above. The problem with washers like the Richard washer is that prints can stick together, float, or get stuck to the bottom. High flow and turbulence rates tend to prevent sticking but not floating so one must pay constant attention to the prints. For single prints or small quantities of small prints a Kodak Print Siphon in a medium sized tray probably works as well as much more elaborate washers. Vertical, so called archival, washers were designed with the idea that each print would be separated from the others and a good water flow maintained at both surfaces. Some work OK, some don't. In general, the rate of exchange of water is too slow because of the relatively large total volume of these washers. Kodak recommends that the water in a washer change in five minutes. Measurements in my Zone VI 16x20 washer show it doesn't come anywhere near this rate. In order to maximize the rate I use it in a bathtub with the exhaust hose removed and flow high enough to cause overflowing. I also pull the plug and drain it about halfway through the wash cycle. I tested it by putting some beet juice in it and seeing how long it took for the dye to dissipate. I can't speak for alternative processes but Ryuji Suzuki makes the point that the materials to be washed out in many of them are not the same as for silver-gelatin so a sulfite wash aid may be useless. I am unaware of any research on either the most effective method of washing platinum or other alt processes or tests for completeness of washing but they may exist. I would expect Judy, who has researched several of these processes and likes reliable data, to have something. --- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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