read what was stated above then went out to verify what is on the Ilford site and found a discrepancy. Ilford in their Hypam rapid fix PDF gives a capacity of 40 8x10 sheets per liter for fiber, 80 sheets for rc.
The document also states that a 2 bath fix is much more economical.
Richard, could you respond to this please and help me clear up any
misunderstanding.
Thank you,
Robert Hall
www.RobertHall.com
The 10 sheets per gallon were in a paper on archival processing using the Ilford Archival wash method for fiber paper. In this the exposure of the paper to the fixing bath is very short to minimize the amount of fixer absorbed into the fibrous structure of the support. The support holds the fixer so that it does not wash out by strictly diffusion as it does from the emulsion. Ilford recommends fixing times no longer than one minute but there is still an advantage of times less than two minutes. Many papers will not fix out in this short a time even in the film strength rapid fixer specified by Ilford. The capacity of ammonium thiosulfate fixers appears to be somewhat greater than for sodium thiosulfate fixers but I've never seen any formal research on it. The capacity of the fixing bath also depends on how much residual silver halide is permissible in the emulsion. The residual halide will eventually change in form and cause staining and loss of the image. After some relatively short period, about two weeks, the residual halide changes to a form which will no longer be removed by subsequent re-fixing. Where the life of the print does not have to exceed about ten years the residual silver level can be much higher than when great permanence is required. Most of the fixing bath capacities are given for such "commercial" levels of residual silver rather than for "archival" purposes. That may account for Ilford's high capacity for its rapid fixer. The use of two successive fixing baths results in an increase in _archival_ fixing capacity of from four to ten times. The use of a sulfite wash aid, such as Kodak Hypo Clearing Agent, subsequent to fixing will extend the capacity a little more because it makes some otherwise insoluble thiosulfate-silver complexes soluble. Since fixing is an exponential process the first bath absorbs most of the silver from the emulsion leaving the second bath fresh enough to complete the fixing process. The routine is to used the two baths until the first becomes too saturated with silver and then discarding it, making the second bath the first and replacing the second bath with a fresh bath. This can be done perhaps five times after which both baths are discarded and one starts out with two fresh baths. The same system works for film or paper. It used to be recommended that the same fixing bath NOT be used for both film and paper. I think this was partly because of the accumulation of iodide from film. Iodide is a slight restrainer of fixing. However, it also helps in washing. In a two bath system the iodide will mostly come out in the first bath. The other reason is probably to avoid dyes from film anti-halation layers or sensitizing dyes which may come out in the fixer. I've never experienced staining from this cause and am skeptical that it exists.
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Richard Knoppow
Los Angeles, CA, USA