Originally Posted by
Doremus Scudder
Strange, I've compared PMK negatives with both TF-4 and Ilford Rapid Fixer and found absolutely zero difference; stain was the same, base fog was the same (visual comparison only, but that should be adequate in this case).
The thing that gets me about simply saying that capacity is "30 8x10s per liter" for TF-4/TF-5 is the discrepancy with the Ilford information. Ilford states that for "optimum permanence," the capacity of their Rapid Fixer is only 10 8x10-inch prints per liter (assuming single bath use) while for "commercial" standards, the capacity increases to 40 8x10s per liter. Here's the relevant section from the Ilford tech sheet:
"If a high level of image permanence is required for commercial use the silver concentration in the fixer should be kept below 2 g/l when fixing FB
papers. This approximates to 40, 20.3 x 25.4 cm, (8 x 10 inch) FB prints. Above this level compounds may remain in the paper base after washing and over time possibly contribute to print staining. For prints that need maximum stability for long term storage a the maximum silver level in the fixer should not rise above 0.5 g/l i.e., approximately 10 20.3 x 25.4cm (8 x 10in) prints." [emphasis added]
If, as it seems logical to extrapolate from the Ilford information, the capacity of the fixer is linked directly to the amount of dissolved silver in the fixing bath, then why in the world should the TF fixers be any different? Is there some "secret ingredient" that allows more thorough fixing at a higher concentration of dissolved silver? And, if so, why aren't the manufacturers/distributors shouting this from the rooftops, since it would make great marketing?
I'm sure Ron Mowrey did his research and has/had all kinds of data about dissolved silver concentrations vs. efficiency and completeness of fixing, but I've certainly never seen any of them. At least Ilford gives you some reasoning for why they recommend certain capacities.
And, if as Michael suggests, the TF products are similar in activity and capacity to Ilford and other rapid fixers, why is the capacity for those fixers listed as some number between Ilford's "commercial" and "optimum permanence" standards?
It's not that I couldn't test myself for adequate fixation using ST-1 or the KRST tests (however imprecise and unquantifiable those are); I do those tests regularly when printing. It's just that I would expect that a product that was so carefully developed for a specific and discriminating market to have much, much better documentation.
I quit using TF-4 for other reasons, but until I really am convinced that TF-5 or whatever is substantially better than the Ilford products I'm currently using, I'll stick with them. PF could go a long way toward convincing me that their products were as good or better if they'd simply publish the research data in more detail.
Best,
Doremus
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