Clearing time for paper?
Is there a process to determine fixing time/fixer exhaustion for paper processing similar to "clearing time" for negatives? I know that fixer exhaustion indicators exist, but they're not available here and I'd like to have a way to measure an optimal fixing time too.
A test recommended by Edwal years ago was to check the clearing time for a piece of Tri-x or equivalent. There was no TMY then. When that clearing time got to be more than 2 minutes for paper strength Edwal rapid fixer, you were to discard the fixer.
There are 2 questions here. One, the capacity of fixer for paper is not about clearing time, but the amount of fixing by products present (silver compounds) that will reduce the stability of the prints (if they are intended to last). Two, assuming that you have fixer that is clean enough to do a good job, what is the optimum fixing time, given that too much time results in the requirement for much more washing to remove the fixer that has soaked into the paper. Oh, I should mention that I'm thinking of fibre paper here.
I read about a test somewhere (maybe in "Way Beyond Monochrome") that you can fix some samples of unexposed paper for various short times, rinse in water, then expose to light, develop, fix again, and see how much density is developed. This is a measurement of how much silver halide was not fixed out in stage 1 of the test.
I should add that you would need to be careful in interpreting your results. I guess that a little bit of fixing would continue in the water rinse. The results would be valid for that type of paper only, and you would hope that future purchases of that paper would be the same (HA!). Once you found the minimum time that produced no visible density, you would not be completely sure that a small trace did not remain. This all leads to having to add some time to make sure.
Originally Posted by john_s
A sulphite wash aid (or KHCA type formula) after fixing paper is always worthwhile.
Well, if fresh fixer clears a piece of film in a given time, the doubling of that time does indeed say something about the remaining capacity of the fixer. In fact it may be a conservative measure of when to discard the used fixer. In view of the fact that it was recommended by Edwal, I suspect it is a reasonably safe measure and is easy for the average person to implement. My error was in not specifying that 2 minutes represented a doubling of the initial clearing time. Sorry about that.
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There are tests for exhausted fixer, residual hypo and residual silver. Ready mixed products are available from Photographer's Formulary. Edwal Hypo-Check is usually available and there are forumlas for fixer exhaustion and residual hypo tests in the Kodak Black & White Darkroom Dataguide.
Gaget, john, Neal - thanks for the suggestions (yes, I use fiber paper).
Neal, here I don't even have a reliable supply of Tri-X, let alone such exotic substances as hypo check. I use discarded military aerofilm processing kits as formulary, and it isn't possible to mix a fixer check out of them.
"...I don't even have a reliable supply of Tri-X, let alone such exotic substances as hypo check...."
I'm very sorry to hear that. If you can obtain potassium Iodide you can make your own "Hypo Check". Per the Kodak Darkroom Dataguide: 750ml of water at 27°C, 190g Potassium Iodide, add water to make 1 L total mixture. To test, combine 5 drops of water with 5 drops of fixer and 5 drops of test solution. If a precipitate forms, the fixer should be discarded.
The test for residual fixer in prints is: 750ml water, 125ml 28% acetic acid, 7.5g silver nitrate, add water to make 1L. To test, squeegee the test print (after full processing) and the place a drop of the test solution on a white area. Allow it to stand for 2 minutes. A slight stain is acceptable.
There are others on this forum that can explain the chemistry. I have done nothing more than copy from the stated source. If these chemicals are not available, keeping track of the number of prints and using a 2 bath fix should work for you.
There are two limits of capacity. There is a quantitative limit;
Originally Posted by varjag
this amount of sodium thiosulfate will complex with and dissolve
that amount of a or a combination of silver halides. The amount
of silver halides that can be complexed and dissolved is in
excess of the permissible silver limits per unit volume.
The permissable silver amounts per unit volume is a second
limit of capacity. Most users of fixer are volume limited users.
Ilford claims 200 8x10s per ltr of their Rapid Fixer concentrate.
That is the quantitative limit. In use though, according to Ilford,
as few as 50 8x10s per liter of concentrate can be fixed if one
wishes for archival results.
I'm not a volume limited user of fixer. I use fix very dilute,
one-shot. I've not tested it to it's limit yet but using straight
from the bottle A. thiosulfate concentrate, I believe 200
8x10s may be the correct quantitative limit.
Read the Ilford Rapid Fixer pdf. The sulfide test they describe
is the same I use to check for complete fixing. I've tested RC
paper at a 1:49 dilution, 2 minutes constant agitation, and
had ZERO stain. Dan
Testing for paper clearing time: in room light cut a 4X5 piece of your chosen photo paper into 1" squares, mark the back of each with time in seconds (for example-10,20,30,40, 50 etc. Place all squares in stop for usual time. Move them to a fresh tray of fix face down with agitation. At the respective times move each square to a water bath receiving a constant flow of water. After the last square has been in the water bath for 2-3 minutes place all squares in your usual developer for 5 minutes with constant agitation then move them to the stop and examine them in good light. You are looking for the square marked with the shortest time that shows paper base white - that is your clearing time. Double that time and you have your fixing time for that paper in that FRESH fix.