FT-1 Fixing Test Solution

[thefizz]

I understand the FT-1 fixing text solution cannot or should not be used on film. Why is this the case?

 

[Doremus Scudder]

FT-1 is a potassium iodide test for fixer solutions. It is mixed 1+1 in small amounts with fixer. If a yellow precipitate appears, the fixer is exhausted. It is not meant to be used on either film or paper. It is similar to (if not exactly the same as) Edwal Hypo Check. What you may be referring to is the bad practice of simply dripping a few drops of the test solution into the fixer tray or bottle and seeing if a precipitate occurs. That introduces potassium iodide into the fixer solution and could be bad for film fixing. The proper method for using the test solution is to remove some fixer to be tested, test it in a separate vessel and then discard it after testing.

FWIW, the potassium iodide test is rather approximate and doesn't give quantifiable results. I wouldn't use it as a test for the final fix for fiber-base papers.

The better test strategy is to use either the Kodak ST-1 or the Kodak Rapid Selenium Toner tests for residual silver and test the paper and/or film itself for adequate fixation. These tests are used directly on the paper of film after fixing and a thorough wash and produce a stain if residual silver is present, which shows that fixation was inadequate. I prefer the KRST test, since I always have the toner on hand.

Hope this helps,

Doremus

 

[thefizz]

Thanks Doremus, I know it’s not placed on the paper or film so I should have worded my question better. I just read many times that it shouldn’t be used to test film fixer exhaustion but never knew why. I can see from your explanation that if used the correct way then film fixer can indeed be checked for exhaustion.

Many thanks,
Peter

 

[thefizz]

And as you said for an approximate result.

 

[Doremus Scudder]

Peter,

Actually, it's fixer for fiber-base paper where the FT-1 test ends up being less-than-adequate, since the allowable dissolved silver levels for it are much less than fixer for film or RC paper. Testing film fixer is actually all I'd ever use FT-1 for.

Best,

Doremus

 

[thefizz]

Peter,

Actually, it's fixer for fiber-base paper where the FT-1 test ends up being less-than-adequate, since the allowable dissolved silver levels for it are much less than fixer for film or RC paper. Testing film fixer is actually all I'd ever use FT-1 for.

Best,

Doremus

Thanks Doremus, so at what amount of silver per litre of fixer would you stop using film fixer?

 

[mnemosyne]

Ilford says: about 8-10g/liter is okay for film, fiber base paper only about 2 g/liter and for highest archival standards maximum 0.5g/liter. I think the differences explain why you cannot use the test for film fixer and paper fixer in the same way. At least not without adapting the test to the different allowable silver levels.

 

[Doremus Scudder]

Thanks Doremus, so at what amount of silver per litre of fixer would you stop using film fixer?

To elaborate on mnemosyne's answer above:

If you carefully read the Ilford tech sheet on Hypam or Rapid Fixer, you find all the information you need. It's here: https://www.ilfordphoto.com/amfile/file/download/file/1833/product/711/

Unfortunately, Ilford is a little confusing with their initial capacity numbers on page 2. There they state that a liter of fix will fix 40 8x10 fiber-base prints; what they don't tell you is that this number is NOT the capacity for optimum permanence, rather the number for "a high level of permanence for commercial use."

Check out the section on Silver concentration on page 4. There you will find all the relevant information. Note that the 2g/liter of dissolved silver in the fixer (and the number that mnemosyne mentions in the post above) is for "commercial use" and allows 40 8x10s per liter. However, for "optimum permanence" the amount of dissolved silver allowable is much less, at 0.5g/liter or less, which works out to 10 8x10s per liter. Note also, however, that these numbers are for a one-bath fixing regime.

Some people are happy to just change fixer every 10 prints, but I find the two-bath fixing regime much more practical (and economical). Unfortunately, Ilford gives no capacity numbers for the two-bath fixing regime, even though they recommend it (p. 3). So, you really have to do your own testing for residual silver to find the optimum capacity for a two-bath regime.

So, the easiest thing would be to just measure the silver concentration of your fixer periodically to determine when to discard it. Unfortunately, there are no tests that are accurate, precise, and practical enough to do that. That's why most of us use throughput as a guide and then test the last print through for residual silver (i.e., adequate fixation) using ST-1 or the KRST tests.

If you want complete information, you need to find the limits of your two-bath regime. Years ago I did just that, overusing the fix till I got a stain using the ST-1 test that I didn't like. IIRC, that was around 50+ 8x10s through one liter each of bath one and bath two. However, as Ilford reminds us, throughput can only be a guide. I like a generous safety factor and have settled on maximum 36 8x10 prints (fiber-base) through 1 liter each of bath one and bath two, or equivalent, as my standard. I now test the last print through with the KRST test every session. It just takes a couple of minutes; I have the test solution at hand and it lasts forever.

There are a whole lot of other considerations when fixing as well, e.g., whether or not to use Ilford's optimum permanence sequence with strong fix and short times or not (for me, it's not), but regardless, the capacity of the fixer remains the same, since it depends on silver concentration build-up in the solution, not on the dilution of your fixer (this latter only affects fixing time).

Hope this helps,

Doremus

 

[thefizz]

Thank you Doremus for your insightful and detailed reply, it is much appreciated.

 

[thefizz]

Ilford says: about 8-10g/liter is okay for film, fiber base paper only about 2 g/liter and for highest archival standards maximum 0.5g/liter. I think the differences explain why you cannot use the test for film fixer and paper fixer in the same way. At least not without adapting the test to the different allowable silver levels.

Many thanks mnemosyne.