Ladies and Gentlemen,
Originally Posted by konakoa
Let's try to read more carefully and give relevant advice. The OP is using a plain sodium thiosulfate fixing bath, NOT rapid (ammonium thiosufate) fixer. Therefore, suggestions using times for rapid fixers are irrelevant and, in the worst case, misleading.
To the OP: Two things come into play here. First is the actual fixing capacity of the sodium thiosulfate, second is the degradation of the fixer over time from oxidation, etc. One of the reasons fixers like Kodak powdered fixers (F-5) and other formulae have other chemicals in them is to keep the fixing agent itself from degrading. A plain "hypo" fixing bath like you are using does not have much of a lifespan time-wise. That means, use it quickly and discard it.
Now, as for times; if you want optimum permanence, use two-bath fixing. Fix 3-5 minutes (Kodak recommendation for F-6 IIRC) in each bath. Capacity is approx. 40 8x10 per liter of bath one (which means you have a bath two of equal size you are not considering). After reaching capacity, discard bath one and replace it with bath two. Mix a new bath two. I would start in fresh fix and the shorter time and gradually increase it to the longer over the life of the fixer. Alternately, you could just standardize on five minutes in each bath and make sure you give an adequate wash sequence.
Since you are using a plain hypo fixer, I wouldn't recommend using the solutions for more than one session. This means one working day and no more. Do use an acid stop bath with a thorough drain before the first fixing bath.
And, since throughput is only an approximation of fixing capacity, you will have to test your own workflow for adequate fixing (residual silver test) and adequate washing (residual thiosulfate test). The most common way of doing this is to use the ST-1 (residual silver) and HT-2 (residual thiosulfate) tests. Look at the Unblinking Eye website page on archival processing for more detailed info: http://unblinkingeye.com/Articles/Ar.../archival.html .
I test the last print in a fixing batch with both solutions every few batches to make sure my process is working correctly. With your home-brew fixer, I'd recommend doing both tests on every batch, pushing to the extreme for a while in order to determine your processing limits, then pull everything back leaving an adequate safety margin. This can then be your fixing regime and you can then test only every several batches as I do as a process control.
You should be aware that using a sodium thiosulfate fixer instead of a rapid fixer based on ammonium thiosulfate will require longer fixing times and, therefore, longer washing times with fiber-base paper. With this process, the use of a wash-aid such as Kodak's Hypo-Clearing Agent is indispensable. Since you have bulk chemicals handy, you can easily mix your own from sodium sulfite and sodium metabisulfite. There are many formulae in cyber-space; just Google. You may also be interested in more sophisticated fixer formulations, especially since they will last longer. I used the Kodak oderless F-6 formula for years. Nowadays, hardening fixers are used much less, so there are likely some better formulations out there. APUG is a great resource and there are many other resources on the web as well.
Hope this helps,
Last edited by Doremus Scudder; 04-10-2013 at 09:15 AM. Click to view previous post history.