It depends on which C-41 fixer you have. The RA Flexicolor fix has more ingredients than the plain C-41 fixer, but they both contain sulfites, EDTA or some sequestrant, and buffers. So it is not a simple solution of Ammonium Hypo.
I use very very dilute ammonium thiosulfate as a bleach for clearing highlights in POP and VanDyke prints...
As PE said, all thiosulfate fixers can bleach the image if used too strong (time, concentration), sodium being less active than ammonium.
There are three two-place variables in currently available fixers: pH, thiosulfate type, and hardening agent. All variations exist, so that's a total of 2 at the power of 3 = 8 possible fixers.
* pH: alkaline or acid. Acid is more readily available. Alkaline washes off more easily
* Thiosulfate: Sodium or Ammonium.The differing uses between sodium and ammonium are based on the type of silver salts you need to fix away. Modern emulsions for both film and paper contain silver iodide, which really needs ammonium thio fixer.
* Hardener: old emulsions were not much hardened, so an extra hardening step in the fixer was useful. Not necessary with modern films/papers.
So you can have:
- Acid sodium w/ hardener
- Acid sodium w/out hardener
- Acid ammonium w/ hardener
- Acid ammonium w/out hardener
- Alkaline sodium w/ hardener
- Alkaline sodium w/out hardener
- Alkaline ammonium w/ hardener
- Alkaline ammonium w/out hardener
I use Ilford rapid fix (non-hardening, ammonium thio, acid) simply because it's available at my corner photo store, and because it's the best for my purposes I can find on the shelves over here. The other choice is Kodak Kodafix (liquid hardening acid ammonium), and Kodak plain fixer (powder hardening sodium thio), none of which meet my needs: I don't need hardener or sodium thio (I use modern films and papers only).
I'm just not that into buying liquid fixer by mail, so I stick with what I can find.
I bought a jug formularys' TF-4 for fixing negs and prints. The fiber print time is listed as 1 minute to fix. I'd never heard of anything that short, usually in the 5-6 minute area. If 1 minute will do it there seems no need in a two bath fix regime.
Well, a two-bath for fiber prints is still better. The idea is for the first bath to dissolve the silver, and the second is to remove it from the fiber base. The actual fixing happens pretty fast, and I have had no problem with 2 30-second baths. The Toning Book says a single bath fix can contain up to .5g silver before fixing is impaired; with two-baths it can contain 2g. With TF-4 or the Flexicolor, that sounds conservative because those baths seem to last forever.
The alkaline ammonium fix with hardener is not practical. You cannot use alums on the alkaline side, and aldehydes tend to polymerize on the alkaline side in the presence of ammonium ion. Therefore, I know of no good alkaline hardening fix based on ammonium thiosufate.
In regard to ammonium 'bleaching' silver faster than sodium, IDK. It depends mainly on pH AFAIK and activity, and so I would think that if bleaching were to take place it would be faster in acid and with ammonia present. I have not tested it personally. Textbooks show this but don't make all of the necessary comparisons to be able to properly define the rate of any fix in bleaching silver. I know that it is rather rapid! More so than most of us would expect.
I have read all the posts and I am somewhat confused about the advantagesof alkaline over acid, specifically in respect of staining developers. One post says no effect on staining with pyro while another says there is an effect. In the case of no effect it is clearly based on pyro and actual experience while I wasn't sure if the other poster was talking from experience of trying both alkaline and acid or was taking the use of alkaline as being better as perceived wisdom.
Closer to home i.e. the U.K. a popular staining developer is Prescysol which may or may not be similar to pyro in terms of the effect of acid fixer and the maker most certainly recommends an alkaline fixer. Has anyone done an experiment of trying a roll in Prescysol with acid then one with an alkaline fixer to see if there is an effect on stain. Clearly if there is an adverse effect on stain from acid fixer then not to use alkaline negates the benefit of using Prescysol in the first place and the case for an alkaline fixer is proven - at least in the case of Prescysol.
Ron, thanks for the correction about hardener, I was unaware of that detail. My only reference for sodium v. ammonium thio bleaching is Haist, so I guess you must have a more up to date one?
It is just experience Michel and the knowledge that formalin reacts with ammonia, especially on the alkaline side.
Kodak alkaline hardener fixes used sodium thiosufate + formalin not ammonium.