Photo Engineer
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Ok, so everyone wants to know how to do this.
Lets start with some facts. Sodium Thiosulfate will form complexes with Silver Halide with up to 5 or so different chemical structures and that means with 5 or so Thiosulfate ions complexed to the Silver. (Mees, Revised Edition pp 566 f, and Mees and James)
These are huge and take a while to form, and a while to diffuse out of the coating. Substituting Ammonium ion for Sodium ion reduces size because the Ammonium ion can replace one or more Thiosulfate ions in this complex and thus it speeds fixation and diffusion. This is nice for fixing and washing. But, the Thiosulfate ion is still big and there can be more than one in the complex.
It has been found that there are at least 4 more chemicals that can speed fixation. Thiocyanate ion is very small and can also replace Thiosulfate in this huge complex. Again, we see a speedup in fixation. We see a disproportionate speedup in washing because this complex is so much smaller and Thiocyanate tends to swell the gelatin allowing much faster diffusion. This is clearly a big gain.
Fixes also can see gains from Thiourea and for the same reasons as for Thiocyanate. Thus, the Super Fix posted here on APUG can achieve fix rates of 40" (clearing in 20") at 20 deg C. Wash times are at the low end of those given by Kodak and Ilford for B&W films and papers. We have found that for dry films, diffusion takes at least 15" which establishes the minimum time in which we can see clearing. Fix times would be about 30".
All of the above assume tests at a pH range of 6 - 8, with bone gelatin which is at its medium swell in that range.
Kodak has shown that a chemical called HTTT (USP 4256826) gives a huge boost to fixation. I was amazed seeing in work in Keith Stephen's lab in the early '90s. It is quite expensive. Other organics as well will speed fixation and wash rates, and I have observed 15" clear times with film (at the diffusion limit) and wash times of 5" in running water. These were tested with the standard retained Silver and hypo tests, and also verified by 10 years of keeping.
I hope to scan the test prints someday and post them here.
All of these chemicals have been tested by starting with the Super Fix posted here and taking off from that starting point to get to the results I mention above.
The cautions are (1) the chemicals may be toxic or cause cancer (thiourea is a suspect, but not proven) and (2) some of them are quite expensive (HTTT - if you can even buy it). Thiourea spills in the darkroom can cause dust which fogs EVERYTHING! You have to be careful with it.
There are many other purported accelerants that are patented, but I'm not going to direct you to them, as I have found most of them to be duds, or I have had notes from others testing them and finding total failure.
This is just the start of a conversation that I hope will improve your knowledge of fixing and washing.
PE
Lets start with some facts. Sodium Thiosulfate will form complexes with Silver Halide with up to 5 or so different chemical structures and that means with 5 or so Thiosulfate ions complexed to the Silver. (Mees, Revised Edition pp 566 f, and Mees and James)
These are huge and take a while to form, and a while to diffuse out of the coating. Substituting Ammonium ion for Sodium ion reduces size because the Ammonium ion can replace one or more Thiosulfate ions in this complex and thus it speeds fixation and diffusion. This is nice for fixing and washing. But, the Thiosulfate ion is still big and there can be more than one in the complex.
It has been found that there are at least 4 more chemicals that can speed fixation. Thiocyanate ion is very small and can also replace Thiosulfate in this huge complex. Again, we see a speedup in fixation. We see a disproportionate speedup in washing because this complex is so much smaller and Thiocyanate tends to swell the gelatin allowing much faster diffusion. This is clearly a big gain.
Fixes also can see gains from Thiourea and for the same reasons as for Thiocyanate. Thus, the Super Fix posted here on APUG can achieve fix rates of 40" (clearing in 20") at 20 deg C. Wash times are at the low end of those given by Kodak and Ilford for B&W films and papers. We have found that for dry films, diffusion takes at least 15" which establishes the minimum time in which we can see clearing. Fix times would be about 30".
All of the above assume tests at a pH range of 6 - 8, with bone gelatin which is at its medium swell in that range.
Kodak has shown that a chemical called HTTT (USP 4256826) gives a huge boost to fixation. I was amazed seeing in work in Keith Stephen's lab in the early '90s. It is quite expensive. Other organics as well will speed fixation and wash rates, and I have observed 15" clear times with film (at the diffusion limit) and wash times of 5" in running water. These were tested with the standard retained Silver and hypo tests, and also verified by 10 years of keeping.
I hope to scan the test prints someday and post them here.
All of these chemicals have been tested by starting with the Super Fix posted here and taking off from that starting point to get to the results I mention above.
The cautions are (1) the chemicals may be toxic or cause cancer (thiourea is a suspect, but not proven) and (2) some of them are quite expensive (HTTT - if you can even buy it). Thiourea spills in the darkroom can cause dust which fogs EVERYTHING! You have to be careful with it.
There are many other purported accelerants that are patented, but I'm not going to direct you to them, as I have found most of them to be duds, or I have had notes from others testing them and finding total failure.
This is just the start of a conversation that I hope will improve your knowledge of fixing and washing.
PE
