Super Fixes and Fast Washing

Discussion in 'B&W: Film, Paper, Chemistry' started by Photo Engineer, Jul 16, 2017.

  1. Photo Engineer

    Photo Engineer Subscriber

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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
     
  2. Prof_Pixel

    Prof_Pixel Member

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    What a great guy Keith was; he left us far to soon.
     
  3. mshchem

    mshchem Member

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    Fascinating. Do these compounds change tone of print?
     
  4. nworth

    nworth Subscriber

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    All very important and true. One note: complexes build up in the fixer with use, the heavier ones becoming significant fairly late in life. If you don't want to invest in a superfix, you can minimize the problems by using fresh fixer and discarding it well before its technical expiration point. I mix up a new batch of ammonium thiosulfate based fixer before each darkroom session. The formula I use (Kodak F-34) is very cheap, and with my less than daily trips to the darkroom, this is an economical and effective solution to the problem.
     
  5. jnanian

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    PE
    sorry to ask ...
    how does one dispose of super fix ?
    i am guessing you cant bring this to
    house hold chemical disposal day considering
    it is made of httt and thiourea
    and people have an adversion to using a waste hauler
    and have extreme property rights views and believe they have dominion claims
    on their yard and septic tank &c ... ( and it shouldn't be put in the drain )
     
    Last edited: Jul 16, 2017
  6. OP
    OP
    Photo Engineer

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    Yes, he and I went back to about 1966.

    PE
     
  7. OP
    OP
    Photo Engineer

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    AFAIK, these fixes behave like any fix with use. They just work faster and allow faster washing.

    PE
     
  8. OP
    OP
    Photo Engineer

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    NO.
     
  9. OP
    OP
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    Well, that is one of the possible problems. AFAIK, basic SuperFix posted here on APUG has no problems. After all, Thiocyanate is used in some toothpastes. As you go beyond that, the problem gets more complex depending on the perception of the local governments. Thiourea is considered toxic in California, but it is a natural product found in wildflowers. So, what do we do? Use your judgment and adhere to local laws.

    PE
     
  10. Jerevan

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    I guess it is standard notation, but what does 30" mean? 30 seconds? In that case, "wash times of 5" in running water" sounds at odds with the rest?
     
  11. Ian Grant

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    One downside to fast fixers and here I'd also include Amfix, Hypam, Ilford and Kodak Rapid fixers etc, is over fixing will change the tone/colour and density of a print, this is most noticeable with warm tone papers. However this is not specifically due to the compounds Ron's talking about rather that Ammonium Thiosulphate is a more aggressive fixing agent compared to Sodium Thiosulphate and will begin bleach the silver itself with overfixing.

    So yes these fixers can change the tone and effectively kill much of the warmth of warm tone papers which have a much finer grained chloro-bromide emulsions compared to bromide papers but only with over-fixing.

    Ian
     
  12. jnanian

    jnanian Advertiser Advertiser

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    this is the problem PE
    trace amounts are found in toothpastes and wild flowers
    but probably NOT trace amounts are used in the fixer.

    this reminds me of aggie who years ago insisted selenium toner was not toxic
    because trace amounts were found in multi vitamins ..

    my best judgement tells me not to dispose of these things willy-nilly ... and to use caution when using them.

    thanks for the post !
    john
     
  13. MattKing

    MattKing Subscriber

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    PE uses 30" to indicate 30 seconds, and 5' to indicate 5 minutes.
    So 5'30" means 5 minutes and 30 seconds.
    It is funny, but while the latter seems familiar, PE's use of them separately always seems unusual to me - but I've grown used to it.
     
    Last edited: Jul 17, 2017
  14. OP
    OP
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    Yep, 5".

    PE
     
  15. OP
    OP
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    At the working pH, these types of fixers do not seem to change image tone. The problem with those who claim image tone changes is that most are working below pH 6.

    PE
     
  16. OP
    OP
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    I did not suggest using them. I was showing how one can obtain a faster fix and wash. These chemicals can be quite expensive. As to the point about carcinogenicity, I just wanted to point out that some of these compounds can be found in nature.

    PE
     
  17. jnanian

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    i realize that PE :smile:

    but unfortunately a lot of folks who do basement darkroom work
    don't think about how to legally and ethically dispose of some of these
    materials .. its been a while, i wonder when someone is going to
    ask about making their own basement silver nitrate ...
     
  18. Ian Grant

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    pH has little to do with it unless a pH of 5.4 means a fixer like Hypam or Ilford &Kodak Rapid fixers actually works faster - image tone, image colour or densiti changes occur with alkali fixers as well, but in all cases only with over fixing.

    The clearing times given earlier in the thread areactually slower than Hypam, and both Ilford and Kodak Rapid fixer when fresh. (Hypam has additional buffering for use with a hardener otherwise it's interchangeable with Ilford Rapid Fixer). The reality is Ilford and Kodak's recommended fixing times in Ammonium Thiosulphate fixers are very short so it's easy to over-fix and cause image bleaching.

    Ian
     
  19. OP
    OP
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    I have observed no bleaching or image tone changes with this fixer. The reports I have read on pH induced changes are with acid fixers in the pH range from 4 - 5.

    Clearing times are limited by diffusion times, which on average are about 15" for most solutions to permeate the back. Therefore 20" is quite good and 15" is the absolute limit for films for the most part.

    Most don't know that Thiosulfate ion is a fairly good buffer at pH 6 - 6.5 so at this concentration the pH is quite stable.

    PE
     
  20. michael_r

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    I think PE is correct specifically re thiosulfate bleaching of silver. Bleaching action requires an acidic pH and the rate of bleaching is inversely related to pH. This is in Haist, James etc. Even at a relatively weakly acidic pH of say 5.5 (Ilford Rapid Fix) bleaching should be quite slow. All things being equal, bleaching should be faster with finer grained emulsions. Note however this is with respect to thiosulfate fixation. Superfix is more complex - I have no idea if/how pH influences things.
     
    Last edited: Jul 17, 2017
  21. Jerevan

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    Yeah, 5'30" I can figure out, I guess it was the one without that threw me, in conjunction with the very short times - amazing! Thanks, gentlemen!