Discussion in 'Film Dev - Staining' started by Ole, Oct 28, 2006.

Pextral's Two-bath

Pextral's Two-bath

  1. Ole

    Ole Moderator Staff Member Moderator

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    Pextral's Two-bath - Pextral's Two-bath

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    Last edited by a moderator: Feb 6, 2016
  2. Sean

    Sean Admin Staff Member Admin

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    Comments from previous article system:

    By Jim Noel - 04:11 PM, 03-06-2005 Edit Rating: None
    Seems amazingly fast, especially to produce such a long scale. I will have to give it a try.
    By Ole - 03:25 PM, 03-07-2005 Edit Rating: None
    BTW, I used FP4+ film in 5x7". Speed seemed good at ISO 100 too!

    The only othe film I've tried it on so far is Slavich PFN-01 glass plates in 9x12cm. It developed those too satisfactorily.
    By jdef - 12:00 AM, 03-11-2005 Edit Rating: None
    My pyrocat was just delivered, so I'm going to give this stuff a try with HP5+ and Forte 100. Wish me luck!

    j
    By jdef - 01:35 AM, 03-11-2005 Edit Rating: None
    I just developed a roll of Edu 100 in 120 format. I exposed the whole roll at EI 100, in a variety of lighting conditions. Speed looks good in all frames, and contrast looks pretty good too, although it's difficult to say with any authority until I print the negs. I snipped a piece of the film to check FB+fog, and it's pretty high at .21 with the V filter, and .27 with the blue filter. I used the B bath right after mixing, and the heat generated by the sodium hydroxide probably heated that bath to around 80F, which undoubtedly contributed to the high fog levels. I will be more patient next time, and allow the B bath to cool to 70F, or so. Maybe I'll throw an ice cube in the water before adding the sodium hydroxide. I'm pretty confident that the fog can be tamed, so if the grain is not objectionable, this might turn out to be a very useful and convenient developer for rollfilms as well as LF! Thaks for posting this very interesting formula, Ole.

    J
    By jdef - 01:50 AM, 03-11-2005 Edit Rating: None
    Oops! I just re-read the instructions, and realized that I developed 1min A, 2min B. This is clearly a very forgiving developer.

    J
    By jdef - 09:32 AM, 03-11-2005 Edit Rating: None
    I printed my negs, and they require quite a bit of filtration to print on VC paper, but would probably print just fine on graded paper. Surprisingly, the grain is very fine, and sharpness is very good as well. Speed was good in all lighting conditions. Just for fun, I made up a second batch of the A bath, but substituted ascorbic acid for the sodium sulfite. I ran a control strip in each version of the A bath for 2 min, followed by one minute in the B bath. The ascorbic acid version showed slightly more density than the original, sulfite version, but I don't know how the ascorbic acid will affect keeping properties.

    J
    By Ole - 06:20 PM, 03-11-2005 Edit Rating: None
    Forgiving it is - and it's also a "true" two-bath devloper in that no development takes place in the first bath, and everything happens quickly in the second.

    I'm uncertain as to the best method to transfer from A to B - whether to throw it over, let it drain for a while, or even wipe it? The A solution sitting on the surface of the film WILL affect development; ideally all the A should be inside the emulsion?
    By jdef - 08:15 PM, 03-11-2005 Edit Rating: None
    Two bath development is a bit of a mystery to me, which is what I find attractive about it, I suppose. Since the first bath is simply an absorbption bath, how will time in the first bath affect density, assuming that the film is in that bath long enough to become saturated? How long does it take for a film to become fully saturated? Time in the second bath seems a little more tricky. Theoretically, development should procede until all of the developer saturated in the emulsion is used up, and no longer, as extending development beyond that point could only potentially produce fog, and decrease contrast. Right? I dumped the A bath back into its bottle, and immediately filled the tank with the B solution, and agitated continuously in both baths. I think that this developer has potential, but I need to get a little more contrast and DR out of it. Any suggestions? Would going to 2min ea. bath help? I'll give it a try.

    Jay
    By jdef - 10:43 PM, 03-11-2005 Edit Rating: None
    I looked around for similar formulae, and found Barry Thornton's Pyrocatechin 2-bath to be nearly identical to Pextral's, with the exception that Thornton adds KBR at the ratio of 1:20 to pyrocatechin in the first bath, and 1:20 to sodium hydroxide in the second, in his formula, and he recommends 4min in each bath, with intermittent agitation. From my initial trials, I would concur with Thornton's changes to the Pextral formula/procedure, and I'll try to verify them for myself, today.

    Jay
    By jdef - 12:30 AM, 03-12-2005 Edit Rating: None
    I just realized that Thornton's working solution is slightly more concentrated than Pextral's. I'll start with Thornton's recomendations anyway, and see what that gets me.

    Jay
     
  3. absolution

    absolution Member

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    This is not original Pextral

    Hello. I just began some experiments with this developer and found very little on Internet, but fortunately I speak Czech and has access to National Library in Prague so I can tell you, that this is not Pextral formula as it ing. Přemysl Koblic published it in 1947.

    In fact, this developer was invented to be as much effective as simple and forgiving. So the differences in concertations, temperature and even time matters very little.

    The original Pextral formula is

    A:
    Pyrocatechol (I hope this is correct english translation of Pyrocatechin) - 1g
    Sodium sulphite ahydrous 0.2g
    water - 100ml

    B:
    NaOH - 2g
    water - 100ml

    In fact ing. Koblic writes, that Sodium sluphite could be in dilution B instead of A or in both or it even doesn't have to be present. It's for control of grain appareance.

    This formula is designed for developing in bowls which is not very much convenient for 35mm. Reason is, that oxygen (or just air) should be present to reaction of dilution B with film emulsion. When developed in tank (where air has no acces to emulsion) the film tends to be veiled and too contrasty. This could be prevented by rising up and down of roll into the dilution in the darkroom or using adjusted formula which ing. Koblic presents as follows:

    A:
    Pyrocatechol - 1g
    water 150ml

    B:
    NaOH - 1g
    water 100ml

    (for sodium sulphite it is the same as before - A,B,both,none)

    Timing of developing is not very important. For A it should be minimally 2 minutes (to let the dilution saturate emulsion completly) and for B 50 to 60 second (but not much happening over that time, in fact evrything happened in the matter of few seconds).

    The last and not least, Koblic also suggest to substitute Pyrocatechol for Metol (Mextral). In that case, the developer is even more staining. Half Pyrocatechol and half Metol is also option.

    Anyway Pextral also could be also used as positive developer, bud I didn't try this.

    So this is it. :smile:
     
  4. OP
    OP
    Ole

    Ole Moderator Staff Member Moderator

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    Very interesting!

    I can't remember at the moment where I originally found the formula I posted here, but I do have a vague recollection of scrolling through a couple of pages of Czech before I found the bit I could read (the chemistry).

    As you said, the timing is very forgiving. I would assume that the original recommendation of a minimum of two minutes in A was made at a time when emulsions were thicker than today, which is why we can get away with one minute now. Still the 2 minute recommendation makes sense, and i would give at least that much. From a brief experiment with orthochromatic emulsion under red safelight, solution B has indeed done most of it's job in a very few seconds.

    I would suspect that the amount of NaOH is B is very "uncritical"; 1% and 2% solutions are much the same in terms of pH. All it needs to be is strongly alkaline, but with sufficient concentration to maintain a consistent alkalinity throughout the emulsion.

    I think I might play around a bit more with this one - and variations!
     
  5. buze

    buze Member

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    Is there a way to make concentrate of these and use them like PMK etc ? (especialy the A, I don't like pyro in solid form :/)
     
  6. absolution

    absolution Member

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    2 buze - Yes, Pyrocatechol concetrated solution is stable for couple of months. ing. Koblics suggest this but unfortunately I did'nt make copy of small publication in which he introduced his Pextral invention so I can't tell how concetrated solution is suggested optimal for stocking.

    Also Pyrocatechol solution could be used until completly soaked into the films - there is no reaction with emulsion so also no degradation of solution. But the working solution should not be stocked more then few days.

    NaOH solution should be prepared immediatly before developing - according to Koblic. And used only once.
     
  7. OP
    OP
    Ole

    Ole Moderator Staff Member Moderator

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    I don't think the NaOH solution "deteriorates" as such (well - a little through reaction with carbon dioxde in the air), but after use it's an unbelievably "gunky brown" colur from the pyrocat reaction products. It's not something you would want to keep!

    There's also the problem of storing NaOH solution: It etches glass.
     
  8. absolution

    absolution Member

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    Note about Koblic

    Well this is forum on chemistry, but I think it's also interesting to note something on the histories connected with this antique formulas :smile:

    It's interesting to note, that Koblic, Czech photographer of sound name that time, invented Pextral during WWII. He writes that one of his motives was to make cheep developer not depending on the chemicals like metol which bacame unavailable during the war.

    Also he writes, that he wanted simple and foul proof formula which will make photography open to the broad masses of all classes (we are on the dawn of communist power grab!)

    Well, he was respected but Pextral formula was nearly forgotten. Even my father prefered in fifties to make D-76 instead of Pextral and after the rise of Foma and availability of Rodinal, people had no need to make their own developers anymore.

    ing. Přemysl Koblic made more photographic innovations. He for example designed Epifoka camera for aerial photography. See here: http://www.liveauctioneers.com/item/988724

    Very interesting indea of Koblic is also his "Polygrad - system of continuos change of paper gradation just by controlling of character of light". This is another thing which seem quiet usable even today. Will come to library to get some info about this next time... :smile:
     
  9. ferod

    ferod Member

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    Hi, somebody can tell me if i must use this formula as "STOCK" and how much film i can developing and how long this solution live??
    Thank you