Microwaving prints!

Discussion in 'Enlarging' started by Anupam Basu, Dec 10, 2005.

  1. Anupam Basu

    Anupam Basu Member

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    Well, ok - not prints, but test strips. Does anyone do this? A search turned up the heartening fact that Ansel Adams did, so I am looking for any tips - how much power, how long etc. Is the effect different than normal drying? Crucially, is the drydown factor exactly the same?

    I am thinking of doing this especially with fiber paper test strips.

    Thanks,

    -A
     
  2. df cardwell

    df cardwell Subscriber

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    Lay some paper towel under the print ( squeegee the print first )

    Set on high, let it rip. It'll not be dry when it comes out, but will evaporate the remaining moisture quickly.

    SOME papers have a deeper black heat dried than air dried. Some papers' highlights dry differently. A quick test will let you know.

    Always know where the cat is when using a microwave....
     
  3. Poco

    Poco Member

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    Every mircrowave is a little different with its power settings so some experimentation is required. Since I only understand and use, like, two buttons on my machine, I zap the test prints in a couple 10-15 seconds bursts on high between which I'll take the strip out and wave it around a bit to cool off. If it gets too hot, the emulsion will bubble off. As it is, the heated drying affects the surface of the paper, making it shinier than normal air drying. That can, as has been mentioned, affect the Dmax and/or color of the test print.
     
  4. Jed Freudenthal

    Jed Freudenthal Member

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    My experience is that it works with with fiber paper test strips. It will not work with RC paper. I use 1 minute and 80% full power. A rinse of about 1 minute before microwaving is necessary. Otherwise, the color will be different.

    Jed
     
  5. Dan Henderson

    Dan Henderson Member

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    Also taking Ansel's advice, I bought an old microwave at a garage sale for $20 and found a place for it just outside my darkroom. My test strips and work prints are dry enough to judge drydown after 1:00. Sure takes the guesswork out of that. Note that if you dry too long the paper begins to scorch (not a good thing), and yes, RC paper will melt (also not good!)
     
  6. noseoil

    noseoil Member

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    I do just enough to "bump" the paper, not anywhere near a full minute for test strips, but 20 seconds or less at a time for fiber. For RC papers, I use bursts of no longer than 8 seconds. At 12 seconds things are difficult to see. When the plastic coating shrinks enough to melt, the print quality changes. I just figure dry-down to be similar to an air dried print, but don't ever "toast" all at one time for a completely dry strip. tim
     
  7. Bob Carnie

    Bob Carnie Subscriber

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    I use the microwave when we are calibrating fibre control strips for the Lambda exposing unit.
    It is a bit tricky , I have destroyed a few. seems high power for short bursts and move the paper around works for us.
     
  8. ScottH

    ScottH Member

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    I use mine for test strips after a short water rinse. Usually 1 minute at 40% power. I've found that higher power tends to dry it too quickly resulting in very bad paper curl, making it harder to view the entire print 'evenly'.
     
  9. photobackpacker

    photobackpacker Advertiser Advertiser

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    I do this but only with pre-production prints - those I judge to be the final product before making a dozen or so inventory prints.

    At the test strip stage, I am making fairly large corrections - both exposure and contrast. There are so many adjustments to be made to the print at that stage of printing refinement - drydown is just not something I am looking at. Having said this, if your drydown factor is large, I recommend reducing it by adjusted the illumination over your wet-print viewing area. By doing this, you can pretty much eliminate drydown.

    By the time I get to the pre-prod print, I am intimately familiar with the image and have made decisions on what are the important elements I am looking for in the final print. These elements guide my fine adjustments.

    The microwave is intended to be a time saver - speeding the drying time before final production. Using it too early will actually add process time and the information it yields at that stage is really not that useful.

    Just to be sure this topic is not misunderstood, microwave drying is not recommended for production prints.
     
  10. Photo Engineer

    Photo Engineer Subscriber

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    Remember that silver is a metal.

    Put some aluminum foil into a microwave and watch the fireworks (no don't).

    The environment around the silver filaments in the print is changed on a micro scale during microwaving. This has a subtle effect on the tone as noted above. IDK how bad or good it is, but I know it takes place. Overdoing it can change the image and create micro defects or other harmful effects as well.

    It is certainly a no-no for film.

    PE
     
  11. Joe Lipka

    Joe Lipka Member

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    Do it all the time for step wedges used in testing Pt/Pd prints. Used it on FB silver paper, too. Didn't work for me when I was doing RC "paper". The emulsion boiled.
     
  12. donbga

    donbga Member

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    What are production prints?

    Don Bryant
     
  13. photobackpacker

    photobackpacker Advertiser Advertiser

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    These are the prints you are producing for display of inventory - the finished product.
     
  14. emanduu

    emanduu Member

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    I use a microwave to test prints, I microwave them for 30 secs let them cool a bit and then do another 15-30 depending on the size of the print. I then let them sit for a few minutes for the remainder of any moisture to evaporate. It works really well, so far I cannot tell much of a difference in the final air dryed print.
     
  15. PeterB

    PeterB Subscriber

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    What change are you talking about? How do you know that it takes place?

    For microwaves to actually induce an electric field (AC current) in the silver metal, the length of the metal should be optimally 1/4 the wavelength of the microwaves (approx 12cm/4=3cm). The dimensions of any 'continuous' length of silver metal in the photo paper is much less than this. In fact the individual and unconnected silver halide crystals which then develop to form filamentary silver are tiny.

    regards
    Peter
     
  16. Photo Engineer

    Photo Engineer Subscriber

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    Peter, although an individual silver halide crystal in a B&W paper may be small, on the order of 0.2microns on a side assuming a cube of AgCl, the filament that is formed may indeed be quite long. This is assuming filimentary silver formation during development as opposed to tabular.

    There are ample photomicrographs of the filaments that form in the literature that I don't feel I have to post one here, but I do believe that they can approach a considerable length, but whether they can react with the shorter wave microwaves IDK. I doubt that anyone has 'unravelled' one of the filaments that form to actually measure its length.

    But, as you say, the operative word is 'optimally' meaning that the highest statistical probability is if the wavelength is as you say, but that does not rule out induced current in shorter or longer filaments. (does it?)

    In any event, I have not personally tried it, as I was dissuaded from using a microwave by my co-workers when I suggested it as a very 'green' researcher many years ago. As a result, no one I knew every used microwaves to dry photo products in KRL. Not even color products which contain no silver when finally processed. (This latter was interesting as I thought of it while writing this reply)

    Now, I may be wrong, and we may have missed something, but I had very pursuading counter arguments made to me by very respected individuals. So, for what it is worth that is the best I can say. I was told it was a 'bad idea' to use microwaves to dry a print or any photo product. Microwaves were never used to dry coatings in a coating machine, but if it were possible that would have been an ideal method of drying a coating.

    In a reply above, someone comments on melting RC prints using a microwave. Heat effects in a developed image would be more evident in RC products due to the relatively low melting point of RC. So, perhaps this was a manifestation of what I believe to be taking place. Also, the change in tone described earlier may be related to this. IDK.

    It may just be that it has nothing to do with the silver itself, but rather a vesicular effect from the micro bubbles formed if you boil water in your wet print causing some sort of translucency. There are many imponderables here that were brought to my attention those many years ago by some very respected fellow engineers with years of experience.

    Final word, you do what you wish, and I do what I wish in my darkroom. I offer only what I have been told in this instance having never tested it personally. I was advised never to try it. I never gave it another thought as we all used hot air drying.

    An added comment. IIRC, it was shown somewhere that microwaving unprocessed photo products had an effect on the latent image. If true, I wonder why? (While writing this, I remembered something regarding this from a publication many years ago and related this to the fact that we never use microwaves to dry coatings) So, make of it what you wish.

    PE
     
  17. PeterB

    PeterB Subscriber

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    PE, I'm not doubting the wisdom of your respected colleagues, my concern was that your proposed mechanism of the paper being "changed on a micro scale" (due to electric currents in the metallic silver inducing heating) has no theoretical basis. I believe that the heating effects are solely due to the water (and any other polar molecules in the paper) heating up.

    I think that a good reason not to dry prints in a microwave is that the uneven heating effects would heat some areas of the paper to excessively high temps. I tried drying an RC print the other day, and sure enough the corners curled up and these curled bits took on a more glossy appearance where I suspect some areas had melted. The problem could be reduced by lowering the power, but even so, some bits of the print would still receive a greater heating effect than other bits.

    The rest of my reply gets into the details.


    It is the two dimensional 'length' of the filament that is important here. As an example, if I take a telescopic FM radio antenna say 1m long (assume it is very thin for the sake of discussion), and crumple the antenna up into a small cube with a side of only 1mm, then the antenna will be useless if we want it to receive the intended FM radio waves.

    Technically it doesn't rule it out, but the current's magnitude will be SO small as to probably be but a few more electrons above the noise floor ! I could go and look up the maths, but we'd be talking pico, no, maybe femto-amps for sure!
    The wavelengths emitted by the magnetron oscillator will have a very narrow gaussian distribution centred around the centre frequency of f=2450MHz. This is the idea behind an oscillator - they have high Q's and thus low bandwidth (i.e. spectral spread). FWIW, the frequency for which 0.2microns is a 1/4 wavelength is f=2500GHz. This is 3 orders of magnitude higher than the microwave's frequency of operation.



    Now assuming an object is 'the right size' (has at least one of its dimensions approximately a few cm) then :
    Metallic (i.e. conductive) objects in a microwave heat up because the electromagnetic field induces a current of electron flow which dissipates heat due to ohmic (resistive) losses.


    Non conductive objects that contain polar molecules (e.g. H2O) in a microwave, absorb energy (heat up) as a result of dielectric heating.

    'shorter' doesn't really make sense here. I assume that the filament length (if stretched out) will still be much shorter than 3cm (=1/4 of the microwave's wavelength). OTOH, if you meant there are sufficiently different length microwaves in the oven to be significant at the filament's length, then that isn't the case.
    I think the relevance of this latter point supports my notion that it is the water that is heating things up and not the silver.

    Quite possibly localised heating could cause this, the object would need non-symmetrical or 'pointy' sections on it for this to happen. This is the case with the corners of the paper.

    My guess is that this is resulting from a weak dielectric heating effect.

    regards
    Peter
     
    Last edited by a moderator: Dec 11, 2005
  18. Peter De Smidt

    Peter De Smidt Member

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    I do occasionally microwave test prints with ilford FB paper. I put a piece of paper towel under the print, and my microwave has a rotating platter. It works fine for this, although the prints done this way do look slightly different from non-microwaved prints. In this case the paper gets slightly glossier and the blacks look a little deeper. Nonetheless, I'd never microwave "finished" prints, as I'd be worried about long-term harm.

    Peter de Smidt
     
  19. B&Wenthusiast

    B&Wenthusiast Member

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    microwaving prints

    instead of microwaving prints, or test strips, I learned something recently that works quickly and well --

    lay the prints out between pages of phone books for a couple of minutes -- the paper absorbs the water quickly and doesn't do anything to the prints!

    A good use for recycled phone books, also!

    :tongue:
     
  20. Photo Engineer

    Photo Engineer Subscriber

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    Peter, your very lucid explanation has convinced me that you must be right and I just never gave the subject enough thought. I do have some additional thoughts that I remembered after the last post.

    First, of course there must be an effect based on all of the above posts, and this must be due to something.

    Thinking back on this last evening, I seem to remember a demonstration of microwaving a step wedge of a B&W material and showing a thickness difference and an imagewise change in tone as a function of step. It was as if the image swelled as a function of the amount of silver and changed in tone quality. That is why I relate, in my mind, the effect of microwave on silver, but perhaps it is due to another effect.

    Your comments above taken with my remembrance of the demonstration would have me consider this imagewise effect to be related to water with some relationship to the silver image, but how, I am at a loss to explain. What I do remember reminds me of a relief image (not exactly) that formed imagewise.

    If it were due to water alone, I would expect this effect to be an even distribution, but I am pretty clear in my memory that the effect was imagewise and the comments here seem to indicate some weak correllation as well with an imagewise change.

    Any thoughts?

    Thanks.

    PE
     
  21. fschifano

    fschifano Member

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    You're right you know. Get all the excess water out of the print, then use a hair dryer. Easy!
     
  22. PeterB

    PeterB Subscriber

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    PE, the only explanation I can give is that there are some chemicals containing polar molecules which remain around the silver after development. As per my previous post, polar molecules do heat up in a microwave and this possibly causes the paper/emulsion to swell. Not being a chemist I haven't really taken a big interest (yet) in all the possible chemicals and reactions that go on in the paper which could contribute to concentrations of a particular substance (besides silver metal) forming around the image. Maybe there is no such thing, in which case I have no explanation for what you remember.

    regards
    Peter