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  1. #1
    Athiril's Avatar
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    Potassium Dichromate Bleach?

    On one of my trips scouring the net I came across this reciple for b&w slide processing (http://personal.riverusers.com/~jdf/.../reversal.html)

    which lists a bleach recipe:
    water 1000.0 milliliters
    potassium dichromate 10.0 grams
    sulfuric acid 12.0 milliliters


    Can someone shed some light on this, if this bleach performs the same function as a colour bleach? and in effect can be used to bleach colour film (regardless of colour shifts).

    I was just thinking for us frugal digital darkroom users that this in terms of economy beats the pants off of even a copper sulfate bleach.

  2. #2
    Ian Grant's Avatar
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    It would work but it's not used for Colour work as it's gross overkill, it's a Black & White reversal bleach. It's more hazardous than Ferricyanide or EDTA based bleaches, the strong acidity is likely to cause a colour shift

    It's function is slightly different in that it converts all the silver to soluable Silver Chromates while conventional colour bleaches convert silver to a halide which is then removed by fixing.

    Ian
    Last edited by Ian Grant; 04-04-2009 at 07:45 AM. Click to view previous post history. Reason: add

  3. #3

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    As Ian told, this is the “classic“ BW reversal bleach. It does dissolve developed silver areas after first developing but does not touch undeveloped silver halides, which shall form the positive image after light exposing and second development.

    It’s very effective, re-usable, quite stable and does not harm film emulsion as bad as its eco-replacement (Potassium permanganate). But it’s toxic for almost every kind of living organism and a carcinogen, despite the fact it was used much more in “former” times.

    Be very careful if ever working with this and dispose it properly, never do a “sewer disposal” !!

    Regards,
    Stefan

  4. #4
    Athiril's Avatar
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    Quote Originally Posted by stefan4u View Post
    As Ian told, this is the “classic“ BW reversal bleach. It does dissolve developed silver areas after first developing but does not touch undeveloped silver halides, which shall form the positive image after light exposing and second development.

    It’s very effective, re-usable, quite stable and does not harm film emulsion as bad as its eco-replacement (Potassium permanganate). But it’s toxic for almost every kind of living organism and a carcinogen, despite the fact it was used much more in “former” times.

    Be very careful if ever working with this and dispose it properly, never do a “sewer disposal” !!

    Regards,
    Stefan
    carcinogen? so if i wear a dust mask, and rubber gloves it's still a bad idea to handle?

    I guess copper sulphate bleach would be much better than.

    What kind of disposal? patch of lawn in the corner?

  5. #5

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    Please read the following MSDS

    http://www.sciencelab.com/xMSDS-Pota...romate-9927404

    This is not really the stuff to begin with :o

    Regards,
    Stefan

  6. #6
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    The stuff is dangerous in that way the EU obliged manufactureres to switch over to Potassium Permanganate in the B&W reversal kits from Tetenal (in the mean time discontinued) and Foma.

  7. #7

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    High athiril

    Definitely it is dangerous, take a break and read that data sheet from “sciencelab”, please.

    What the EU says and the way they set priority is somehow relative. They just banned Mercury for thermometers etc. in April and now spreading it over whole Europe through CFL’s because they see a risk in incandescent bulbs. Strange world…

    Off topic, I know, but couldn’t resist

    Regards,
    Stefan
    Last edited by stefan4u; 04-04-2009 at 12:58 PM. Click to view previous post history.

  8. #8
    Ian Grant's Avatar
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    It's not THAT dangerous as long as you're careful. If it gets near a cut or scratch it will cause the wound to not heal properly & ulcerate, so use gloves.

    There are far worse chemicals that can be used for photography, I was responsible for running a laboratory (precious metal testing) and had to draw up the Health & Safety (COSSH) procedures to meet EU Regulations, they were quite sensible but thorough.

    Ian

  9. #9

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    I haven't used it but Tim Rudman mentions its use several times in his excellent: The Photographer's Master Prining Course" It clearly forms a valuable weapon in his printing armoury. His book was first printed in 1994 so not in "the dark ages" as far as omitting to mention the need to take sensible precautions against harmful chemicals. He mentions it as being corrosive and causes skin ulceration as do several darkroom chemicals that require sensible precautions but to the best of my knowledge he is still in the rudest of health.

    So a sense of proportion may be needed here. Oh and as far as I am aware his title of Doctor refers to his medical background so he is likely to know what substances are just too dangerous to even think about using for the amateur darkroom enthusiast. It wouldn't appear to fall into that list.

    pentaxuser

  10. #10

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    Dichromate is definitely not very dangerous if used with reasonable care. In fact, in the dilute solutions we use it photographic applications it represents almost no threat after it goes into solution. Also, it is used in very minute amounts in photographic applications.

    Main precautions, take care in mixing the dry chemical into solution, don't drink the solution, and wear gloves to avoid dermal absorption.

    Sandy King








    Quote Originally Posted by Ian Grant View Post
    It's not THAT dangerous as long as you're careful. If it gets near a cut or scratch it will cause the wound to not heal properly & ulcerate, so use gloves.

    There are far worse chemicals that can be used for photography, I was responsible for running a laboratory (precious metal testing) and had to draw up the Health & Safety (COSSH) procedures to meet EU Regulations, they were quite sensible but thorough.

    Ian

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