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  1. #1

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    May 2011
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    Converting dichromates for gum and related processes

    I stumbled across an article on water treatment for chromium (VI) contaminated effluents. The authors successfully used sodium sulphite solutions at fairly low pH to reduce Cr (VI) to Cr (III), the latter of course being relatively harmless. I haven't seen in my literature any suggestion that gum prints should be first washed in an acidic sulphite bath before further washing, but it seems like good practice to at least investigate whether that would make the bichromate processes more environmentally friendly.

    There are a few questions worth asking though.

    i) Will the low pH bath negatively affect the gelatin sizing or gum layers?
    ii) How much post washing, or what alternative treatment, will be required if multiple layers are to be applied? Residual sulphite in the paper might consume some of the bichromate before UV exposure, rendering the sensitivity lower or affecting the contrast etc of the subsequent layer. If it is uneven, it would be worse.

    These are of course only ideas at this stage. I will definitely look into it when I do gum printing again (soon, I hope!), but would welcome the response of regular gum printers.

    Other reagents such as ferrous sulphate or ferrous ammonium sulphate may also be used, but since most APUGgers have sulphite stashed somewhere, I thought it would make for a neat practical way to reduce our environmental footprint.

  2. #2

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    Jul 2010
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    Before disposing of any unwanted dichromate solutions they should be treated with sodium sulfite. The solution color will change from orange to green showing that the Cr (VI) has been reduced to Cr (III). Treating prints however may cause problems and is not recommended as it may damage the print.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  3. #3

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    I think there is no Chrome(VI) left in the processed gum print, since you wash out the sensitizer and the hardening is done by means of Chrome(VI) being photoreduced to Chrome(III). Chrome(VI) in the wash water is another issue, but I don't know exactly how to reduce Chrome(VI) to Chrome(III) because I use very little (2-3 ml 5% for each layer) and flush with plenty of water. (Makes max. 0.04-0.06g Chrome(VI) in 2-3 liters of water when it goes into the city's sewer system, not taking into account the water that gets added when flushing thru the WC. This is an extremely unsignificant amnt.)
    Last edited by Loris Medici; 03-23-2014 at 08:33 PM. Click to view previous post history. Reason: Adjusted the amnt. of Chrome(VI)



 

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