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  1. #1
    mrtoml's Avatar
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    Potassium carbonate as bleach for cyanotypes?

    I have been trying to get some washing soda aka sodium carbonate to use as bleach for cyanotypes rather than using ammonia. All the brands I have seen locally seem to have other things in them such as surfactants. I don't know if these are bad or not. I have some potassium carbonate already and thinking back to my schoolboy chemistry days was thinking that this might act in a similar way to sodium carbonate. Anyone know?

    Cheers,
    Mark
    Mark Tomlinson

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    hi mark

    can you get baking soda ?
    baking soda is washing soda but with more water absorbed
    into the powder.
    if you put some in a oven at a low temp you can evap the moisture out of it
    and convert it to washing soda.
    i can't say much about the potassium stuff never used it ..
    g

    good luck !
    john

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    Hi Mark,

    Just use soduim bicarbonate (bicarbonate of soda) from the cake ingredients aisle at Tesco's; dissolve it in hot water and then dunk the print. Make a test print to give you an idea of what happens at different strengths. I also find the washing soda from Tesco's works fine too, despite having a few other ingredients. Prints on different papers will also respond differently too, so it is worthwhile trying a few different paper brands.

    Best regards,

    Evan

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    Quote Originally Posted by jnanian View Post
    hi mark

    can you get baking soda ?
    baking soda is washing soda but with more water absorbed
    into the powder.
    if you put some in a oven at a low temp you can evap the moisture out of it
    and convert it to washing soda.
    i can't say much about the potassium stuff never used it ..
    g

    good luck !
    john
    Baking soda (sodium bicarbonate) and washing soda (sodium carbonate) aren't just different because of more/less absorbed water. That's the difference between anhydrous/monohydrate/decahydrate forms of sodium carbonate. They're different compounds.

    But jnainan is correct that baking soda will decompose on heating, slowly, starting at 70c, more quickly at 200c, into the anhydrous form of sodium carbonate. Left open after cooling, it will eventually absorb some water and convert to the monohydrate form, the most stable.

    --Greg

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    mrtoml's Avatar
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    Thanks for all your replies. I can get baking soda no problem, but as that is bicarbonate was not sure whether it would work.

    I will try it.

    I may as well try the potassium carbonate as well as I have a big pot of it, but just wanted to know whether anyone else had used it.
    Mark Tomlinson

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    Baking soda is less alkaline than washing soda. It will probably work, but it might take longer to achieve the same amount of bleaching, and your final image tone might be different than if you used wshing soda.

    The big point is that with baking soda, a glass baking dish, and an oven, you can make highly pure washing soda from baking soda with just an hour or so of your time.

    --Greg

  7. #7

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    Mark, potassium carbonate should be perfectly fine for bleaching cyanotypes.

    Regards,
    Loris.

    Edit: Use exactly 30% more (by weight, compared to sodium carbonate) if you're going to use a specific formula / strength that states sodium carbonate. OTOH, it's not that much critical...
    Last edited by Loris Medici; 07-04-2012 at 02:32 AM. Click to view previous post history. Reason: Added extra information...

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    mrtoml's Avatar
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    Thanks a lot for your help. I'll try both now I know how to make Sodium Carbonate and see if it makes a difference.
    Mark Tomlinson



 

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