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  1. #41

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    Quote Originally Posted by Marco B View Post
    Hi Katharine,

    I came across this link on:

    http://www.handprint.com/HP/WCL/waterb.html#PB27

    It has some interesting information regarding Prussian and other bleus, including some (light) fading tests of different pigments.
    Hmm, now we seem to be going in circles. Handprint is of course one of my trusted sources for information about watercolor paint, and I cited that site in the post earlier in this thread where I wrote about the variability in lightfastness among watercolor paints containing the Prussian blue pigment, PB 27. As I said then, it seems that the variability among watercolor paints has to do with extenders and fillers that are used in some paints, that compromise the permanence of the color. That's why I used pure powdered PB 27 to do the second test.

    I think I'm willing to accept for now the possibility that although Prussian blue pigment PB 27 and the compound that makes up a cyanotype are both ferric ferrocyanide (FeIII[FeII(CN)6]) there is a variability among materials having this structure, and perhaps there is enough of a variation in the generic cubic lattice structure to allow the compound as produced in the cyanotype process to be vulnerable to alkali and the pigment PB 27 not to be vulnerable in the same way.
    Katharine
    Last edited by Katharine Thayer; 09-05-2007 at 02:30 PM. Click to view previous post history. Reason: punctuation

  2. #42
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    You may want to consider the fact that copper sulfate hydrate is bright blue, but the anhydrous salt is colorless. This shows how drastically a colored pigment can vary as a function of water. The same is true with complexed ammonia when one views the color of ferric ammonium EDTA at different pH values (amount of ammonia). It varies from orange to beet red.

    PE

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