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I'm working on some plaster carvings, following the steps in the answer I was given about making plaster.

These blocks dried in their container for about 8 hours, then have been air drying for another 48, as of this morning.

Last night, they were fine. As of this morning when I woke up, even the carving that I started has its sharpest edges turning orange. These edges do not match the edges of the initial mold. They seem to be the most completely dried portions of the plaster. The thinness of the edges might have sped up the drying process, which could be part of the issue.

As of now, about 7 hours later, my WIP carving has started turning white wherever it has dried out.

I used blue, liquid food coloring dye.

Why are my edges turning orange? What could I do to prevent it?

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It seems to have stopped turning white as of 72 hours or so. The orange is gone, but blue remains.

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  • What type of blue dye did you use? Brand or chemical ingredients would help.
    – BeaglesEnd
    Commented Jul 1, 2016 at 14:53
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    @BeaglesEnd Added a link to product
    – user24
    Commented Jul 1, 2016 at 14:59
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    Thanks, it's Blue 1. If it had been Blue 2, then as a redox dye it would change colour based on the ph level. Plaster can get to 13+ ph, and Blue 2 changes colour from Blue to Yellow at about a ph of 11-13.
    – BeaglesEnd
    Commented Jul 1, 2016 at 15:08
  • Interesting -- it definitely looks like it's heading to yellow...
    – Erica
    Commented Jul 1, 2016 at 15:18
  • Just to add, you probably would have seen it turning colour when it was added to the plaster. There are some blue redox dies, like DCPIP, that turn pink in acidic conditions but that is not a food colouring!
    – BeaglesEnd
    Commented Jul 1, 2016 at 17:16

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The dye you have linked to is known as Blue 1, or Brilliant Blue FCF, and is, unlike my comments under the question, a synthetic dye. It is a dye that can be consumed by humans. It is not defined as a redox dye (indicator), so should not react to changes in pH, but it is derived from Methylene blue which does change colour from blue to clear in it's reduced form.

I cannot provide a definitive answer on why the dye appears to have broken down but the probable causes are a reaction to the plaster, air, sunlight, or a combination. Nor can I suggest how to stop the colour change once it has started.

What I can suggest is to use dyes purposely manufactured for use with gypsum, limestone, or portland cement plasters. There are quite a few colours available, such as blue, and can be easily obtained from building supply stores and websites. The link I provided is for the UK, but I'm certain that they are available world wide.

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