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Acid For Plaster of Paris

  1. Oct 25, 2006 #1
    I'm looking for an acid or whatever that will react with plaster of paris that has already set to a solid to dissolve it back into a liquid state. Basically, I want to cast something in a plaster of paris mold, but instead of breaking the plaster mold off I'll "melt" it off. I need to do it in this unorthodox way because what I'm going to cast is fragile and a very complex shape that I doubt I could remove from the mold intact any other way.
    I've read that citric acid can dissolve it, but this was in reference to cleaning small amounts of it off tools, not dissolving a solid mold that weighs ten or more pounds. The acid (or whatever) needs to be fairly cheap and not horribly toxic (no gas mask and thick rubber gloves needed), if possible.
  2. jcsd
  3. Oct 25, 2006 #2


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    Do you have time on your side? How about dilute acetic acid (vinegar)?
  4. Oct 26, 2006 #3
    You are asking too much. It's Very difficult to find even a toxic chemical that can easily dissolve CaSO4.
    Anyway, you should tell us of exactly which material/materials is made what you want to cast, otherwise we could suggest you an acid that dissolve that material as well!
    For example, you could try with hot oxalic acid (I don't know how effective it is, however) but some metals dissolve in it as well.
    Last edited: Oct 26, 2006
  5. Oct 26, 2006 #4
    It's similar to this stuff: http://www.delviesplastics.com/casting_resin.htm
  6. Oct 26, 2006 #5

    Sure, or rather maybe, depends on how long it would take. Would this take weeks, couple months, more?
  7. Oct 26, 2006 #6


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    I'm no chemist but I do a lot of remodeling. How about drywall Joint Compound? It resembles plaster (white, chalky and hard) but doesn't set up chemically--it dries out, and will dissolve in water even when hard. It has high shrinkage, so you'd probably have to build up thin layers and let each dry.
  8. Oct 26, 2006 #7
    You could try some of the acids available from a home improvement store for cleaning grout of ceramic tiles -- Muratic Acid is available from Home Depot. If your plastic can handle it, you can get Sulfuric Acid drain cleaners from most places as well (be very careful!!!!).

    I would wear gloves and safety glasses with any acid when you are reacting this much material. Better to be safe!

    Otherwise, could you instead make your mold out of silicon?
  9. Oct 2, 2009 #8
    try HCL (muratic acid) from home depot its very corrosive though and will dissolve many other things, will probably not dissolve polyester resin

    HCL + CaSo4 -> H2So4 + CaCl
  10. Aug 18, 2011 #9
    I've done extensive experiments on this subject using many acids and caustic chemicals. My finding which is contrary to popular belief, is that acids had no effect on Plaster of Paris regardless of the concentrations. Plaster of Paris did dissolve however with chemicals on the basic end of the ph scale, not acids.

    What dissolved it better than anything I tried was warm water saturated with sodium bicarbonate, or Baking Soda.

    I then used baking soda in warm water to dissolve a thick plaster of paris mold from a piece of delicate plastic.
  11. Aug 19, 2011 #10
    "Dissolve" seems quite a big word. Do you mean that the solution became limpid?
  12. Aug 19, 2011 #11

    Good point Lightarrow. It softened it up a layer at a time, on the stove top over low heat, like about an 1/8th of an inch, so that it could easily be brushed away. I had to change the water and baking soda periodically. Dissolve might not be the best word here then.

    Potassium cyanide also had a similar effect at room temp on a tiny test chip, and Clorox had some small effect.

    There was a final layer which contained a vegetable oil parting compound, and the baking soda could not dissolve, or loosen that up, even with time, and I had to sand that layer out.

    I had a delicate plastic model, due to unexpected foaming, or otherwise it would have easily separated from dry plaster with the parting compound.

    I did various tests with hydrochloric acid, nitric acid, sulfuric acid, acetic acid, various mixes of hydrochloric, nitric and sulfuric, under various temperatures and lengths of time, with various sample pieces, almost without any success. This whole process of experimentation took place over several weeks, until I hit upon baking soda, and "brushed away" the big block in about one day.

    There was one occurrence however that I can't explain, and it points towards a quicker process still. I had been experimenting on both the main block of plaster that I needed to dissolve in small patches, and also on small chips of pure plaster. There was an area on the main block that got hit with several acids in various places earlier and rinsed, and perhaps hit with nitric or bleach just before, and when I applied sulfuric acid to that patch, it dissolved a large section instantly, about one inch deep. I tried to replicate it for a day and couldn't. That tells me that there is some combination of what I did that might be useful to people who need to do this.
    Last edited: Aug 19, 2011
  13. Aug 20, 2011 #12
    It is very difficult to sat what could have worked. The only thing it comes in my mind, and it could be plain wrong, is that previously used conc. sulphuric acid had deydrated the plaster so when you used it again, there was no water to react and it could transform calcium sulphate into (soluble) hydrogen sulphate.
  14. Aug 30, 2011 #13

    I need to grow moss on plaster for an art piece.

    Moss thrives in an environment with a low pH, but from what I've been reading new plaster carries a high pH.

    Does any one have any ideas of how to lower the pH in the plaster without corroding it? My brother-in-law is a physicist and suggested possibly applying lemon juice after the plaster was dry, but was concerned that mixing anything into the wet plaster would prevent it from setting.

    Any suggestions on how to create an acidic environment for my moss to grow in? I live in the Northwest so our damp environment with low light is ideal, but am quite concerned about the high pH of the plaster.

    An acid that is TOO strong will also likely cause the moss not to grow.

    Thanks so much!
    Last edited by a moderator: Aug 31, 2011
  15. Sep 3, 2011 #14


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    I assume you are using a lime-based plaster? Any white portland blended into it? If so, the pH will be high and acidifying it will cause it to go to mush. If you use a lime only based plaster, you might treat it with CO2 to encourage the carbonation reaction which is how lime only based plasters 'dry'. You only need to 'acidify' or carbonate the surface of the plaster so you don't need to worry too much about acidifying the whole. This is how concrete, marble, stone and so forth are degraded with time.
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