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Topic: Disintegration of gypsum molds in presence of wood ash  (Read 3274 times)

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Offline martinwilson1203

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Disintegration of gypsum molds in presence of wood ash
« on: March 22, 2015, 06:46:29 AM »
Hi,

As a sculpture student I am currently experimenting with casting ash (from wood fires) in Plaster of Paris (gypsum) molds. The process is similar to ceramic slip casting, in that I pour a slurry mixture of ash and water into the dry and porous molds, and then allow the water to be absorbed into the thick mold walls leaving the insoluble ash particles in the mold cavity.

This process is working rather well, except that after only a small number of repeated castings the inner surfaces of the gypsum molds become weak and begin to crumble. I need to work out why this is happening, and come up with a way to increase the longevity of my molds.

The problem is not very well defined due to the large variety of components found in wood ash, but I do know that the ash that I use is alkaline (as it reacts with fizzing when adding vinegar).

What could be reacting with the calcium sulfate in my plaster molds, and what could I try to prevent it? Would it be helpful to neutralize the ash with an acid before pouring it into the molds?

Thanks so much!!

Offline Borek

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Re: Disintegration of gypsum molds in presence of wood ash
« Reply #1 on: March 22, 2015, 07:23:53 AM »
If it is fizzing it contains carbonates. Calcium carbonate solubility is orders of magnitude lower than the solubility of the calcium sulfate. My first guess would be that you are replacing some of the CaSO4·2H2O with CaCO3. The latter has about twice lower molar volume, so your plaster becomes less dense and much more porous.
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Offline martinwilson1203

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Re: Disintegration of gypsum molds in presence of wood ash
« Reply #2 on: March 22, 2015, 08:30:08 AM »
Thanks for your quick reply Borek.

Are you saying that the presence of CaCO3 is making the CaSO4·2H2O dissolve faster than it usually would - compared to if I were just filling the molds with water? As though the CO32- ions are actively replacing the SO42- ions in the solid gypsum crystals?

And if this is the cause, do you expect that first reacting the CaCO3 (in the ash) with an acid would be beneficial? Or could I 'rinse' it out with excess water and then filter to be left with just the non-reactive solid particles in the ash?

Thanks again.

Offline Borek

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Re: Disintegration of gypsum molds in presence of wood ash
« Reply #3 on: March 22, 2015, 10:01:25 AM »
Are you saying that the presence of CaCO3 is making the CaSO4·2H2O dissolve faster than it usually would

Not presence of CaCO3 but presence of carbonates in general - wood ash contains a lot potassium and sodium, I expect sodium and potassium carbonates.

Quote
As though the CO32- ions are actively replacing the SO42- ions in the solid gypsum crystals?

Yes, that's the problem I suspect.

Quote
And if this is the cause, do you expect that first reacting the CaCO3 (in the ash) with an acid would be beneficial? Or could I 'rinse' it out with excess water and then filter to be left with just the non-reactive solid particles in the ash?

As explained above not CaCO3, but carbonates in general, but yes, treating them with acid will remove CO2. To keep things simple I would use sulfuric acid, that's a common ion with the CaSO4, so the interference would be as low as possible. Trick is, after treating with the acid ash will be no longer the same ash you started with, so its properties can be completely different. No idea what it will mean for your application.
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