[martinwilson1203]

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!!

[Borek]

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.

[martinwilson1203]

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 CO₃^2- ions are actively replacing the SO₄^2- 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.

[Borek]

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.

As though the CO₃^2- ions are actively replacing the SO₄^2- ions in the solid gypsum crystals?

Yes, that's the problem I suspect.

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.