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Plaster traces on slip casted pieces

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#1 Cadaola


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Posted 09 November 2012 - 05:37 AM

I made plaster molds and casting slip (porcelain), I checked specific gravity and viscosity of the slip and that was ok. Then I made the slip casting process and some pieces came out very well, but others carried some traces of the plaster on them. Does someone know why?
Is that because the mold is not dry enough?
Or should I put some "protection" on the mold surface that will receive the slip? I noticed that the molds that gave good results were spreaded with some release agent such oil or wax (becuse the object I molded needed it). I attach some picture to clarify. If I remove the plaster from the pieces, can I fire them? Or the contamination is "impossible" or very difficult to clean up?
Thanks a lot! Paola

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#2 Marcia Selsor

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Posted 09 November 2012 - 06:11 PM

The plaster is on the surface and not imbedded into the body of the piece. You could leave it and it would fall off or clean it off of the surface.
Do not put anything on the plaster to seal it or you will ruin the plaster mold.
Marcia Selsor, Professor Emerita,Montana State University-Billings

#3 Mark C.

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Posted 09 November 2012 - 08:46 PM

As noted above the plaster can be brushed off later when piece is dry.

A reason this may be happing is the mold was made with a bad (poor or old) plaster mixture.
Do not seal or mold soap the molds as this will not help dry the slip and in fact will work against you.
If you plan on making a lot of these pieces from the mold make that mold over again doing all the right steps with plaster mixing.
Use fresh plaster and read up on how to mix it.
Mark Cortright

#4 BeckyH


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Posted 10 November 2012 - 10:23 AM

You have to let your molds dry between pourings. If you make several casts in a row, the plaster will degrade. As for your current pieces, just scrape the bits off with a sharp craft knife and sponge the surface smooth.

#5 Cadaola


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Posted 10 November 2012 - 12:32 PM

Thanks a lot for your help and advices :-)
I'm happy that I can fire the pieces, they are my first casts and the whole process is very exciting!
Thank you very much.

#6 Wil



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Posted 13 November 2012 - 12:02 AM

I have been working in slip cast ceramics for over 40 years. I have found that as the mold ages, it will wear. The condition which you are describing could be caused by casting too soon. Reguardless of what you have been casting or pouring, it is known that porceline can be cast more than non-porceline casting in a regular period of time. The pieces of mold could be caused by fatigue by the plaster. Remember, you made the mold. The general rule of thumb is when the cast item is dry, the mold can be cast again.

I am a certified ceramic instructor and have been so for many years.

As for putting something on the surface of the mold to protect it, do not. The slip will not set up in that area and you will ruin the mold. I am manufacturing some molds of my own and I am keeping it simple. I make the forms from hard wood and then produce the mold with good plaster. If there is a problem with my mold, I make another one and destroy the bad one.

Good luck in your actions and career

#7 perkolator


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Posted 14 November 2012 - 08:11 PM

that looks like a little more volume of plaster stuck on than the typical "plaster haze" you might get from slip casting -- my guess is your mold is slightly punky and didn't set up properly; caused by either bad mixing ratios or old plaster.

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