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Oil-based Clay, plaster and heat


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#1 DHL

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Posted 17 May 2012 - 12:54 PM

I'm a ceramic newbie with a question: can the heat produced by curing plaster cause oil from oil-based clay to leech into the walls of a two-part plaster mold?

I've been making simple two-part molds and casting solid plaster objects wqithout problems. Now, I'm experimenting with making two-part molds for slip casting with Liquache, a liquid paper mache product and AM experiencing problems. Something in the mold-making process appears to be affecting the absorption qualities of the plaster molds. It occurred to me that the same oil-based clays which work fine for making molds and casting solid plaster objects may not work as well for molds intended for slip casting where water must pass out of the slip and into the plaster walls.

The problem I'm having suggests that parts of the mold aren't allowing an even absorption of that water.

So question 1: Is oil-based clay appropriate to use in the process of making plaster mold for slip castings, i.e., is its oil-content a problem? and

Question 2: What kind or brand of non-oil-based clay might best be used for this purpose?

#2 Lucille Oka

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Posted 17 May 2012 - 01:26 PM

Oil and water don't mix. Be sure to allow the plaster mold to cure and cool completely before releasing the model. However, do not allow the mold to dry with the model still encased or smearing of the oil based clay may occur.
A good oil based clay is Sculpture House Roma Plastilina Sulphur free; available in four consistencies hard, medium hard, medium and soft.

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#3 perkolator

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Posted 17 May 2012 - 01:51 PM

personally, i would never use oil-based clay as my positive to cast plaster from, especially for slip-casting. for a press-mold you might be ok. the oil will/may leave a film on the plaster that will definitely hinder the wicking properties that you're trying to achieve with plaster.

i was taught to make plaster molds with just regular ceramics clay object or found object. use same clay body for imbedding @ midpoint or section. quick coddles made from either clay, vinyl, or tar paper. pure lube mold soap for release agent. pottery plaster #1 as the plaster. when the mold's released, do a quick wash/wipe out with vinegar to remove mold soap, then dry mold while strapped together.

#4 Lucille Oka

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Posted 17 May 2012 - 02:39 PM

One other thing, oil based clay can give you a real smooth surface on your model if you rub it with a silk sponge wet with water. This smooth surface of course translates to your final slip casting.
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#5 neilestrick

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Posted 17 May 2012 - 03:11 PM

Ditto what Perkolator said. There's a darn good chance the oil from the clay has sealed off the plaster mold to some extent, and is preventing it from absorbing properly.
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#6 Marcia Selsor

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Posted 17 May 2012 - 03:29 PM

I have used oil based clay for making sprig molds which I use for press molds.
I never have noticed any adverse effects from the heat of the plaster curing. I use Pottery Plaster.


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#7 Stephen Robison

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Posted 17 May 2012 - 06:00 PM

I have not had much problems with oil based but usually just use water based clay. Just use the water based clay for your models if you are having problems and see if it fixes them.
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#8 Matt Oz

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Posted 17 May 2012 - 06:39 PM

You could try putting a coating of Murphy's oil soap (or similar) on your positive, to prevent absorption of oils. Then do a wash/wipe out, as perkolator suggested.

#9 Denice

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Posted 18 May 2012 - 08:45 AM

I have also made slip molds with pottery plaster and a oil base clay model and didn't have any problems but I wash it with lots of dish soap when I clear the clay out. I pour the soap in the mold and a little water and rub and scrub with my fingers and rinse well. I don't remember the brand of the clay but it was a expensive one. I split an order of it with a professional sculptor, I had just started working with the oil clay and he offered to help me get started the right way. Denice

#10 Lucille Oka

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Posted 18 May 2012 - 07:03 PM

Be sure your mold plaster mix is correct so that you will have a good sturdy mold.
I agree with Denice. I sometimes scrub my molds with a soft bristle tooth brush or a cellulose sponge.
If necessary wash the mold with a diluted mix of Dawn and hot water. I recommend saturating your mold with plain water prior to using the Dawn to prevent the mold from getting soapy.

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#11 Diana Ferreira

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Posted 20 May 2012 - 11:44 AM

Ok, me again. I would rather advise you to clean your molds with vinegar. The vinegar will unclog any blocked pores in the plaster mold. I frequently use plasticine when I make molds. Either to create a shape, or use it as a filler if there is tiny undercuts (or I made a boo-boo while I were cutting the master on the wheel, and cut an accidental groove in the surface). I use soft soap and a release agent on all my masters. And we discovered that a local supplier sells a cutting oil as a mold release agent. I have tried it - found no problem with the mold afterwards. But then I also squirt/spray water on the master just seconds before I cast my plaster into a mold.
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