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Dennis Hetland

Stuck In The Mold

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I assume that these are moulds you made yourself? Are you copying an idea you've seen?

 

The drying clay does shrink onto inner surfaces, so it's a compromise between too early release

(with the consequent distortion which might be correctable) and the casting firmly stuck on to the

inner mould surface..

 

Dusting the difficulty surfaces of the dry mould before casting does help. IIRC the usual recommendations

are talc for low-fire and neph syenite for high-fire. Both worked well for me. Maybe dab the mould with a

twist of nylon stocking with a little of the powder in it. 'ware dust.

 

Are you are double-casting (the clay is solid cast between the two halves of the mould).? If so the outer

mould will give a lot of support if you can remove the inner mound first. ... unless the inner mould is firmly

stuck by the time that the solid casting has started to set.

 

If you are single casting  (giving you an inner and outer wall) it sounds very tricky unless the shapes are well-

chosen. In my limited experience immature flat surfaces are very inclined to distort under their own weight.

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Awesome!!! I think that sounds like the answer. Thank you.

I just happen to have a bag of neph syenite.

Let me clarify a few things.

I should apply the neph syenite dry? Dust my inner mound with it? Not make it into a paste like kiln wash?

The slip won't wash it down the sides when I pour it?

 

This is all my original design. I've lost count to how many cardboard models I made before I got to this point, but I'm not quite done shaping my design. I plan to make a plaster positive and reshaping/refining it slightly. So another question I have is if I coat my mold with tincture of green soap will that interfere with my mold absorbing water from my slip during future castings? I want to use plaster for my positive because I'm on a budget, but if it's going to damage my mold I'll spring for a poly cast product to make the positive from.

What would you recommend?

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Awesome!!! I think that sounds like the answer. Thank you.

I just happen to have a bag of neph syenite.

Let me clarify a few things.

I should apply the neph syenite dry? YES Dust my inner mound with it YES? Not make it into a paste like kiln wash?No paste

The slip won't wash it down the sides when I pour it?Will not matter

 

This is all my original design. I've lost count to how many cardboard models I made before I got to this point, but I'm not quite done shaping my design. I plan to make a plaster positive and reshaping/refining it slightly. So another question I have is if I coat my mold with tincture of green soap will that interfere with my mold absorbing water from my slip during future castings? I want to use plaster for my positive because I'm on a budget, but if it's going to damage my mold I'll spring for a poly cast product to make the positive from.

What would you recommend?

What is suggested is put some talc (for low fire) or neph sye (for high fire) in a bit of ladies stocking and dust (shake) onto trouble release areas of mold .

Green soap will not affect your plaster .

Hope this helps

Mark

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I recently made a positive from a plaster mold using cheap dish soap as a release and plaster bandages to make the positive. The soap washed off with no issues, just give your mold enough time to dry. Not sure about using straight plaster.

 

If you brush the neph sye on in paste form, you'll leave a brush mark texture, and be adding more moisture to your mold. A dusting of it shouldn't wash off the side unless you're washing the casting slip down the interior sides of the mould. If you pour your slip into the centre of the opening, it should rise evenly up the sides.

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Just to clarify,  I put about an eggcup full of the powder in the toe of an old pair of tights and tied it

off to give a full bag.  I then patted this onto the mould surface. [Actually very similar to the bags

of charcoal traditionally used for pouncing.] I then poured as usual. Actually with my mould I couldn't

avoid pouring straight onto the treated part of the mould.

 

I found that this definitely helped separation, but wasn't a panacea.

 

BTW the choice of talc or Neph Sye is to minimise any visible effects on the fired pot. So by all means

test with neph sye, but if you low-fire you may want to switch to talc later.

 

Actually the first time I emptied an unused tea-bag, filled it with powder, then sellotaped it closed. You just

need something porous enough.

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You can buy it a drug store -use it as a mold release with any type of mold.

I use a mold release from Laguna called Pure Lube which is made for thois purpose.

You can order it at the ceramic shop online.

Mark

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A question to the more traditional mould makers out there.

 

If you use the traditional mouldmakers-size treatment on the casting face of a

mould does it influence either casting rate or the ease of mould-release?

http://www.scarva.com/en/Scarva-Raw-Materials-Mould-makers-Size--Soft-Soap/m-132.aspx

 

Background

Green-soap/soft-soap/mouldmakers-size has traditionally been used as a parting agent

when casting plaster onto plaster. AFAIK it's a fairly time and labour intensive process

which relies on the potassium soap changing the top layer of the treated surface to

calcium stearate (ordinary or hard soap contains sodium rather than potassium).

[For most dirt-removal purposes detergents performed better than  traditional green soap,

which was phased out as a main-stream cleaning product. AFAIK not everything now sold

as green-soap is the traditional potassium soap; as used by Victorian housemaids, and still

widely sold in my childhood (in hard green blocks).]

 

Personally I've got by using a slightly diluted detergent mix on both non-absorbant and

plaster surfaces.

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Years ago I saw an article about casting something really difficult, pure alumina I think.

It had practically zero green strength, which made it difficult to extract from the mould.

So they first cast a very thin layer of paper onto the mould, then the slip. I cannot imagine

that this is too good for the mould, and it may reduce mould life by blocking the pores.

 

If you have a mould you are willing to sacrifice, might be worth a try. From my paperclay

making efforts I would recommend cheap white toilet-paper for making the paper-slip.

It disperses quite readily in water, espicially hot water. [Expensive zeta-potential controlled

paper pulp would presumably be better.] Perhaps you could arange only to cast paper onto

the interior part of your mould.

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Hi Mark

To clarify; Can you pour the slip on green soap treated plaster?

When you use a product like Murphy Oil Soap or some of the other mold release soaps sold from different ceramics suppliers, they recommend you clean the mold soap off with white vinegar.

Scrubbing a detailed mold with vinegar is a pain and it would be a time saver if scrubbing off mold soap was avoidable.

 

PeterH I have seen a lot of people praise green soap. It was nice to finally see that the potassium in the soap is the reasoning behind why it works as a release.

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Just out of curiosity, my pottery backround is weak but I’ve use vasaline (heated with a heatgun to thin it and make it go into nooks and such) for mold release on many materials. It’s a problem if you need to paint the cast piece, you have to clean up the residue first but in pottery I would  expect the vasaline to be anialated in the first firing. Would this be a good (and cheap) alternative?

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Seems to me like a lot of good advice above, but you have to trawl through it all to make sense of it.  

 

I hope this summary will help.

 

Plaster moulds do need release agent while being produced.  Each part of the mould needs a release agent applied before the next part is cast, otherwise the plaster sticks to itself.  Also some models need a release agent to stop the plaster from sticking to the model.  Damp clay and plastic models do not need a release agent.  Most other materials do.  I use soft soap (bought from a pottery supplier) when making moulds, and then scrub the mould with vinegar to remove every last scrap of the release agent before allowing the mould to dry thoroughly before using.

 

Plaster moulds do not need any release agent when filled with slip.

 

Some say that an extremely dry mould does not cast well, and they recommend spritzing the surface with water just before pouring the slip.

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