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Question about mold casting

A friend of mine inherited a huge cache of plaster molds from an aunt who was much into casting all kinds of doo-dahs in them, then firing them and glazing them. He also inherited an automatic electric kiln (the one I use occasionally) along with them. He is not into ceramics at all, and has zero knowledge of the craft, but is very interested and wants to learn at least enough to allow him to use these molds.

He asked me for advice (talk about the blind leading the blind) and I told him I thought he just poured casting slip into the molds, slosh them around for awhile, pour it out, let it dry and open. He'd never even heard of casting slip. I offered to try and help him out, hence this post... There are a few questions I cannot answer:

  1. Is there any particular kind of casting slip he should get? The molds are things like vases, still lifes, basically the kinds of articles people refer to as "gifts."

  1. Should some kind of mold release agent be used?

  2. How long should the slip be left in the mold? Should it be rotated? Topped off? Should the hole be sealed with some kind of tape?

  1. To what cone should the casting be fired? Is this bisque fired forst, then glaze fired, or is it all done at once?

I appreciate any advice. Many thanks...

Fred

Reply to
Fred
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Dear Fred,

In article , Fred writes

Look for Hobbycraft casting slips for a start, like Exel, that should get him started.

No

If it's a big mould it will probably need topping up. Test the clay thickness at the top of the mould where there is a *lead in* or *Spru*. Use a teaspoon to sample the wall thickness, NOT a knife! When it is judged to be thick enough pour out and drain.

the temperature etc. should come with the Tech. sheet relating to the slip. Usually work is bisqued first.

Steve Bath UK

Reply to
Stephen Mills

[Casting slip comes in low-fire (cones 06 - 03, earthenware) and high-fire (cones 5-10, porcelain and stoneware) types. Either will work. High-fire ware is more durable, lowfire can have brighter colors. Of course, it's less expensive to fire to a lower temperature.]
[No, slip casting depends on the plaster absorbing water from the slip; a release agent would interfere with that.]
[It needs to stay in there long enough to form a sufficient thickness of clay. The time this takes depends on the piece, its intended use, the dryness of the mold, etc. Rotation shouldn't be necessary unless there are blind areas in the mold that would trap air otherwise. Topping off isn't usually necessary, and you don't need tape - just pour it out when it's thick enough, and leave it to dry until it's firm enough to be removed without damage.]
[That depends on the type of slip (see question 1 above). Most potters bisque-fire first, as it makes glazing much easier.]
[No problem, Fred. Making this sort of item was popular in the 'fifties (and the designs usually reflect that era), then the hobby went into a long decline. But with these "glaze your own ceramics" places popping up everywhere, your friend might find a market for them as bisqueware.]

Andrew Werby

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Reply to
Andrew Werby

One more tip on using plaster molds (falls under question # 3):

The wetter a mold gets, the longer you will need to leave the slip in to acheive the desired thickness. Plaster molds work by pulling the moisture out of the slip, leaving behind a thin layer of clay around the inside of the mold. The more times you use a single mold in a row, the wetter the mold will get, and the harder it is for the plaster to continue to pull the moisture out of the slip.

If you plan on producing from the same mold multiple times per day, just remember that each time you use it, it will need to sit a little longer with the slip in before you pour it out, and it will take a little longer before the slip cast form is dry enough to pop out.

(Granted, I make my own molds, so yours may be better/more absorbent than mine are... I am far from an expert at mixing plaster correctly!)

Good luck!

Reply to
anna

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