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Fixing up little holes in a plaster mold for slip casting


Polyhedron
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  • 3 weeks later...
On 11/25/2021 at 2:21 PM, Chilly said:

The holes will from bumps on the cast item. 

Might be just as easier to rub them smooth after casting.

It's unlikely that tiny amounts of plaster will stay in place.  Might be just as quick to re-make.

 

Thanks for the info. There are some small pointy details that are breaking off, are there any tips for reinforcing them? Maybe by using a different mixing ratio for the plaster? Or is it because I use too much mold release, does that affects things?

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3 hours ago, Polyhedron said:

any tips for reinforcing them?

might be the plaster is too weak, or wrong type of plaster.  Or just the pointy bits are too small to survive slip.

 

3 hours ago, Polyhedron said:

because I use too much mold release,

I don't know if that would cause that.

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A picture would help. Not sure if the "small pointy details" are plaster or clay?

Too much mold release. Are you sponging off  any excess mold release? Are you pouring while the release material is wet? 

Its best to make sure the surface being molded is dry. If its wet the mold release can absorb into the plaster surface.

I use Murphy's and I sponge off the excess lather and then use a fine brush to remove any bubbles that remain.

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Rereading your subsequent posting I'll offer this: if the "pointy details" are areas in the mold that have fine detail/sharp edges I would suggest you design these out of the mold. In other words, fill them in with clay, on the model, BEFORE you make the next mold. 

When making a mold, for myself or others, I try to create a mold that will result in consistent results. A mold that gradually wears away, after being exposed to casting slip, is not an effective mold. Instead I fill in the deep areas, before pouring the mold, and then instruct the customer to dig these out AFTER the piece has been cast. That way the piece comes out of the mold consistent with itself and its just a subjective decision how much "detail" is added to the finished pieces.

I also contend that this detail usually is not noticed by the customer. I rarely hear people complain about sculptures not having enough detail.  They do, however, notice/complain about inconsistent results.

Regarding filling air bubbles after pouring plaster: its a nuanced process but I find it best to thin the plaster down to a "milk like" consistency. I then pour it into the air pocket. Quickly the water is absorbed and the plaster remains. The plaster does shrink, quite a bit, so you then need to apply another coat.  I repeat the process until I have filled the hole. After the water is completely absorbed I then use a scraping tool to gently level off the plaster. If timed right I can get a nice smooth surface to the filled area.

 

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  • 4 months later...
On 12/14/2021 at 7:14 PM, Jeff Longtin said:

Regarding filling air bubbles after pouring plaster: its a nuanced process but I find it best to thin the plaster down to a "milk like" consistency. I then pour it into the air pocket. Quickly the water is absorbed and the plaster remains. The plaster does shrink, quite a bit, so you then need to apply another coat.  I repeat the process until I have filled the hole. After the water is completely absorbed I then use a scraping tool to gently level off the plaster. If timed right I can get a nice smooth surface to the filled area.

I stepped away from doing it this way because I always have this mess around the pocket as well. Instead, I fill the pocket with plaster and add a drop of water. It does shrink as well, and needs repetition, but cleaning up is more easier.

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