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Bowl (used as model) stuck in plaster after pouring


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Hello,

I followed https://www.instructables.com/Plaster-Mold-for-Slipcasting-a-Bowl/ this tutorial to make my own 1 piece plaster mold. I used a similar bowl (ceramic bowl glazed on the outside) and followed all the steps (except for the slip reservoir part) and used Murhpy's Oil Soap as my release agent and rubbed it onto both the sides of the bucket and the bowl. I was able to rather easily release the actual mold from the bucket after it set for around 45 minutes by flipping the bucket and tapping the bottom. However, I can't seem to get the bowl out at all. I tried getting the bowl to shrink a little by pouring ice water into it, but that didn't work, and I spilled some onto the still hot and very moist plaster.  I'm not sure if spilling did anything, but the mold has been setting for 2 days now, and it's barely hardening at all. I also drilled a small hole into the bottom of the plaster mold (don't want to damage the bowl) and applied air pressure, but to no avail. I've now read a little and learned that it's best to wait until the plaster sets fully as it will expand (and increase the gap between itself and the bowl), but I'm at a loss at what I can still do. If necessary I can break the mold and repour it, but it would be great to have some tips on how to prevent the same thing next time. 

Edited by oola
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Does the bowl curve inward at all at the top? Are there undercuts anywhere on the foot? I'm wondering if it's trapped in the plaster or if it's stuck to the plaster. If the plaster hasn't hardened, then you may have mixed it wrong. What type of plaster did you use, and what water-plaster ratio did you use?

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51 minutes ago, neilestrick said:

Does the bowl curve inward at all at the top? Are there undercuts anywhere on the foot? I'm wondering if it's trapped in the plaster or if it's stuck to the plaster. If the plaster hasn't hardened, then you may have mixed it wrong. What type of plaster did you use, and what water-plaster ratio did you use?

No, the bowl curves outward at the top. It's a handmade bowl, so there might be a slight undercut, but I think the foot is mostly straight. This is my 2nd attempt at making a mold, and the first time I used a bowl that definitely should have worked (progressively wider towards the top, just a flat bottom with no inward foot) but experienced the same issue. I used pottery plaster #1 with the correct 7/10 ratio. Just to clarify, the plaster is definitely hardening, it just doesn't feel like there's much of a difference between an hour after I first poured and 2-3 days. 

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I've had luck with running water between the form and the plaster then tapping fairly firmly with a rubber mallet to get a stuck model out. Can you spin the bowl around within the plaster? If you can but it still won't release then I'm guessing there probably is enough of an undercut to stop it coming out. (It's super frustrating isn't it?!)

2 hours ago, oola said:

Just to clarify, the plaster is definitely hardening, it just doesn't feel like there's much of a difference between an hour after I first poured and 2-3 days. 

Depending on the humidity and airflow I don't think that's an issue. It can take me several weeks to dry out a plaster mold if I don't put it in a cupboard with a dehumidifier. Same size mold dries in a week with the dehumidifier.

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I had the same with an owl mould.  Tried water, soap, air.  Nothing shifted it.

In the end we sawed the foot off, and then put it back together with dowels.  It worked fine as a slip-casting mould. 

We never determined the problem.   The master was cast-Iron, and had several coats of sealer and copious amounts of release.

Maybe it just likes it's new plaster coat.

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Without photos I'll make a few assumptions: more than likely the rim/lip of your bowl form is rounded. (correct?) Regardless of the angle of the wall, if the lip is rounded, that is likely where the form is "catching".  Running a fettling tool, around the rim of the form, will remove the undercut.

Then, with one hand in the bowl, holding it as best you can, pounding on the mold rim, with a rubber mallet, will loosen the plaster mold. You can do so with a steel hammer but that will damage the mold rim. Sometimes a gentle pounding releases a form immediately, sometimes it takes a few minutes, but either way, its the only method to encourage separation between a stubborn form and the plaster mold. 

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Must have a under cut and or not enough mold release (pure lube is one of the best-https://www.axner.com/pure-lube.aspx

Now just wait a few weeks or cut the rim as noted in above post

Its usually an undercut and there can be NO UNDERCUTS IN MOLD WORK

since you said it may have a undercut-thats the issue most likely.

The 1st rule is NO UNDERCUTS IN MOLD WORK

second rule is -NO UNDERCUTS IN MOLD WORK

third rule -well, its the same as the rule 1 and 2

 

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