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clay lover

advice on plaster molds that won't release, please

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I did my first foray into pouring plaster into rigid forms to make hump molds yesterday.   I use 4 small dishes from the junque store that were shapes we wanted to use.  Two were sloped sided,  two were rather straight sided. None of them had undercuts or ridges on the sides that would prevent the set up plaster from coming out later.   I used mineral oil, brushed on to all 4.  Today, the two slanted sided ones came out with a bit of upside down tapping.  The straight sided ones WILL NOT RELEASE!   What now?  The forms  that came out are cool to the touch, still feel damp, but solid.   I used the formula that Gypsum plaster provided, and followed all mixing , pouring instructions.

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Could be they're "vacuum locked".  In order to get the plaster out, air has to get in.  

With sloped sides, as soon as the plaster moves even a tiny bit, it separates from the sides, and air can easily get past it into the dish and break the vacuum.  The straighter the sides, the farther it has to move to reach a place where the plaster is narrower than the dish, so that air can get in.  The mineral oil may actually be working against you in this situation, as it creates a flexible seal between plaster & dish.

If you're not up against a deadline, you may find that just waiting a day or two will give the plaster time to shrink a bit as it dries, and create the needed gap.

If you don't care about the dish,  drilling a small hole or two in the bottom might do the trick.

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@clay lover, I just made my journey into plaster mold making as well.  I was taught to use Murphy's oil soap, but I am certain the mineral oil would be fine also!  We had a little trouble with some of the dishes releasing.  Once they were cooled, I just took a hammer to it and tap tap tapped and they released.  Only one dish broke and that was ok.  It also, was from a thrift store!  I hope yours release!

Roberta

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The original formula Murphy's Oil Soap is a reliable release agent, in my (not extensive) experience. Mineral oil may not be as effective, overall. 

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I'm going to wait a day or 2 and see if anything changes.  The mineral oil suggestion came from a book called "Mold Making and skip casting",.   I thought about drilling a small screw into it for pulling force.  .  I'm not in a hurry and would rather not break the forms until all else fails, so we'll see. 

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Why don't you make a replica piece out of clay and that way you don't have to fight with the plastic form.  Was your plaster mold totally dry?  If it wasn't you may have to wait awhile.   If it is humid and my mold is drying slowly  I will put it in  a 175 degree oven any hotter will damage the plaster.  I don't know what temperature your plastic platter will withstand.  Denice

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10 hours ago, clay lover said:

nothing has worked.  I even drilled into the plaster and set screws to pull on it with.  Guess I get to try and chip it out and repour.

I mentioned this before, but did you try drilling a hole in the bottom of the dish ?  If air can't get in, the plaster's not coming out, no matter how hard you pull (at-least not in one piece).  It's the same effect as a suction-cup firmly pressed onto a mirror or smooth piece of metal:  If you pull on the center of it, it's going to stay - but if you slip a fingernail under the edge, so air can get in, it will pop right off.

What are your 'forms' made of?  If they're plastic or metal, you should be able to drill a small (1/8") hole that will let air in, but not prevent being able to use it again as a mould by covering the hole with a piece of tape on the outside.

Something else to look at: Are you certain the sides are 'straight', and not tapered inward ever-so-slightly ?  If it's even a tiny bit narrower at the top than at the bottom,  you're fighting a losing battle - even if you do get air in from the bottom.

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VICTORY!  as Marcia suggested, I heated them, by putting them in a skillet with boiling water around them, until the plaster was hot.  After removing them from the water bath, they would not budge, so I figure I would start this morning on trying to chip the plaster out and start over.  I had drilled holes and set screws nto the plaster earlier,  trying to pull the form out, unsucessfully.  this morning, I pulled on the screws in the now cooled plater and both of them released.!!  thanks for all the suggestions.  Now to pour more , with better release agent

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On 8/13/2018 at 1:52 AM, oddartist said:

I have nuked molds in the microwave to heat them up/dry them out faster.

 

What's your microwave temperature and for how long you let them cooking?

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Steven, yes, they were fine.  Chilly, it was just poured into some junque store cassarol dishes that had pretty straight sides, thus making getting them out tricker.  Nothing special.

I think I will now try adding 2nd layer to increase depth of hump mold options.  Book says, make some deep scratches in the plaster first, them add fresh pour on top of it. we will see.  I WILL use  better release agent this time!;)

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Alanhoff, just 30 seconds at a time until they heat up enough to draw the water out. Then I pull them out and let them air for a bit. Repeat as needed.

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Just buy some lube made for this process-its cheap enough

https://www.axner.com/pure-lube.aspx

or you can struggle with corn starch and olive oil or  wd 40 or whatnot

also picking the right forms also helps as not all forms can be cast well.

Edited by Mark C.

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