clay lover 170 Posted July 27, 2018 Report Share Posted July 27, 2018 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. Link to post Share on other sites
Rockhopper 161 Posted July 27, 2018 Report Share Posted July 27, 2018 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. Link to post Share on other sites
Roberta12 447 Posted July 28, 2018 Report Share Posted July 28, 2018 @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 Link to post Share on other sites
LeeU 1,343 Posted July 28, 2018 Report Share Posted July 28, 2018 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. Link to post Share on other sites
clay lover 170 Posted July 28, 2018 Author Report Share Posted July 28, 2018 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. Link to post Share on other sites
clay lover 170 Posted August 2, 2018 Author Report Share Posted August 2, 2018 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. Link to post Share on other sites
Denice 760 Posted August 2, 2018 Report Share Posted August 2, 2018 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 Link to post Share on other sites
Rockhopper 161 Posted August 2, 2018 Report Share Posted August 2, 2018 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. Link to post Share on other sites
Marcia Selsor 1,905 Posted August 4, 2018 Report Share Posted August 4, 2018 try putting some heat lamps on the mold. it could heat up the oil. It has worked for me. Marcia Link to post Share on other sites
clay lover 170 Posted August 5, 2018 Author Report Share Posted August 5, 2018 Thanks, cold, freezer, did nothing. Link to post Share on other sites
clay lover 170 Posted August 6, 2018 Author Report Share Posted August 6, 2018 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 Link to post Share on other sites
Stephen 531 Posted August 6, 2018 Report Share Posted August 6, 2018 did you end up with usable molds? Link to post Share on other sites
Chilly 901 Posted August 10, 2018 Report Share Posted August 10, 2018 Would love to see pictures, then we can all learn........ Link to post Share on other sites
oddartist 12 Posted August 13, 2018 Report Share Posted August 13, 2018 I have nuked molds in the microwave to heat them up/dry them out faster. Link to post Share on other sites
alanhoff 1 Posted August 14, 2018 Report Share Posted August 14, 2018 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? Link to post Share on other sites
clay lover 170 Posted August 14, 2018 Author Report Share Posted August 14, 2018 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! Link to post Share on other sites
oddartist 12 Posted August 19, 2018 Report Share Posted August 19, 2018 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. Link to post Share on other sites
Mark C. 4,862 Posted August 19, 2018 Report Share Posted August 19, 2018 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. Link to post Share on other sites
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