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bluecreekpottery

Why is my clay sticking to my plaster mold? And how do you keep yours from sticking to your molds?

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It has been a long time since I have done anything with molds as I am mainly a wheel thrower. I have a plaster fruit bowl mold  (hump mold over a year old but have never used it before until now) and my clay won't release from it and I'm scared that if I don't take it off soon that it will crack (its been on the mold for almost a full day uncovered). I can't get it off, what should I do? And how do you keep your clay from sticking to your plaster molds?

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The mold needs to be bone dry-the clay should not be sticky wet. whats your climate and is that mold super dry?What is the studio temp? Are you in AZ or Maine?It all matters.

Edited by Mark C.

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I have found with hump molds that often the very edge of the slab needs to be gently picked loose from the edge of the mold/form so that the slab is not hooked over the edge and can slide upward slightly as the overall slab shrinks while drying. Usually the rest of the slab will release easily when it has dried slightly, but don't take it off until it is dry and strong enough to hold its shape on its own.

Rex Johnson likes this

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Hump molds are just a bit more fussy than slump as with a slump the piece shrinks away and does not crack. With a hump it can crack as it shrinks. You have to be on it with hump molds to prevent that cracking-Dick has a good point on the edge catching.

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dick is right, i was taught at a workshop to run a fingernail under the edge of the clay on a glass mold to release the edge and allow shrinkage.  the glass mold had WD-40 brushed all over it to prevent clay from sticking.  do not use WD-40 with plaster but do release the edges.   at some point you can slide the clay all over the mold and remove it once the edges stiffen enough to lift without tearing.  flip the whole thing over onto a clean bat if you can handle the weight.

(people wonder why i have 7 totally bitten fingernails and 3 longish ones.)

Edited by oldlady
i only have 10 fingers, cannot do math! 8 +3=11
Rae Reich and Chilly like this

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....what Dick said, and yes, the humidity makes a difference too. I work outside using 18-20" hump molds, and I have to keep on them. That is, I have to pay close attention to how fast they are drying. I try to catch them right at leather hard. I normally trim the lip right at the edge of the mold or higher up from the lip so there's no overhang.

When they are just right, I'll pull at the edges, alternately one side then the opposite, until the shape releases or 'slides'.

Once that's done, I can leave it until it stiffens a bit more or pull it set it upright if it's stiff enough.

As a note I don't use any release agent (WD40 or whatever). My molds are commercial molds made of Hydrocal plaster.

I wipe them down with a sponge before each use and directly apply the clay.

 

IMG_4485-XL.jpg

Edited by Rex Johnson

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