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My plaster of paris molds never work

I tried making one part and also two part (push) molds with plaster of paris. I followed the directions and mixed two parts of plaster to one part of cool water and poured into a box (dunno the technical name for it) over my shape. But the plaster sticks to my shape no matter what. I can't get my shape (positive) out without breaking my mold b/c it won't release. I've tried mineral oil, silicone release spray, and vaseline. The vaseline has worked the best but I can't use it on every part. I've tried making a mold of a piece of wood and a concrete paver and neither of which worked. 

I don't know what I'm doing wrong. It seems fairly straight forward. Lubricate the positive (what I want to make a mold of), put a slab of clay down, put the box around the positive and imbed into the clay to prevent leaking from the bottom, and then pour the plaster over the positive. 

Kiteman3 years ago

"Sticks to"?

Does it adhere to open surfaces, or jam in crevices?

(Also, see caitlinsdad's comment about shape.)

caitlinsdad3 years ago

Casting 101: Your pattern piece or thing to be molded needs to be designed in the first place so it can pull out of the plaster easily. You cannot have pieces that the plaster forms around and locks the piece in or in the way and obstruct another part from pulling straight out. Look at an ice cube tray. The sides are ever so slightly slanted, funnel like so you can pop the ice cube out. If you take a block of wood without rounding the corners and edges a bit, it will certainly lock in or snag when you try to pull it out. I hope that helps. Good luck.