Plaster Mold Of Wooden Object
Posted 22 March 2010 - 03:04 PM
"I was wondering how I would go about making a plaster mold for a clay form if I wanted to use a ready made wood molding? How would I prevent the wood from sticking in the plaster? Would I need to use a pliable substance instead of the plaster whereby being able to pull out the clay once formed? Thank you for your help."---Janet Gattsek
Your mind is guiding you in the right direction: You need to keep plaster from sticking to the item you are making the mold from by using a release agent. You also need to avoid undercuts so that the mold can release from the clay when the plaster mold is in use.
If only a pliable casting media will make it possible to get the cast separated from the mold, then it is not a mold that will work for ceramics. Place the item you want to cast on a flat surface and look straight down on it. Run a finger or tool against all of the now vertical surfaces of the item, starting at the table surface up the side toward you, ending at the top. Do this for every surface. If at any point the tip of your finger or tool disappears behind part of the form, then it will not release from a one-piece mold---that point will "lock" the form in the mold. You will need to cast several parts to the mold, so that each will release cleanly at different angles, designed around these undercuts.
A good basic mold release is a 70/30 dish soap (or Murphy's Oil Soap) and water mixture. You'll want to brush it on your model and mold forms, being carful not to raise suds, and let it dry before casting the plaster. Usually one good coat will do it---you don't want to use too much, because it can then "melt" into the plaster when the plaster is curing.
Porous surfaces need to be sealed. If the wooden item you are talking about is not already sealed somehow (with some sort of varnish, paint, or polyurethane), then you will likely want to do that and let it cure before using it to make a mold---and you still need to use the mold release. Wet clay does not need to be sealed, because it does not stick to plaster, but almost everything else does.
If you want to make your wooden form as it is, just made of clay, then the plaster cast will need t be taken from the form itself. If, on the other hand, you want to make a negative, then you can press clay onto the wooden form itself and you're done! The wood (if not sealed) will absorb some of the water from the clay and release it. You could also bisque fire the clay impression of this wooden object and use that as a mold. It acts very similar to plaster in that it will absorb water from wet clay pressed against it and will release it. The detail is often not quite as fine, and you have to take the shrinkage of the clay into account, but it can be a good alternative to plaster.
Hope this helps!
Posted 04 April 2010 - 10:31 AM
Posted 19 February 2012 - 01:39 AM
Posted 19 February 2012 - 09:03 AM
I also hand made reverse copies of these pieces in clay and cast them too.
Marcia Selsor, Professor Emerita,
Montana State University-Billings
Marcia Selsor Studio in Brownsville, Texas.
Posted 20 February 2012 - 12:14 PM
Posted 20 February 2012 - 02:28 PM
Plastic wrap works really well over wood.Just tape together on the back of the wood. You may have to scour the finished mold just a bit to remove wrinkles from the plastic but it is not much of a problem.
Remind me why I'm doing this????
-Beth Ward, Crone Potter
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