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Guest Sherman

Plaster Mold Of Wooden Object

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Guest Sherman   
Guest Sherman

Molds are like magic, and like magic, they can be mysterious. One CAD subscriber wrote in with a questions that is typical of folks just getting into making molds:

"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

 

Janet,

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!

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I found Pam Spray Cooking Oil works well as a release agent, Mineral Oil works good, I've also used Vaseline but you have to be careful not to get it too thick. Using Murphy's Oil Soap is a good idea I haven't used yet but will give it a try. Regular Dish Soap didn't work very well for me I had sticky molds.

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I find that vaseline works the best regarding a non treated wooden surface. Oftentimes wood has crevices which are too deep or sharply angled, causing the plaster to adhere. Currently, I am creating molds from very old pieces of wood which are extremely dry and extremely absorbent/porous. I use a fine wire brush to remove excess surface particles and then with my hands really smear the Vaseline on. With those sharper angles and crevices the Vaseline fills them sufficiently and is easily malleable. If you have the time, let it sit over night. If you don't, then simply rub the Vaseline deeply into the wood with your hands or a dry sponge.

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Marcia Selsor    1,301

I got a box of antique wooden furniture decorative pieces and cast them in plaster to use as sprigs. I use wd-40 for the release agent.

I also hand made reverse copies of these pieces in clay and cast them too.

Marcia

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If you use Vaseline, it is a good idea to heat the Vaseline with a hairdryer, to remove any streakiness that would be transferred to your plaster mold. I use soft soap that I dilute a bit with hot water, and rubbed off after a few minutes. I normally repeat this 2 or 3 times. (If you have a compressor, and have an acetone based paint (automotive paint) you will create a very good separation layer between your master and mold. It works very well on plaster masters too.) After I have applied the soft soap, I use a commercial mold release fluid, and just before I pour my plaster into the master, I spray the master lightly with water. It also helps to add a little blow-hole by inserting a thin stick into the poured plaster (as soon as it starts to set). Touch the master, and pull the stick back about 1 mm. Leave it in until the plaster starts firming up. If you keep it in longer, it might be difficult to remove the stick due to expansion. When I want to release the mold from the master, I force air into the hole (expect to see lots of water). most times the mold will lift of by itself.

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MadMudder    2

I use Armoral. It seems to work fantastically well.

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.

 

Beth

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