This is the cheapest and easiest method of preparing a mold for lost wax casting. All you need is your wax sculptures with sprues, plaster of paris, water and containers.
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Step 1: Make a plaster mixture

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Use two parts plaster powder to one part water. Most plaster packaging instructions say to mix water into plaster. On the contrary introducing plaster powder to water will minimize the amount of bubbles. So start with water and gradually add plaster. Note- plaster powder shrinks as it soaks, you'll use a lot more then you think so be supplied with a big bag before you start.

Step 2: Stir

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You can use a stir stick of some kind but I use my hands to be able to brake up all the lumps fast before the mixture starts to set. Be thorough and add a little more water or plaster until it's smooth and the consistency of pancake batter. If there are bubbles you can tap the bucket lightly on the work surface and watch them raise to the surface and pop. 

Step 3: Imbed wax sculpture

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Gently and slowly insert your wax piece into the plaster mixture. Sculpture down, sprues up. Sprues should remain visible on the top surface. Your piece should not reach the bottom or any walls of the container. Hold your piece hovering in position until you feel it's being supported by the surrounding plaster. 

Step 4: Allow time to set

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Allow the plaster to set completely, the longer the better ( I left it overnight).

Step 5: Pop the mold out of the container

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For one piece I used a plastic container. For the other which didn't fit into a plastic container I improvised with aluminum foil. Remove whatever you used as a holding vessel and get to the bare plaster mold.

Step 6: Bake the mold

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Even though the plaster mold appears to be dry, there is moisture trapped in it. In order to dry it out and pre-heat it before melting the wax out use a conventional oven and very slowly increase the temperature. Heat shocking it will cause the mold to crack so be patient. Follow my next instructable on burning the wax out of the plaster mold for lost wax casting for further instruction. Have Fun!
Nice one! Check my instructable on making a vibration box to get the bubbles out! Thanks
chakra3 years ago
one ever gnawing doubt.. it is said that plaster of paris is a bad mould material if u r casting anything with a higher melting point than of lead.. and disaster for silver or copper.. suggestions?
Miss Cabbit (author)  chakra3 years ago
Good question! I have casted silver and aluminum in plaster molds with great results. There is a particular method of doing this though: You have to heat treat your molds. For investment molds a conventional oven works well. Over several hours gradually increase temperature to 400 then gradually decrease to cool as to not crack your mold with temp shock. With lost wax without a vacuum chamber you'll have to get it much hotter to burn out the wax. I use a kiln and ever so gradually increase the temperature to 1000 degrees. Then gradually bring down to cool and leave in the kiln over night. It always works despite varying opinions. Good Luck!

wouldn't it make more sense to cast, high temp metals while they are hot? to reduce the high gradient shock, then reduce the temp gradually? in a kiln with a programmable controller, designed for annealing glass.

hey!! thanks for the tips!
will let u know next time i cast!

I know this is two years late. Great Instructable. I have cast bronze and aluminum in a plaster lost wax mold. It's how I originally learned. I've done it for small (jewelry scale) sculptures and larger scale sculptures. The largest size is 18"X18".

mslade12 years ago
Very informative! I just wish it was longer.
aholbrook2 years ago
Wow. Thank you for this write up. I've been looking into doing this for some time (been using lost foam casting) and now this weekend I will try my hand in lost wax casting. Thank you again.