Recently I have had several inquiries from sculptors who want to experiment with pouring their own bronze using the lost wax process. While ceramic shell is the best way to go it is also more expensive, I have a simple formula and method that works well for pouring in a plaster investment.
Mix equal parts molding plaster and 200 mesh silica flour by volume in a large bucket or container. Dry mix these two ingredients well. Gate and spru your wax pattern, hollow or cored wax works better for this method. Build a wood box around your sprued pattern leaving at least 2 inches clearance on the sides and top. Mount the wax pattern on a board pouring cup down, place the box over the pattern, use 1/4 inch hardware cloth and make hardware cloth box that fits just inside the wood box. Make sure that the wire is not touching the pattern anywhere. Seal around the the bottom of the box with hot wax or hot glue.
Next we need to calculate the amount of plaster we will need to fill the box. Multiply the height x the width x the depth of the box, this will give you the cubic inches inside the box. Divide the cubic inches by 26.7 (one pound of plaster mix fills 26.7 cubic inches), then divide that by 16 to get the pounds of plaster and silica mix you will need.
The box is 16 1/2 inches high by 6 inches wide by 6 inches deep (16.5 x 6 =99 x 6 = 594 divided by 26.7 = 222.4 divided by 16 = 13.9 pound of mix to fill the box.
Here is the water to plaster mix ratio; 100 parts plaster to 50 parts water, i.e 13.9 pounds of plaster mix and 6.95 pounds of water. Place the water in one bucket and you plaster mix in another, Make sure your box is sitting level and is at a comfortable height for you to pour into. Pour the plaster into the water and let is soak a minute, then slowly stir until no lumps are present, pour into your box, tap the sides to insure their are no air bubbles. Let dry over night, the next day remove the wood box and allow to dry another 24 hours. The mold may then be place in a bake out oven to remove the wax, bring the heat up slowly to prevent cracking in the mold, I generally use an old electric kiln to bake this type of mold out. I generally bring the kiln temperature up 40 degrees per hour until it reaches 1000 degrees, I hold it at 1000 for another 24 hours to insure all the carbon is baked out, at this point the mold is ready to be removed, set in sand and poured.
Note the cost of the plaster mix runs around 25 cents a pound and cost to run the kiln for two days is $15 making the total cost for the mold I just describe $19.
Hope this helps!