Lost wax, or investment casting, is one of the oldest methods for casting items from metal. Basically, a wax model is made and then it is surrounded (invested) with clay or plaster. This is then heated to melt out and burn away the wax, leaving a cavity that is the exact shape of the original model. The wax is lost, thus the term lost wax.
Lost wax is used for jewelry, dental work and industrial manufacture. You can do it in your workshop with a minimum outlay. The process I will describe here uses a vacuum to draw the molten metal into the mold. There are several other methods such as steam and centrifugal that also work well.
What you will need:
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Step 1: Make your model. Remember, anything you can carve, mold or otherwise form from wax will work. You can even cast "found" items. I saw an insect that was invested and burned out (stinks!) and the hairs on the legs were reproduced! For my example, I will just use a finger ring for the model.
Step 2: Add the sprue. The sprue is the channel that the melted metal will follow to get into the cavity. For vacuum or steam casting, you will melt the metal right on top of the plaster so the sprue must be small enough that the melted metal will not drip through it. For large models, several sprues may be necessary. I find 1/16" diameter works well for jewelry size items.
Step 3: Tree your model (models) together by joining sprues together. You can cast one thing at a time, but it is common practice to "pack the flask" by putting as many models in as possible.
Here are four wedding bands cast together.
Step 4: Invest your wax. You can use a commercial sprue button that fits your flask to hold your wax while you invest or just make a shallow cone from modeling clay. Position your wax so there is at least 1" between it and the bottom of the flask. If you use clay, press your flask into it a bit to make a good seal to keep the investment from leaking out. I extend the flask with wide masking tape to keep the plaster from frothing over when I vacuum it. Mix the plaster and pour enough into the flask to cover your wax by about 1/4" and vacuum. Vibrate or vacuum the mix to draw out the air bubbles. Any bubble on your wax will be a metal bump on your casting. After you have de-gassed the mix, gently pour in plaster to within 1/8" to 1/4" of the bottom of the flask. Lightly vibrate it to level and let it set up.
Step 5: When the plaster is set (wait 3 hours), I remove the flask from the button. I like to carve away the exposed sprue flush with the plaster. This cleans up the pouring/melting basin sprue hole interface.
Step 6: Burn-out. I find getting the wax out is a two step process. I have a tuna can with a couple of rods across the top. I put a little water in it and place the flask on the rods upside down so the sprue is facing the water. This goes in the kiln and run at low temp (300F). The water steams up and melts out most of the wax and the wax drips down and collects on top of the water in the can. This will get the bulk of the wax out without turning it into fumes. I steam for about an hour. Then the flask alone is put into the kiln and heated to about 1200F and any remaining moisture and wax are driven off. When no more smoke or fire comes out of the sprue, it is ready for the metal.
Step 7: Have your torch ready! Put the hot flask onto your vacuum rig. Put your metal to melt in the basin. Be careful not to chip any plaster that could fall down the sprue. Melt the metal and give it a little superheat. When you think it is hot enough, engage the vacuum. The melt will get sucked down the sprue and fill your cavity.
Alternate Method: You can melt your metal in a hand crucible and pour it into the flask.
Step 8: Shake-out: After a few minutes, when you are sure the casting is solidified, just drop the whole thing in a bucket of water. It's hot, so you may want to lower it slowly. The plaster turns to paste and you will find your casting somewhere inside.
References: Stuller Jewelry Supply
Practical Casting - Tim McCreight ISBN:0-9615984-5-X
McCreight's book focuses on studio casting in general and gives a good overall view of methods and techniques, including, but not limited to, lost wax casting.