There are two basic types of waxes: solid and hollow. Solid waxes are the easier of the two because the mold is simply filled up with wax and allowed to cool. When cast, this will in turn produce a solid bronze. While this is fine for small or thin pieces, larger pieces will begin having problems with shrinkage. For this reason, larger pieces use the hollow wax method.
Hollow waxes have the advantage of producing lighter castings and minimizing shrinkage, but are more time consuming than solid waxes. Rather than filling the mold with wax, only a fraction of the mold is filled. The mold is then moved in a circular fashion while changing the orientation of the mold. As the wax cools and solidifies, it sticks to the interior surface of the mold. Excess wax is then poured out of the mold. This is referred to as slushing. After slushing a coat of wax, the wax in the mold must be allowed to cool before the next slush. The thickness of the wax is determined by the number of slushes it goes through before being pulled from the mold.
On some pieces, there is a combination of the two methods used. On the pieces above, for example, the women's torsos are hollow while their heads are solid. The torso and head are cast hollow to prevent shrinkage in the body. The head is then filled with wax so that shell does not get trapped in the confined space, unable to dry.
Regardless of whether the piece is hollow or solid, after it is pulled from the mold, it must be chased. Using various tools, the wax chaser removes the seam lines from where the mold came together. They then correct any other defects there might be in the wax.
After the wax of the piece is chased, it then needs to be prepared for the shell process. The first step is to cut and plug the piece as necessary to make the piece a workable size and allow air flow for drying in shell. The pieces and plugs are then sprued and vented.
Spruing and Venting
Sprues are attached to parts to be cast and then to a cup to act as pathways for the bronze to get into a piece and in some cases for the air to get out. The red bars in the waxes above are sprues. After being sprued, a piece is vented. The vents allow air to escape areas that are extremities or places that, when cast, are above the last sprue for that area. If a piece in not properly vented, the bronze will not flow into that area. This is referred to as shorting. You'll notice that the sprues all go to the top of the cup, while the vents go to the bottom of the cup. Once the shell is made and is being poured in bronze, the shell is flipped over and the cup becomes a funnel to pour the bronze into. This arrangement allows bronze to be poured into the cup and down the sprues without blocking the vents. This allows the vents to remain unobstructed so as to permit the escape of gases from the mold without having to go through the bronze poured in. Once this is done, it is ready for the ceramic shell process.
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