Tuesday, September 4, 2012
05 Molding Design Nº 5
The two main things that Martha Armstrong-Hand used plaster for were for making plaster rough shell molds over the oil-clay doll parts for casting carving wax, in order to translate one material into another material; and to make plaster slip-casting molds over the refined carving wax doll parts for casting porcelain slip, in order to make cast porcelain doll parts to make the finished dolls.
In order to use plaster molds to cast carving wax, the plaster rough shell molds are immersed in water until the plaster has absorbed enough water to be thoroughly saturated. Since wax and water do not mix, carving wax may be poured into a water-saturated plaster mold without the wax sticking to the plaster. This is an important thing to remember if you are going to use plaster rough shell molds to mold the oil-clay doll parts, for casting carving wax doll parts. When molten carving wax is poured into a water-saturated plaster mold, the carving wax begins to cool and thicken along the walls of the mold. When the carving wax along the walls of the mold have thickened sufficiently, the excess carving wax may be poured back into the wax pot, leaving a hollow carving wax casting.
The plaster slip-casting molds, on the other hand, are used after they have thoroughly dried, and have the ability to absorb water. It is plaster's ability to absorb water that makes it useful as a slip-casting mold. When the porcelain slip is poured into a dry plaster mold, the walls of the mold begin to absorb water from the slip, causing it to thicken. When the slip reaches the desired thickness, the excess porcelain slip is poured back into the slip container, leaving a hollow porcelain casting.
With both types of molds, it is plaster's ability to absorb water that makes it a useful mold material.
For oil-clay doll parts in-the-round, the molds will need to be made in two or more pieces. Each mold piece is made one piece at a time. There are several mold design considerations that need to be addressed when making a plaster mold, whether the mold is to be used for casting carving wax, or porcelain slip.
The first thing to consider is the size of the doll part that needs to be molded. The mold itself needs to have a certain wall thickness. Molds that have very thin walls are very easy to break when they are opened for the first time, and also have a tendency to break easily with repeated handling when pouring and casting. In order to obtain a mold that is thick enough for strength, the diameter of the doll part must be measured, so that a mold can be made that has walls at least 1.5 inches thick all around the doll part. The mold must also have room for a spare, which is used to pour the carving wax or slip into the mold. The spare may be made of clay, and incorporated in the clay build-up, or it may be carved into the plaster after it has set. One important thing to remember when making the spare is that it must be large enough to pour excess wax or slip through, after the wax or slip has thickened in the mold.
Usually, the size of the mold is determined by the cottles that are placed around the doll part to be molded. Cottles may be made from wood, clay, linoleum, or another material that can hold the poured plaster until it has set up. It all depends on how big the doll part to be molded is. Small doll parts can use clay cottles, which are slabs of clay fastened together around the doll part. Larger doll parts may need wooden or linoleum cottles. Usually, cottles made of wood are sealed with several coats of shellac or another sealer before being used. The important thing is that the cottles should not absorb any water from the poured plaster which may change its consistency.
Another thing to remember when designing a multi-piece mold is to make registration keys. Registration keys may be clay keys placed on the clay-build-up, before the plaster is poured, or the keys may be carved into the plaster mold piece after the plaster has set up. At least two, preferably three or more keys should be placed between each mold piece. The keys keep the mold pieces from sliding around during a pour. If that happens, the cast part will be offset.
Preferably, the clay build-up should always be perpendicular to the surface of the doll part. The clay build-up determines where the mold seams are. The mold seams should always meet the surface of the doll part at a 90 degree angle. The clay build-up should follow the parting line on the doll part. The parting line divides the doll part into sections that eliminate undercuts, so the doll part does not become locked in the mold. The parting line may undulate as it follows the division of the doll part. One half of the doll part will be submerged (hidden) in the clay build-up. The cottles are placed around the clay build-up and all edges are sealed so that the poured plaster cannot leak out. Once the plaster has set, the cottles are removed, and the clay build-up is carefully removed, leaving the doll part embedded in the set plaster.
For molds that have more than two parts, clay build-ups are made over all the doll part except where the plaster mold piece is being poured. As each piece is poured, more of the clay build-up is removed, until the last mold piece is poured. The angle of withdrawal must always be kept in mind when designing multi-piece molds. Indentations in the surface of the doll part dictate withdrawal possibilities.
Because plaster adheres to plaster, it is important to remember that all plaster surfaces that are going to come into contact with freshly poured plaster must have a mold separator, or parting agent applied to them before pouring the plaster. If this is forgotten, then the fresh plaster will adhere to the set plaster, and the doll part will be locked inside. Registration keys that have been carved into the set plaster must also be coated will mold separator.
If for some reason, a doll part does gets locked in a mold piece, a saw may be used to cut the mold piece until almost touching the doll part inside, then wooden wedges are tapped into the saw kerf to break the mold piece along the saw kerf. Then, a two piece mold becomes a three-piece mold, or the cut mold piece is discarded, and a new mold piece is made along a new parting line. The good news is, the doll part is not lost forever, encased in hard plaster.
Sometimes, rather than deal with overly complex molds, it is easier to remove the complicating member from the body of the original model, mold it separately, then reattach it later, after all the parts are cast. This is especially true for ears, fins, horns, and so forth.
It is also a good idea to make the first mold piece over the most detailed area of the doll part. This way, the detailed area is embedded in plaster first, and the clay build-up can be more easily removed from the less detailed area. For example, an oil-clay torso may have the back embedded in the clay build-up, and the front is poured in plaster first.
Make time to design and make a mold properly. If you have spent days, weeks, or months modeling a figure, there is no reason to rush through making a mold. Plaster mold making is a Craft Skill and deserves to be given the time considerations that it deserves. Why take a chance on ruining your sculpture by rushing through making the molds?
Next, I will cover the hot-pour moulage molding material.
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