Introduction to Molding and Casting

Molding and Casting techniques can be very useful in theatrical fabrication. Being able to easily replicate sculptural forms can be useful to stage, prop making and costume crafts. In this introductory project, students will learn to create a one piece plaster "dump" mold to create a permanent casting of their sculpture.

The Sculpt- As we enter this project; students have been introduced to a series of figurative exercises to build technique with working with oil based clay. Each semester will approach a different theme and create a low relief sculpture. Students will have a chance to utilize some of the design concepts we have discussed in the onset of the semester while creating their composition. Here are some examples of different relief styles:

Assyrian Low Relief Carving

An example of "sunken relief", a style almost entirely unique to ancient Egypt where all figures are pushed down below the surface of the stone face.

This example of high or "broken" relief created by Michelangelo would prove to be problematic for the process we will be approaching.

The dark shadows around the heads of figures indicate possible "undercuts" that would likely make it difficult to remove the casting from the mold.

The leg of the nearest horse breaks free of the background. In a one piece mold, this leg would become locked in the mold.

The Mold- A mold is a negative form in which the positive form of a finish casting will be created. A general rule of thumb in creating molds is that if you want a rigid final product you will want to use a flexible mold, and if you want a flexible casting, a rigid mold will be ideal.

One of the easiest molding materials to use is plaster. Gypsum plaster (or plaster of Paris) is produced by heating gypsum to about 300 °F (150 °C):

CaSO₄·2H₂O + heat → CaSO₄·0.5H₂O + 1.5H₂O (released as steam).

When dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing; but not full set for about 72 hours. Temperature will vary setting time. Heat will accelerate the process: hot water can be used to make the plaster react faster, and putting a hardened mold in an oven at low temperature will "cure" the mold faster by pushing out the remaining moisture.

If plaster is heated above , anhydrite is formed, which will also re-form as gypsum if mixed with water.

Plaster of Paris can be used to impregnate gauze bandages to make a composite material called modroc. This material was commonly used to create orthopedic casts. As a mold making or sculpting material it is used similarly to clay, as it is easily shaped when wet, yet sets into a strong and lightweight structure.

It is possible to use a plaster "waste mold" to create a rigid casting. This is accomplished by creating a thin shell which will be broken away from the final casting. In this project we will be creating walls around our original sculptures to create a single solid "dump" mold created by pouring plaster over and around the sculpt.

The Casting- We will be using a latex casting compound that will yield a semi-rigid end product. (Holden's Latex: HX-200 Casting Latex & HX Filler Hardener mixed 50/50)

A cured mold will be filled with the compound. The plaster will begin to draw moisture from the liquid material and after approximately 30 minutes remaining material can be poured out and re-used, leaving a relatively even skin within the mold. Generally, castings will be de-molded 24 to 48 hours later while still flexible. One of the downsides to latex is that castings will shrink. This is why it is ideal to not wait several days to remove the casting from the mold. Shrinkage can cause tearing of the casting, or for the casting to lock around more complex forms. The flexibility is ideal as it makes it possible to create forms with slight undercuts possible. Upon de-molding, the compound will continue to harden for a few days. Like plaster, low heat can be used to push moisture out to expedite full curing of latex casting.

Within this project each semester will approach a different them or objective. Each student must create a basic sculpture, plaster mold, and finished casting that must be painted and have some means of hanging or display to be considered complete. Assessment of student work will look at the design, execution of sculpture, and also completion and presentation of castings.

Considerations within the design :

• Work should be connected to the theme of the semester.
• Complexity of designs should appropriately challenge the student and showcase their abilities.
• Compositions can take elements of space, scale and texture to create emphasis.
• Line, shape, and texture can be used to create contrast or harmony within the composition.

Considerations within execution of sculptures:

• Realistic rendering of form.
• Shadow created should create a sense of depth and dimension.
• Attention should be given to detail and texture.

Within presentation of final castings:

• Problems that might have arisen during the molding process should be addressed. Edges should be trimmed and flaws patched to make it fully presentable.
• Any paint should ideally work harmoniously with the form of the sculpture and not obscure the dimension that has been created.