Casting Handout
Ceramics & Sculpture
Casting is the process of filling a mold with a liquid material that will then harden. It is
the replacement of one material (the original) with another, and it is used to create
sculptures in bronze or other metals, cement, glass, plaster, resins, or any other material
that can exist in a liquid state and then set in a hard state.
In the past, the most common core objects were of bronze. Scholars believe that by about
2000BC, cultures in the Near East were using earthenware molds to cast bronze. Many of
these early sculptures of gods and goddesses were solid and therefore small in size.
Cultures throughout the ancient world, from Greece and Rome to China, and from India
to Africa, refined the technique of casting in bronze. During the Renaissance, bronze
casting reached new heights with the work of such artists as Donatello, Andrea del
Verrocchio, and Benvenuto Cellini.
There are many cements and plasters available for casting. Plaster is used for sculptures
that will be displayed indoors, while most cements can be used for sculptures that will be
displayed outdoors. Some casting mixes have an aggregate added. An aggregate can be
any hard substance such as rock, granite, crushed marble, or even glass. Aggregate is
used fro color and/or strength.
Plaster Casting
Plaster and cast stones (cements) require similar processes for casting, but
difference materials pose different problems. Plaster sets fast, but some cements require a
prolonged time to set up. Rapid setting requires the sculptor to work at a quick pace but
also enables the work to be finished more quickly within an often easier-to-work surface.
Slow-setting cement allows more detailed surface work prior to setting.
Plaster and cements are cast in either solid or hollow masses. A solid casting can
be cast with or without reinforcement. A hollow casting can be slushed or laid-up by
hand. A slush mold casting is a hollow casting. It is created by pouring a liquid casting
substance into the mold, allowing it to partially set, and then draining the surplus. This
leaves a thin surface coast resulting in a lighter, more manageable casting. A laid-up
mold casting is also a hollow casting, but one created by adding casting material directly
by hand or by brush to the interior mold surface. Solid casts are must heavier and often
stronger, but not always practical due to weight and material costs. Hollow casts are weak
unless reinforced. However, with good reinforcement, a hollow cast can be strong. The
size of the mold and the opening into the casting area help determine the casting method.
Solid casting in plaster is done using a mold with a good release agent applied to the
mold pieces before they are assembled. (Petroleum jelly is the preferred release agent for
plaster casting.)
Cement
Cement is a building material known for its hardness and durability. It is activated by the
addition of water. Cement with building aggregates added is known as concrete. Cement
sculpture can be displayed outdoors; plaster cannot. Being much heavier and harder than
plaster, cement is more stable and less susceptible to damage from the elements. For this
reason, many sculptors prefer working with cement. The use of aggregates achieves a
stone-like appearance. A proper concrete mix includes cement, sand (preferably white
sand) , and various sizes of aggregates. Aggregates are added to the mix to strengthen
and/or add color or design value. Strength, especially for outdoor or larger works,
requires a variety of aggregates particles sizes. The strength is achieved by the differentsize aggregate particles filling all the spaces so that no void is left between fragments.
Bronze Casting
The exact beginning of bronze casting is uncertain, though it dates back beyond
3000 BC in Chinese, Indian, and Babylonian civilizations. All early castings were small
because they were cast solid. By about 2500 BC, the Greeks discovered how to cast with
hollow forms, allowing much larger castings. The lost-wax method of bronze casting was
refined by 1500 BC in the Chinese Shang dynasty; clay molds were used to withstand the
hot metal heat. When Rome became a world power, bronze was frequently used for
official portrait works.
Bronze casting was practiced as an inherited craft until the middle of the twentieth
century. This is when colleges and art schools began small teaching foundries, using
much the same methods as those passed down from previous generations. In the 1960s a
national sculpture conference was held to discuss metal castings. Here, artists and
instructors first learned about the ceramic-shell process, which is the most widely used
method by professional art foundries today. Although colleges still practice the traditional
plaster investment (mold) method, many are now converting to the new ceramic shell
(mold) process. The difference is that the mold was traditionally made of plaster and
sand, while the new process involves a more refined silica coating as well as higher heat.
The actual art of mold making and bronze casting has seen little change. It is still
called “lost wax,” because the wax artwork is lost in the process. A heat-resistant mold is
formed around the wax sculpture. The wax is melted out, leaving a cavity that is filled
with hot molten bronze. Once the metal solidifies, the mold is removed to leave a hard
bronze surface. Then the casting is finished, sometimes with a surface patina. Patina is
the color or finish on a surface. It can be natural or created by paints, chemicals, or
weathering.