Bronze Casting

THE LOST WAX METHOD

After I finish sculpting a piece in either clay or plasteline I take it to my local Art Foundry which I have been using for the last 25 years. The casting process alone takes 12 weeks, and cant be rushed. This is why it takes so long:

What is Bronze? First, lets understand what Bronze is. It's an alloy of copper with the addition of tin, lead, zinc, or other metals. In contemporary metallurgy, sophisticated alloys have been developed such as copper-silica, which is corrosion resistant. Bronze may range in color from copper to white, depending upon the alloy. For example, nickel silver, which resembles sterling, is actually an alloy of copper and nickel.

What is Patina? It's the industry name for the color of a sculpture. The rich patinas (color) found on bronzes and coppers are caused by surface oxidation or chemical combining on the surface with other oxides, nitrates or metal salts. These patinas are delicate and should not be touched, as the acids and salts present on our own hands may chemically react and alter the color. Patina colors range from rich greens to browns, reds, blacks, and blues. When created by an artist or foundry, the chemicals may be applied to the surface cold or by blow torching the surfaces and daubing the chemicals on. The surface color "develops" in a way similar to photographic development in a chemical bath.

Steps to casting in bronze:

Step 1, Master Mold

If the sculpture model is done in other than hollow wax formed in the proper thickness, a first mold must be make and the object cast hollow in wax. For a multiple editions, this mold will be designed for repeated use. The surface of the model is covered with a layer of rubber or plaster after "parting lines" are established. The parting lines must be placed so that the mold may be separated to remove the wax master easily and without damage; hence the mold may consist of a number of interlocking parts.

Step 2, Wax Master

Molten wax is poured into the mold which is rotated so that a layer of wax is built up on the inside surface to the intended thickness of the eventual bronze. This is the wax master.

Step 3, Bronze Mold:

The wax master must now have a mold and core built around it. This is usually a "wasted mold", meaning it will be destroyed during each casting. A funnel-like cup and thick rods of wax are first affixed to the wax master.

Step 4, Burning Out Wax and Moisture:

Next, the wax must be melted or burned out and the mold made chemically free of water. Plaster is made from gypsum which is fired in a kiln to drive out the water molecules. When mixed with water, it forms plaster as the water molecules recombine with the gypsum. Contact with molten bronze would re-release the water molecules in the form of vapor and ruin the bronze, so the mold is placed in a kiln and brought to about 1000 degrees, which vaporizes out the water molecules and the wax but a the same time leaves the mold weakened and fragile.

Step 5, Metal Melt and Pouring

The molten bronze is now quickly poured in the mold, hopefully in one pour and without mishap. The bronze must have been brought to the right temperature, and it is difficult to manage the white hot and extremely heavy crucible brought to the mold by a chain hoist and manipulated with a pair of large tongs.

Step 6, Removal of Bronze Mold

Now begins the excitement. Did the parts all fill? Are there large gas holes? How will it look in bronze? The mold must be broken and chipped away and the core chipped out - a big, messy job as the mold material sticks to the bronze surface.

Step 7, Removal of Vents, Sprues and Pouring Cup

Next, all of the networks of tubes and the pouring cup must be sawed off. Some of these parts will be saved for the next step. The steel pins used to hold the cores in place are pulled out, a bronze metal bolt. A piece of one of the vents or sprues is now ground down to the bolt size and threaded to fit the threaded holes in the surface. A piece of the casting is used so that the color with match perfectly.

Step 8, Chasing

The surface will now be "chased", with small chisels to remove burrs or to fill in gas holes or pits. Bronze is malleable and the surface may be easily worked with these chisels. The points are which the metal bolts were inserted may be chase over lightly and they will disappear.

Step 9, Patina

The bronze must now be cleaned three times - mechanically, chemically, and with soap and water. Most of the mechanical cleaning was done during the chasing process, but it may need sanding or sandblasting as well. A chemical bath or a solution of hydrochloric acid is given and quickly removed. Next comes a good thorough soap and water cleaning and the work is ready for patina.

Step 10, Sculpture Base

The bottom of the sculpture may have tabs cast in which are drilled and threaded for the attachment of a base which must be selected or manufactured.

Marilyn Ines Rodriguez

The Human Figure & Horses