Bronze Casting
by Gail Lang

Bronze is an alloy made of various combinations of copper, zinc, and tin. The combination varies according to the sculpters needs. Nickel, aluminum, silver, or manganese can be melted in with the molten bronze to produce a lighter colored cast. Bronze containing phosphorous will produce a red-colored bronze. Other chemicals can be applied to bronze to change the coloring to blue, red, brown and copper. The color of bronze can also be affected by weather. It will turn green or black when left outside over a period of years.

Bronze casting was developed over 5000 years ago. Egyptians and Chinese both developed sand casting techniques which are still used today. In recent times other forms of casting have been developed including: ceramic shell casting, styrofoam casting, lost wax and centrifugal casting. Newer metals, including aluminum and stainless steel are also cast now.

The basic principle in casting is to fill an empty mold that was created from the original model. Molds are made from numerous materials: sand, plaster, rubber, glue, metal, ceramic, clay and more. The materials are used to create a negative impression from a positive form. This negative form is filled and is called a ‘cast'.

While many materials can be used to create the mold it is important to remember that it must be strong enough to hold and contain the cast material. Bronze molds must be prepared carefully and materials must be chosen that can withstand the tremendous heat of the metal and the pressure of escaping fumes and gas.

Chemical combinations to create various colors

These combinations require that the metal be dipped into the solution if possible
Blue
Sodium Hyposulfite - 60 grams, Nitric Acid - 4,Water 1 quart
Yellow Green
Sodium Thiosulfate 1 gram, Iron Nitrate 8 grams, Water 1-1/2 quarts
Brown
Same as above, but brush until brown appears.
Blue Green
Copper Sulfate 5 ounces, Cupric Acetate 5 ounces, Copper Carbonate 5 ounces, Water 1-1/2 quarts

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These combinations requre that the mixture be heated then brushed onto the surface
Matte Brown
Barium Sulfide 1 ounce, Potassium Sulfide 1 ounce, Ammonia 2 ounces, Water 1-1½ quarts
Green
Ammonium Chloride 7 grams, Copper Acetate 8 grams, Water 1 ounce
Light to Dark Brown
Ferric Nitrate 1 tsp, Water 16 ounces
Brown to Black
Antimony Sulfide 2 grams, Sodium Hydroxide 4 grams, Water ounces
Purple
Early stage of previous recipe
Light Green
Sodium Chloride 5 grams, Ammonia 4 grams, Glacial Acetic Acid 4 grams, Ammonium Chloride 5 grams, Water 1 quart
Antique Green
Copper Sulfate 12 grams, Ammonium Chloride 2 grams, Water 8 ounces

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For these combinations Heat bronze and brush onto surface
Antique Green
Cupric Nitrate 40 grams, Ammonium Chloride 40 grams, Calcium Chloride 40 grams, Water 1 quart
Green
Cupric Nitrate 1 tsp, Water 16 ounces
Antique White
Bismuth Nitrate 2 tsp, Water 16 ounces
Blue
Potassium Sulfide 15 grams, Ammonium Chloride 200 grams, Water 1 quart

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Heat solution to a boil. Bronze should be immersed and solution stirred
Black
Copper Carbonate 2 grams, Ammonium Carbonate 4 grams, Sodium Carbonate 1 gram, Water 1 quart
Golden Yellow
Sodium thiosulfite 1/4 ounce, Ferric Nitrate 2 ounces, Water 1 quart

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Immerse bronze into solution until dull green appears
Verde
Copper Nitrate 1 gram, Ammonium Chloride 1 gram, Calsium Chloride 1 gram, Water 1 quart