The coloring and finishing of art metalwork is a very important factor in its success whether viewed from a commercial, artistic, or educational standpoint. It is of importance commercially because articles well colored and finished will sell more readily than those that are not. It is of importance artistically because of the color harmonies and tone values that are involved. It is of importance educationally because of the direct correlation with an important school and industrial subject, namely, chemistry.
Copper has an especially strong affinity for sulphur and oxygen. It combines readily with the moisture and carbon dioxide of the atmosphere to form basic copper carbonate and basic copper chloride. There is no other metal known upon which so many, or so beautiful colors can be produced readily and easily by means of its own compounds. If a piece of copper is cleaned and exposed to the air for a few weeks, it will assume a dark color (frequently called the "Patina") caused by its combination with the oxygen of the air, or with the hydrogen sulphide if soft coal is burned in the neighborhood, or it will turn a green color caused by its combination with the moisture and the carbon dioxide present in the air.
There are three important points to keep in mind when coloring and finishing art metalwork: first, the work must be perfectly clean; second, all of the chemical solutions when not being used must be kept in well corked bottles; third, no matter what color is obtained the final operation must be to protect and preserve the color by a coat of lacquer or wax.
To color successfully art metalwork, whether copper, brass, silver, or gold, one must always keep in mind that the metal must be perfectly clean and free from any trace of oil or grease. Even so slight a thing as the moisture from the hands is a frequent cause of failure along this line. The metal may be cleaned by friction, that is, by rubbing with emery cloth, powdered pumice, or a wire brush, or polishing on a lathe; or it may be cleaned by dipping for a few seconds in strong acids.