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Anodized Aluminum

Anodized Aluminum.  A coating, actually plating on aluminum which unlike other plating can be done in color. Light weight metals cannot be plated with normal metals – gold, silver, copper – but by using the same electroplating equipment aluminum can be treated, depositing a film of aluminum hydroxide on its surface. The anodic film has the ability to absorb dyes; thus pieces struck in aluminum can be so plated in color. The film and color are permanent even though the film is very thin. In the numismatic field aluminum has replaced white metal as a low-cost coin and medal composition and special medals, like the Mardi Gras medals – aluminum throws – are made in color.

Anodizing aluminum in color.  Pieces struck in aluminum are degreased, washed in detergent, rinsed and dried. Each piece is hung on an anode rack of pure aluminum with pure aluminum connectors. The rack is immersed in an electrolyte (of dilute sulfuric acid and sodium sulfate) and current applied. The oxide film coating forms in about 30 minutes. The rack containing the coated pieces is removed from the electrolyte and washed in cold water. At this state the aluminum surface appears cloudy. A second bath is prepared with the dye to add the color. The cold pieces are immersed in a boiling hot solution containing the dye for about 10 minutes; they are then rinsed in hot water and dried.

The color and film at this stage are fixed. The color is permanent and will

withstand temperature extremes, even boiling water again. Any color dye can be used (but no chemicals, like vinegar, or metals, like copper, can be present). The color is the responsibility of the finishing department and may vary from batch to batch (thus color anomalies are finishing errors).

Anodizing is noted for its low cost token production. An even lower cost process

is to anodize and dye the strip first before blanking and striking. The low weight tokens for use in mass mailing direct mail pieces (like Reader's Digest tokens) have been made by this method.

Minor scratches will remove the film (and color) but the light-weight composition does not elicit nicks and dents like heavier alloys would do.  See aluminum throw.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


Roger W. Burdette, Editor

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