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Interference Coloring (Three-Step)

Interference coloring, the newest development in the aluminum finishing story, offers anodizers the ability to produce a wide palette of stable colors with an anodized finish. These colors offer a whole new world and dimension to architects, designers, and others. They are a real departure from the traditional bronzes and black of the two-step electrolytic process. These colors are produced by the refraction, reflection, and absorption of light hitting the surface. Because of this they are impervious to the effects of ultraviolet radiation and therefore are stable and will not fade with time and exposure in outdoor applications. By modifying the anodic coating prior to the deposition step, a prismatic effect is achieved. This in theory will allow for the production of any color in the rainbow. However, in practice, to date, it has been found that colors such as greys, blues, and greens are more readily repeatable than colors such as yellows and reds.


One of the earliest methods of interference coloring was achieved by first anodizing with sulfuric acid and then anodizing a second time in a phosphoric acid, followed by the deposition of nickel into the pores. This process produces a blue-grey color that is the combination a blue interference color and the standard electrolytic bronze. This process has been carried out in Great Britain for some 16 years now.

Another process creates light pastel colors, some of which even change color with the angle of observation. This process is more of a four-step method, as it involves a final anodizing after the modification and deposition of a metal. This process, although having many steps, is said to give very good color uniformity. It gained some recognition and use in Japan in the early 1990s.

More recently, three processes have emerged in the marketplace. The colors created by this new breed of technology are of a solid intense nature, somewhat like the appearance of dye work; however, they are stable and lightfast. These new processes all give similar results, but by different approaches. They all require and rely on three distinct steps: sulfuric acid anodizing, electrochemical modification of the anodic pore, and metal (tin) deposition. Where their approaches differ is in the configuration of the tanks and equipment for the process. They range from one tank per step to one tank for anodizing and modification, to one tank for modification and metal deposition. There are a few of each type of installation around the world at this time.

Regardless of which process is chosen, the level of success will greatly depend on the individual shop's ability to control all aspects of the anodizing process to very tight tolerances. Anodizers and their customers should also define, ahead of time, the color parameters that will be accepted. Color variations tend to be more a variance in hue of the color rather than a variance in intensity of shade (light to dark).

Applications and Markets

The applications and potential markets for this finish are as wide and varied as the imagination of the designer. A large market that is keenly interested is the architectural field, which is looking for a lightfast, durable substitute to high VOC paints and organic coatings and yet has a wide range of colors. Certainly the world of sports equipment, auto manufacturing, decorative trims and showcases, as well as light fixtures and consumer durables, all offer potential applications for this finish.