Anodized metal can be found on nearly every type of new building—from storefronts to high-rises. In most cases, an anodized finish is a flat or satin finished piece of aluminum in either clear (silver), a shade of bronze (light, medium, and dark), or a black finish. Most anodized aluminum is not typically reflective. Anodized aluminum is created by dipping mill finished aluminum in a series of specific chemicals at a certain temperature for a certain amount of time. There are many factors that create an anodic coat on aluminum. The more factors, the more likely there will be variation. 

I grew up working at my family’s aluminum finishing plant, and I learned one lesson above all others: we are all at the mercy of our metal. Let me state one thing first and foremost about anodizing: expect a color variation. It may be one out of one hundred pieces or it may be one out of two pieces. 

The Differences

Why is this so? Aluminum has many different elements in it. As with any metal, there are different alloys. My company uses 6063 alloy aluminum for extrusions and 5005 alloy for sheets. These are anodizing quality. This does not mean that they are the same however.

The first and most common variation in color arises from the fact that different batches of metal will anodize differently. It is not only the finish that should stay the same, but the metal as well. If a customer wants us to anodize their metal as well as SAF’s or wants us to anodize to match their already anodized material, expect color variation. Since different batches of anodizing quality aluminum have different amounts of particular elements in them, they will appear different after being anodized. Because of the number of variables involved in anodizing, the slightest difference in what constitutes a particular alloy of aluminum can cause a different appearance. 

To many architects, variation is a problem because a uniform finish is desired. If an owner/architect/general contractor wants a uniform finish, they should choose paint and not anodizing. Painting costs more money, but anodizing offers a natural depth of color that paint is used to avoid. 

Alternate Alloys

The other less frequent problem that arises when architects want a uniform anodized finish concerns different alloys. Specifically, the problem arises when matching anodized sheet to anodized extrusions. As I explained, a batch of metal for a particular alloy has different elements in the aluminum, and different alloys have even greater differences. The majority of anodizing quality extrusions are 6063 alloy. The majority of anodizing quality sheets are 5005 alloy. The clear and dark anodized aluminum for both are relatively well defined. Whether 6063 or 5005, clear and dark bronze anodized material will have less variation and will be more likely to match. The 5005 light and medium bronze, when compared to 6063 light and medium bronze, will appear different. Again, this is because the elements in the alloys are much more different. Compared to 6063 medium bronze extrusions, medium bronze sheet comes out with a green tint. Light bronze is slightly easier to match. The key factor in both situations is having an anodizer that can take a target sample and use that target to match. We are lucky that in this day and age we have the technology to define how close the match for a batch of anodized material comes to its target.
 
But buyers, contractors, architects, and owners should be aware of the risks of color variation.

USG
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