Volume 38, Issue 7, July 2003
What You Need to Know
The Basics of Buying Architectural Aluminum
by John McClatchey, Ph.D.
Glaziers buy aluminum extrusions, sheet and brake shapes on most commercial jobs. The purpose of this article is to cover the basic considerations when getting quotes and making these purchases.
When appearance is important, it is crucial that the glazier keep in mind the required finish. For anodized finishes, only certain alloys of sheet and extrusions can be anodized for best results. The longer the metal is in the anodizing tank, the thicker the surface-protecting finish will be, and the thicker it is, the more durable it will be. Class I is the most durable. Since the metal will be covered with paint finishes, the alloy does not affect the quality of the finish. Normally, if aluminum is going to be anodized, a 5000 alloy is best for sheet and a 6000 alloy is best for extrusions.
Because anodizing is a process of chemical conversion of the aluminumís surface, matching colors can be tricky. If color match is important, try to get your metal and finish from the same source and let the vendor know the details of the installation. If the vendor gets all his metal from the same batch and anodizes with the same finisher, mismatched pieces will be less of a problem. Of course, extrusions and sheet normally will not match one another perfectly because they are made of different alloys, but thatís not usually a problem, as architects rarely design anything for a seamless match of sheet and extrusions.
Paint finishes do not depend on the alloy for their color, but color match can still be an issue, especially with metallic or mica flake paints. Even when colors match, the gloss can vary, and when gloss and color do match, light reflecting from different angles can make perfectly matched paint appear to be different colors.
In the last 20 years there have been great strides in color measurement. Most anodizers and paint applicators have color measurement equipment to help remove subjectivity from the evaluation of appearance. In addition to measuring gloss, color can be measured with a colorimeter, which uses three-dimensions corresponding to the three primary colors: red, blue and yellow. Incidentally, since color can vary three ways, having two range samples can cause problems.
If a glazing contractor is purchasing brake metal or factory-fabricated sheet metal, additional problems can arise. When prefinished metal (either anodized or painted) is bent in a press brake, the metalís outer surface is stretched. When good design comes together with proper alloy, hardness and temper, the metal itself should not crack or craze. Stretching of the outside of the bend, however, will stress the finish. Paint will withstand the stress better than anodizing because anodizing is a very hard, but brittle, finish. Therefore, if you have fabricated shapes that must be anodized, the edges will look better and last longer if they are anodized after fabricating. Some fabricators, such as our company, also do anodizing. Even if the metal is painted, the edge can break, craze or discolor. The thicker the metal and the thicker the finish, the more likely the paint is to stretch past its breaking point. For best appearance and lowest risk, any finish should be applied after bending.
Once you know the basic considerations, your job will be easier. The most important job a purchaser has is communicating with the vendor. He should be told as many details as possible about the end use of the aluminum and about the design of the building. Most full-service aluminum finishers, suppliers and fabricators will be able to tailor their products and services to your applications. If you can find a vendor who does fabrication, metal supply, anodizing and painting, he will understand the best way to solve your problems. Like so much in life, communication is the key. You just canít tell the vendor too much about your application.
John McClatchey, Ph.D. is the president of Southern Aluminum Finishing of Atlanta.
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