Volume 35, Number 9, September 2000



Delicate Matters

handling light and medium bronze anodized orders

by Todd Hamilton

Light and medium bronze anodized finishes can be beautiful and long lasting on aluminum. Glazing contractors often choose such finishes because when they are handled correctly, they are of surpassing beauty. However, because expectations can run high for these finishes, light and medium bronze can also become a nightmare for the anodizer, the contractor, and the building owner. These colors are subject to severe color variation from piece to piece.

With lighter colors, the human eye perceives more variations in chromaticity and hue (two key components of color). This variation is far less noticeable with darker colors. Lighter colors also develop very quickly in the anodizing tanks. Sometimes, just a few extra seconds in the tank will make a difference between a light and medium bronze color. This creates a smaller margin for error. While these problems are frustrating, there is a way to help. The problems with anodizing lighter colors can be minimized if the orders are handled carefully.


Variable Affects

Anodizing color is affected by numerous variables, such as alloy type and temper, load time, etch time, tank temperature and coating thickness, just to name a few. Anodizers can control many of these variables through their processes. However, anodizers often are unable to control the types of alloys they receive to finish. Careful alloy selection can help improve the color consistency of light and medium bronze anodized jobs. Certain types of alloys anodize better than others. For flat or fabricated sheet items, 5005 produces the best results. For extrusions, 6063 is the best alloy choice.

To take alloy selection a step further, supplying metal (sheets, extrusions and fabricated parts) from a single lot of material can improve color consistency. When aluminum is produced, each lot has slight differences, such as alloy constituents, temper, extruded temperature, grain, rolled temperature, and so forth. These slight differences can have a big effect on anodizing color. For example, material from one lot of anodized light bronze might produce a color to the light side of a range. Whereas material anodized from another lot might produce a color near the middle of the range, even though both lots were anodized using the exact same process and tank time. Thus, anodizing material from one single lot will minimize the chances of color variation.

Anodizers are often asked to match the color of a specific piece of material to the color of an extrusion. Unfortunately, this is impossible, especially with light and medium bronze. Different alloys take on slightly different appearances when anodized. Thus, an anodized piece of 5005 sheet will look slightly different than an anodized piece of 6063 extrusion. Furthermore, color range samples are often supplied on 5005 alloy sheet. If an architect is looking to match storefront extrusion to 5005 range samples, the color will not match. For best results, the anodizer should create range samples from the lot of material they plan to anodize, thereby providing the most accurate depiction of expected color range.


Target Colors

Producing a perfect match with light or medium bronze anodizing is impossible. Because so many variables exist, a certain degree of color variation is inevitable. Given that some color variation will exist, a way to minimize the color variation is for the anodizer to use a target color when anodizing the job.

In an anodizing shop, there is usually a dichotomy between sales and production. An anodizer’s production staff must have a target to do their job, but salespeople need something called “range samples.” Range samples really are a technique to adjust the expectations of contractors and customers and are not helpful to the anodizer’s production staff. The reason is because the color is just one of the aspects of appearance and not even color can be measured on a light-to-dark axis. What one might describe as “lighter,” for example, might really be “yellower.” Range samples are only useful in providing the specifier with a feeling for the degree of variation that is possible, and they are not scientifically defined limits of color. The bottom line is that anodizers require a target color on all light and medium bronze projects.

For best results the next time you have a light or medium bronze anodize job, remember these key points:

• Supply a target color for all light and medium bronze anodized projects;

• Expect some degree of color variation;

• Use 5005 alloy for sheet and 6063 alloy for extrusion;

• Do not attempt to match sheet with extrusion;

• Supply material from one lot of metal;

• Have range samples developed from the same lot of material you plan to provide on large jobs;

• Agree upon an acceptable color range with your anodizer.

Todd Hamilton is the inside sales manager for Southern Aluminum Finishing Company in Atlanta.


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