Why is there no white anodized aluminum? (Anodized Aluminum 101)

At least a couple times a month, customers ask us: “Why are there no white aluminum jump rings?” The simple answer is that it’s not possible.

It seems like it should be possible because coloring aluminum is done by dye, and there are white dyes. However, these dyes cannot be used in anodized aluminum. When Jen and I visited our anodizer, he walked us through the process, and explained why this is so.

First, let’s look at what anodizing does to the surface of jump rings. The electricity in the anodizing process makes the metal porous, thereby preparing it to receive dye. Below are two images zooming in on the surface of the anodized metal.

anodized aluminum surface microscopic surface of anodized aluminum

To be more specific, anodizing creates an oxide layer that has roughly one million microscopic pores on the surface of one square inch of aluminum. If you’d like to get your nerd on with more technical information about this process, SubsTech has lots of great articles (open-source!) on their website. The image below, from their page on anodizing, shows how the color is added to “fill the holes” in the surface of anodized aluminum.

Anodizing process

Now that you know a little more about the technical side of anodizing, let’s get back to our original question. It’s important to understand that not all dye molecules are created equal, and some colors are bigger than others. Blue molecules have no problem filling in the holes.
blue dye filling anodized aluminum pores
Red molecules are bigger than the blue ones, but they still fit, no problem.
red dye filling anodized aluminum pores
Unfortunately, when we get to white molecules, they are so large that they do not fit into the holes created by the anodizing process! There’s no way to squeeze a white molecule into the pore!
white dye unable to fill anodized aluminum pores

Another way to look at the size comparison of molecules:
Imagine a pore the size of a basketball net.
A gold dye molecule is the size of a golf ball.
Blue dye is about the size of a tennis ball.
Red and black dyes are about the size of a baseball.
Scientists haven’t been able to get white dye molecules smaller than beach ball size, which is too large to fit.

And that, fellow maillers, is why there is no white anodized aluminum.

Now some of you may think that you’ve seen white anodized aluminum. It’s true that our anodizer can get a color fairly close to white, by etching the aluminum and then sealing the surface just as would be done after dyeing colored anodized aluminum. But the color most definitely is not a pure white, and can vary from a muted grey to a dirty off-white. If you see someone selling “white” aluminum, they have most likely used this acid-etched process.

Blue, red and white molecule illustrations by Artie Keefe and Erik Watson.

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15 Comments on "Why is there no white anodized aluminum? (Anodized Aluminum 101)"

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8 months 21 days ago

Hey, I would like to use these illustrations in my thesis, which is focusing on making white anodised Al.
Thanks a lot.

2 years 2 months ago

As of now, I’ve heard three theories why white anodized aluminum doesn’t exist. The first one is the one mentioned in this article, having to do with the size of the white pigment molecule, which I think is uninformed.

The second theory, to which I subscribe, is that since we are talking about dyes, they are by definition translucent, and therefore show the gray metallic look of the aluminum base. Therefore, if you were to dip the rings in white dye, even if the dye were absorbed, it would be like having milk on metal: you can still see the gray metal.

The third theory, which I haven’t researched a lot, is that there is no such thing as a white organic pigment. Notice, in order to do Type 2 sulfuric acid anodizing as it’s done with these rings, the pigments in the dyes have to be organic.

3 years 1 month ago

How fascinating. In digging through some color and quantum physics sites, could it be that to produce white, you need to combine the three primary colors, red, blue and yellow, this would add the electrons and multiple bonds of all three “dyes” making it a very big molecule. In over my head here but fun to think about. Can’t wait to hear from your anodizer!

Scott Anguish
4 years 1 month ago

This is a great explanation. The “white” I purchased elsewhere wasn’t near white.

But, here’s a question. Why don’t you have white Enameled rings? :-)

4 years 1 month ago

I found this article to be so interesting! I’m not generally one for scientific stuff, but the way you wrote this makes it so easy to understand. *hugs the poor white dye molecule* What makes the white so large and the others so small?