Sep/11

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

9 Comments · Posted by Rebeca in B3 General News

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.

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.

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.

Red molecules are bigger than the blue ones, but they still fit, no problem.

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!

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.

anodized aluminum · Anodized Aluminum 101 · anodizing · chemistry · dye · molecules · white aluminum · white anodized aluminum

9 comments

Sarah (Saturday Sequins) · September 26, 2011 at 3:22 PM

Aww! Poor little white dye molecule! He looks like he needs a hug.

Awesome illustrations. Adorable.
Marguerite · September 26, 2011 at 4:17 PM

Wow! Now how cool is that…and I have to agree: that poor little white molecule needs a hug.
L. Bergin · September 26, 2011 at 6:10 PM

I also love the illustrations….might I have permission to use them in a future science lesson?

Thank you again for sharing!
Author comment by Jenna · September 27, 2011 at 9:14 AM

Of course you can use it in a science lesson! The creators of the images give you their blessing
Kimberly Balque · September 30, 2011 at 12:10 PM

Thanks so much for this. The science nerd in me loves this kind of stuff!
Mabel Tanner · October 3, 2011 at 10:00 AM

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?
Scott Anguish · October 3, 2011 at 10:18 PM

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?
Admin comment by Rebeca · October 5, 2011 at 10:24 AM

When we first started carrying enameled copper, the wire supplier did not offer white, so we decided not to carry it. We would like to offer this as an option someday, though!
Admin comment by Rebeca · October 5, 2011 at 4:11 PM

Mabel – wow, you ask the tough questions, don’t ya! We’re not sure why the white is so large. That’s just the way it happened to work out. It’s sort of like – why are grapefruits bigger than grapes? Um….that’s just the way it is…?

However, we admit that answer doesn’t quite satisfy us, either. We’ve asked our anodizer, and he’s meeting with his dye crew soon and will ask them. Maybe there is an explanation other than “that’s just the way the molecule crumbles” ;-P. If we find out anything useful, we’ll be sure to post here!