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ted_yosem
Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
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Aluminum in the 1930's: "Deplated Finish"

61775-1ext

Q. Finishing folks,
I work designing restorations of old buildings and have been researching something that may be of interest. I am also hoping this group can help me understand it!

For a brief period in the early 1930s, prior to the introduction of their 'Alumilite' anodized finishes, Alcoa advertised a 'deplated' finish for architectural purposes. (See their 1932 publication 'Aluminum in Architecture', available online at HathiTrust ⇨
The finish was described as dark, slate gray, and the process was proprietary. This has puzzled me and people in my world, since these features have long since weathered and the term has disappeared from use (at least in this context). The big question: what did these elements look like when new?

Fortunately, I was finally able to find some instructions in Alcoa archives out in Pittsburgh. I am not a metal finisher, but I am overconfident, so I followed the directions in my backyard. I am not sure if I can attach the original text so here is the gist:

-bath of 7.5% by vol. sulfuric acid
-work is anode, 1100 aluminum is cathode
-applied current density of 0.3 amps per square inch for 20 minutes at ~75° bath
-Worked as described. Here is a photo of the result.

61775-2

The key to the process seems to be that the work was Alcoa's #43 alloy (today 443), 95% Al / 5% Si, which was hard for me to get. The 'deplating' removes the aluminum from the surface and exposes the silicon, resulting in the dark gray appearance.

My questions to you all:
-Anyone ever heard of this specific way of finishing aluminum before? Or did it completely die out?
-My big question: this process, to my untrained eye, seems very similar to anodizing. But why was I 'deplating' my aluminum instead of anodizing it? At first I assumed the anode and cathode were reversed in the two processes, but that doesn't seem to be the case.

Hope this is perhaps interesting. I can't tell if this process was a step on the way to modern anodizing, or a weird offshoot. Thanks in advance for any thoughts.

Preston Hull
Architectural Conservator - Philadelphia, PA
November 15, 2024


A. Following up on my own thread because I may have found the answer to my own question.

If I am doing my math correctly, I think I was applying roughly twice the current density to my aluminum samples as is typically done when anodizing today. I found a 1987 article (Thompson, Xu, Skeldon, Han, & Wood, "Anodic Oxidation of Aluminium" Philosophical Magazine B 55, no. 6: 651-667) that states that at a critical current density, the Al from the anode goes into solution faster than the O2/OH can bond with it at the surface. So that's my best guess -- perhaps this finish came first because it was easy to see the results, and it took Alcoa awhile to figure out that if they ratcheted back the current, they were developing a coating that was actually protective. Don't know how they could have really known what was going on at the surface in the early 1930s, at the micron-level. Anyway, I welcome corrections if anyone knows better.

Preston Hull [returning]
- Philadelphia, Pennsylvania




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