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  for Metal Finishing since 1989
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Aluminum in the 1930's: "Deplated Finish"

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.

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