-----

Users are the most important part of the industry.

I would guess that the black deposit is copper & silicon, and the other alloying elements in the casting. As you dissolve away aluminum with repeated applications, the surface becomes ever richer in these other materials. It is possible to make pure aluminum mirror-bright by chemical bright dipping followed by anodizing; in fact, this is how most reflectors on outdoor lights and medical lights are made. Alloying elements make it more difficult to obtain mirror brightness, but not necessarily impossible--it depends.

Albright & Wilson [Richmond, VA] is a major supplier of proprietary aluminum bright dipping solutions. You might also check the yellow pages under anodizing, plating, or metal finishing to see if any local shops do aluminum bright dipping, as well as reviewing the Shop Directory here.

Whether the alloy in question can be made permanently bright by immersion bright dipping and anodizing is a question I can't answer--experimentation would be necessary. But my feeling is that it will be impossible to get and keep a mirror finish unless the parts are disassembled, and properly bright-dipped and anodized in immersion tanks using chemicals you wouldn't want to mess with except in an industrial setting.

Everything you could want to know about aluminum bright dipping and finishing can be found in "The Surface Treatment & Finishing of Aluminum and its Alloys"