How to polish Sherwood amps copper case

[FauxHall]

The technical term is "corrosion", being primarily oxidation and sulfidation.

Anytime someone calls it "patina", an extra thirty percent has been added to the base price. In the mideast the bluish copper-oxide coating on brass artifacts to be sold to tourists is achieved with used cat litter, an inexpensive source of ammonia and some organics to add color.

Chemically and metallurgically you are correct in calling it corrosion. However, it is a protective layer something like the transparent aluminum oxide that forms on freshly cut aluminum metal exposed to air.

Patina is more of an architectural and interior decorator term. You know, "artsy!"

For copper and copper-based alloys, the pH and minor chemicals will affect the color. A bluish patina indicates sulfur, as in air pollution with sulfur dioxides. I imagine cat piss qualifies.

So if you want that bright and shiny copper color to endure, coat it with a clear shellac or lacquer to block air and contaminants. Sorry but I can't recommend any products specifically.

 

[Retrovert]

Chemically and metallurgically you are correct in calling it corrosion. However, it is a protective layer something like the transparent aluminum oxide that forms on freshly cut aluminum metal exposed to air.

Right, but the color depends on the form and the real world is never so clean.

Corundum is cystalline and, absent impurities, clear. But the corrosion on aluminum can also have a white Al2O3. The sulfides are black or gray. An outdoor aluminum frame, window or door, demonstrates the extremes of corrosion, including various colored layers, all of which add up to "not selling this house in this condition without a steep discount". Hence metal polish and a buffing wheel in a drill. Oooh, shiny!

Patina is more of an architectural and interior decorator term. You know, "artsy!"

Yeah, I know. Which is why I wrote, "Anytime someone calls it "patina", an extra thirty percent has been added to the base price."

Many times the surface metal is chemically treated for color. I have used a variety of darkening agents, including selenious acid on iron which turns it black (iron selenide). Similar to bluing. This is fragile and requires a clear coat.

I've seen HiFi gear where the faceplate was anodized aluminum, where an oxide layer was rendered porous, dyed, and the pores closed. It is physically and chemically fragile.

But that's not what is being discussed for a bright-brass chassis.

So if you want that bright and shiny copper color to endure, coat it with a clear shellac or lacquer to block air and contaminants. Sorry but I can't recommend any products specifically.

Ages ago the common coating was a nitrocellulose lacquer or organic shellac (insect shells), both of which are fragile and gas permeable, and consequently only lasted a few years. I'm sure you remember having to remove that with a solvent to fix the underlying corrosion.

Modern lacquers are more stable with time, typically either acrylic or polyurethane, both with an added tarnish inhibitor. That greatly increases the stability. No coating will forever last, particularly outdoors.