Letter 3273

Green Tarnish on Copper Courthouse Dome 

-

We are cleaning our copper dome on top of the courthouse. We have had good success at removing two coats of old paint using just plain paint stripper. Under the paint is that old green tarnish. In testing, Muriatic Acid [link is to product info at Amazon] seems to remove this green tarnish very well. But muratic acid is very dangerous. Any recommendations on how to remove this green tarnish more safely? How do we keep this green tarnish from returning too quickly? All suggestions and comments are welcomed. Thanks Butch Bridges.

Butch Bridges
Carter County Courthouse - Ardmore, Oklahoma

Wow. You've got a tough one there. You're absolutely correct that Muriatic (Hydrochloric) Acid is very effective at removing the tarnish. The problem is that copper is such an active metal that it forms oxides very quickly. I am not aware of any practical or cost effective method that will inhibit the tarnishing process on such a large scale.

I work with copper plated parts every day and anything short of lacquering or chromating the part do you have a chance of inhibiting the tarnishing process. The green patina that you see on copper is a naturally occuring chemical process that will occur unless you apply a film or barrier over the copper. Many people find this patina film to be an enhancement to the that has occurred over the years. Unfortunately, if you like bright, shiny copper; you must protect the polished surface immediately or the tarnishing will occur very quickly.

On anything as large as a building dome the only alternative I would see would be an organic coating of some sort. It would be extremely impractical and environmentally hazardous to attempt this. My advice is to clean up the copper as well as you can so you can see the copper color for a short period of time but you can be assured that the patina will return. Good luck to you.

Daryl Spindler
- Portland, TN

They grit blasted copper with sodium bicarbonate to clean the Statue of Liberty. Commercial units are available. Don Mattox

Donald M. Mattox Society of Vacuum Coaters Albuquerque, NM   Ed. note:             Mr. Mattox is             the author of --

I WANT to make the green tarnish on copper for crafting. Any ideas on a quick method? or just leave the copper out in the weather for a while? Thanks for any tips.

Connie Simmons
- Raymond, MS, USA