Copper is generally chemically cleaned with nitric acid.
Some twat in the government worked out that nitric acid was pat of nitro so it is a scheduled product in OZ and you need a good excuse to be allowed to use it.
Killed spirits or spirits of salts is zinc chloride.
Either bought from an ironmongers or made by dropping zinc ( usually old bent galvanized nails ) into hydrochloric acid. Muratic acid is an impure form of hydrochloric acid.
You should be able to get a copper cleaner from the kitchen wear section of a good cooking supply store.
Otherwise, stainless cleaners will work almost as well. Make sure that you wash it all off really well.
Vinegar will also work.
If you have any film thicker than a translucient blue-yellow colouration ( much like light pipe blueing ) you are cooking the copper at way too high a heat.
I generally do it in a "furnace ' consisting of 5 silicon bricks ( from inside a wood stove ) and when finished I plop the "roof" brick on top os the gasket . By doing this you end up with a reasonably clean surface. Lightly spraying with water will also dislodge the furnace scale.
