True brass is an alloy of copper and zinc. It tends to oxidize (tarnish)
quickly when exposed to air, which is a major reason why most brass is
given a clear coating of lacquer to prevent this condition.
Additionally, most metal polishes contain solvents and detergents to
remove the tarnish, mild abrasives to polish the metal, and oils to act
as a barrier between the raw metal and air.
Brass turns "black" when cleaning due to over-use and misuse of polish.
The biggest challenge to the upkeep of most metals, including brass, is
the removal and inhibition of tarnish. All substances, especially
metals, oxidize when exposed to air. Once tarnish is removed, a chemical
barrier should be created between the bare metal and the air to inhibit
the process from re-occurring. |