Magnus:

NOTE; USE GLOVES AND EYE PROTECTION DOING THIS!!

Use some old battery acid, test with some scrap first. The sulphuric acid will attack the copper in the brass, and darken it quickly. You might even need to dilute the battery acid more to make it work at a comfortable rate. Alternately, if the battery acid doesn't work fast enough for you, just get some reagent sulphuric and dilute it to get the effect you want.

NOTE: IF YOU HAVE NEVER TAKEN A CHEMISTRY CLASS AND LEARNED TO HANDLE ACIDS DO NOT USE ANYTHING STRONGER THAN NORMAL BATTERY ACID!!

Bill.