I have just tried to clean a brass door knocker by electrolysis. I place the 
item in a plastic bath of salt water solution and then with a piece of 
copper as the anode and the brass object as the cathode used at 12v battery 
charger (with iron croc clips) to pass a current through the system. Lots of 
bubbling and rapid discolouration of the water. The brass did clean up but I 
feel that this may have been because I created and HCl solution. Does anyone 
know how to do this properly please?

Peter