Cleaning Brass

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?
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