If you have hardened flux either use sulphuric acid (dont see that much in Australia now) or hyrdrochloric acid just paint it around the joints get rid of the flux and boil the copper chiller with some white vinegar and a lemon cut up and it will come up all shiny and bright.
Yeah, vinegar and lemon, or one or the other. Copper/silver solder is most common materials used in home distillers. There's another forum on distilling/building distillers and it's standard protocol to do a vinegar run or sacrificial run to clean in the internals which are otherwise pretty much unreachable.