Before solder wire had a core of flux in it you had to use a paste, or a liquid flux. The liquid was Zinc Chloride. It was made by getting some hydrochloric Acid (Muriatic Acid) and dropping zinc into it. The most common source of zinc was bits of galvanised iron. When you dropped the zinc into the acid bubbles would come off. When the bubbles stopped, the zinc had replaced the Hydrogen in the acid. The Hydrochloric acid was then called “killed spirits.” The killed spirits was great flux to solder together, say, brass or copper – or even galvanised iron. Of course this was potentially dangerous but that was what you did in the good old days! P.S. don’t forget to wear safety goggles if you attempt this.