When joining metals together in jewelry-making, there are three important elements: solder, flux and heat. When metal is heated, solder, which is also made of metal, melts to join the pieces together. But during this process metal is also exposed to oxygen, which cause the metal to oxidize or tarnish, and tarnish can prevent solder from melting. Flux works as a catalyst in this process. It helps inhibit oxidization and enables solder to melt. Flux is commercially available in jewelry supply stores, but you can make your own at home.
Put about 2 cups of water in the pot. Add the tri-sodium phosphate, boric acid and borax. Mix using the wooden spoon.
Place the pot on the the stove. Turn heat on high. When the mixture boils, turn the heat down to low. Let the mixture simmer for three to five minutes.
Turn off heat. Let mixture cool. Pour the liquid flux into a glass jar or plastic bottle. Close the lid tightly until you are ready to use the flux.