Flux is a soldering compound that keeps the soldering joints clean by blocking oxygen, which causes oxidation. Flux cleans, coats, and protects the piece and enables the solder to flow easily between the joints when heated.
After pickling the item, coat/dip it first in a boric acid flux that protects the entire piece from oxidation and firescale (heavy oxidation that forms on the jewelry from copper in the metal). Firescale is very difficult to remove.
Boric acid flux is 60% boric acid and 40% denatured alcohol and it yields an excellent and inexpensive flux. Add the boric acid to the alcohol until it stops dissolving and forms a thin paste forms. Dip the entire item into the solution, then dry it with gentle heat from the torch; do this three or four times before soldering. Boric acid and denatured alcohol can both be found at a grocery store.
Now, you need a flux at the solder joint to assist with melting the solder without oxidation. This is called the flow flux. Batterns and Handy Flux are the top choices. Boric acid and water is also used.
- Batterns Self-Pickling Flux is used for hard soldering (solder that melts at a high temperature) of gold, silver, and platinum. It can be diluted with distilled water if it is too thick. Don’t be fooled by the name, you will still need to pickle the item before soldering. It is available from Amazon in 1, 3, 8, 16, and 128 oz jars.
- Handy Flux is used for easy and medium soldering (solder that melts at a lower temperature) of gold and silver. It can be diluted with distilled water if it is too thick. It’s available from Amazon in 7 oz, 1/2 lb., and 1 lb. jars.
- Borax and water is a flow flux used by jewelers for years. It’s cheap, but is leaves a glassy coating on jewelry that’s tough to remove. You can purchase borax in the detergent aisle at the grocery store.
Use enough flux to protect the joints from oxidation, but not too much that it prevents the solder from flowing. Flux will bubble when the torch first touches it, and too much flux will really create a lot of bubbles. The bubbling may cause your solder piece to shift, so preheat the flux a bit and then add the solder piece.
Flux dries out easily. Keep enough flux in a small vial to complete the job. Store the rest of the flux in an air tight container.