Borax has been the jeweller’s flux of choice for centuries. It is cheap, effective and works on most precious and non-ferrous metals.
Why Borax Works as a Flux
When heated, borax loses its water of crystallisation and melts at around 740°C into a clear glass. That glass coats the metal, dissolves oxides and blocks air, so the joint stays clean and solder can flow by capillary action.
How to Use Borax Powder as Flux
- Clean the metal by pickling or sanding so the joint is bright.
- Make a paste: mix borax powder with a few drops of water (or methylated spirit for faster drying) to a creamy consistency.
- Apply a thin layer to the joint and the solder pallions.
- Heat gently at first. The borax bubbles as water escapes; keep heating until it turns glassy, then bring the piece up to soldering temperature.
- Pickle afterwards to remove the glassy flux residue.
Tips
- Add a pinch of boric acid if you need extra protection against firescale on silver.
- For brazing and forge welding steel, use dry borax powder sprinkled on the hot metal.
- Store borax sealed and dry; it slowly loses water if left open.
Safety: work with ventilation, wear eye protection and gloves, and keep borax away from food and children.