Borax flux is one of the oldest and most reliable fluxes in metalwork. Jewellers, silversmiths, blacksmiths and brazers have used it for centuries to keep joints clean and help solder flow.
How borax flux works
When heated, borax first bubbles as it releases water, then fuses at around 740 °C into a clear glassy layer. That molten layer dissolves surface oxides and stops oxygen reaching the metal, so solder or filler metal can wet and flow into the joint.
Which metals is it used for?
- Gold and silver soldering (hard and medium solders)
- Copper, brass and bronze brazing
- Forge welding of steel
- Melting precious metals in a crucible (see our guide to borax for melting and casting metal)
It is less suited to low-temperature soft soldering, aluminium or stainless steel, which need specialised fluxes.
How to prepare borax flux
- Paste: mix borax powder with a little distilled water to a creamy paste and brush it onto clean metal.
- Solution: dissolve borax in hot distilled water (roughly 1 part borax to 5–6 parts water) and dip or brush parts before heating.
- Dry powder: for forge welding and crucible melting, sprinkle dry powder onto metal already at red heat.
Many jewellers prefer anhydrous (pre-fused) borax to reduce bubbling (see borax decahydrate vs pentahydrate vs anhydrous for how the forms differ). It is made by gently heating borax powder in a crucible until it stops foaming, then grinding the glassy result once cool.
Tips for clean joints
- Clean and degrease metal first; flux does not remove grease or heavy scale.
- Heat the whole piece evenly before focusing on the joint, so bubbling flux does not dislodge solder pallions.
- Remove fused borax residue afterwards in a warm pickle solution.
- For forge welding, flux at bright orange heat and weld at welding heat.
Safety
Work with ventilation, wear eye protection and gloves, and avoid breathing fumes or dust. Borax is classified Repr. 1B (H360FD) and is supplied in the UK for professional and trade use with an SDS. See Is borax banned in the UK? and our wider guide to borax uses.
Need borax for professional use? Our 99.9% pure borax powder is available in 900g and 3kg packs with SDS and free delivery to the UK. For home laundry and cleaning, choose our borax substitute.