Borax has been the metalworker’s flux for centuries, and it is still the base of many brazing and hard soldering fluxes sold today. When it melts, borax turns into a clear, runny glass that dissolves the oxide film on hot metal and shields the joint from air, so that solder can wet the surface and flow into the gap. That makes it a high-temperature flux: it suits silver (hard) soldering, brazing and forge welding, but not low-temperature soft soldering. This guide explains how borax flux works, which form of borax to use, how jewellers, brazers and blacksmiths apply it, and how to handle it safely in a UK workshop.
New to the chemistry? Our guide What Is Borax? Sodium Tetraborate Explained covers the basics. This article looks only at borax as a flux.
How borax flux works
Every metal you heat in air grows a thin skin of oxide, and solder will not stick to oxide. A flux has three jobs: to stop more oxide forming while the metal heats up, to dissolve the oxide that is already there, and to let the molten solder or filler wet the metal and run into the joint.
Borax does all three once it has melted. Anhydrous borax melts at about 743 °C, and molten borax dissolves many metal oxides to form glasses (source: Borax, Wikipedia). The glassy layer covers the hot metal, keeps air off it and carries the dissolved oxide out of the way as the filler flows in.
Why borax is not a soft soldering flux
Because borax only becomes an active flux once it has fused, it is matched to processes that run hot enough to melt it: hard (silver) soldering, brazing and forge welding. Soft solders for electronics, plumbing and stained glass melt at far lower temperatures, long before borax has fused. For those jobs use a flux made for soft soldering and follow its own label and Safety Data Sheet.
Which form of borax makes the best flux?
Borax is sold as the decahydrate, the pentahydrate and the anhydrous (fused) form. They differ only in how much water of crystallisation they hold, and that water matters when you heat them.
| Form | What happens on heating | Typical flux use |
|---|---|---|
| Decahydrate (standard borax, CAS 1303-96-4) | Gives off its water first, swelling and bubbling, then fuses into glass | Mixed with water as a paste for brazing and hard soldering; sprinkled dry for forge welding |
| Pentahydrate (CAS 12179-04-3) | Less water, so less swelling than the decahydrate | Mainly bulk industrial use |
| Anhydrous / fused (CAS 1330-43-4) | No water to lose, melts straight to glass | Pre-fused flux where puffing would disturb small parts or solder pieces |
The swelling of the decahydrate is the most common complaint about raw borax. As the water boils off, the flux puffs up and can lift small pieces of solder off the joint. Two habits help: apply a thin, even coat rather than a thick one, and warm the work gently at first so the water leaves before you bring the joint up to temperature. If you work with very small parts, a pre-fused flux avoids the problem altogether.
For most benches the decahydrate is the practical choice because it is the form sold in small packs. Our checklist Buying Borax in the UK: How to Read the Label, SDS and Spec Sheet shows how to confirm which form you have from the CAS number.
Ways to prepare borax flux
Borax paste
Powdered borax mixed with a little water to a creamy paste is the simplest flux. Borax dissolves poorly in cold water, so the paste is really fine powder suspended in a saturated solution. Warm water gives a smoother mix; our borax solubility chart explains why. Keep a small, labelled pot for flux only and make up fresh paste as you need it.
Borax cone and slate dish
The traditional jeweller’s method is a solid cone of borax ground against a slate or ceramic dish with a few drops of water, giving a thin milky flux that is painted onto the joint with a brush. It produces a smooth, even layer with very little dust.
Dry borax for forge welding
Blacksmiths usually sprinkle dry borax straight onto hot steel. The powder melts on contact and spreads across the surfaces to be joined.
Ready-made fluxes
Many commercial brazing and hard soldering fluxes are borax-based blends with other ingredients added to change their working range. These are separate products with their own labels and Safety Data Sheets. Do not assume their hazards or uses are the same as plain borax.
Using borax flux for brazing and hard soldering
The method is broadly the same whether you are silver soldering a ring, brazing brass or joining copper:
- Clean the joint. Flux removes thin oxide, not grease, paint or heavy scale. File, abrade or degrease the metal first.
- Fit the joint closely. Solder and brazing alloys are drawn into tight gaps. Flux cannot make up for a poor fit.
- Apply a thin coat of flux to the joint and the surrounding metal, and to the solder if your method calls for it.
- Heat gently at first so the water in the flux evaporates without puffing, then bring the whole joint up to temperature evenly. Heat the metal, not the solder.
- Watch the flux. When it turns to a clear, glassy film the metal is protected. Solder flows towards the hottest, cleanest area.
- Remove the residue. Cold borax glass is hard and slowly soluble. Soaking in hot water softens it; jewellers then usually pickle the piece by their normal method, following the pickle’s own Safety Data Sheet.
Using borax flux for forge welding
Borax is often used as a flux for forge welding. It lowers the melting point of the iron oxide scale on the steel, so the scale turns fluid and is squeezed out of the joint by the hammer blows (source: Borax, Wikipedia). A typical sequence is:
- Prepare and clean the faces to be welded, and shape them so the weld closes from the centre outwards.
- Bring the steel to a red heat and sprinkle borax over the faces. It melts and forms a glassy coat.
- Return the work to the fire and bring it up to welding heat.
- Set the weld with firm, quick blows, working outwards so the fluid scale and flux are pushed from the joint.
Molten flux is very fluid and can fly off the work when it is struck. Wear eye and face protection, and stand so that spatter is directed away from you and others. Over time, molten borax that drips into a forge can attack its firebrick or lining, so many smiths protect the forge floor or clean it regularly.
Common borax flux problems
| Problem | Likely cause | What to try |
|---|---|---|
| Flux puffs up and solder pieces jump off | Water leaving the decahydrate too fast | Thinner coat, gentler first heat, or a pre-fused flux |
| Solder balls up and will not flow | Metal not clean, joint not hot enough, or flux burnt off | Clean again, heat the whole joint evenly, refresh the flux |
| Flux turns black and stops working | Heated too long or too hot; flux exhausted | Shorter heating time, fresh flux |
| Hard glassy residue after cooling | Normal fused borax | Soak in hot water, then clean or pickle as usual |
| Lumpy powder in the pack | Damp storage; borax decahydrate cakes in humid air | Break lumps up inside the closed bag and keep it sealed |
Safety: handling borax flux in the workshop
Safety information. In Great Britain borax (sodium tetraborate) is classified as toxic for reproduction, category 1B, with the hazard statement H360FD: may damage fertility; may damage the unborn child. Melting it into a flux does not remove that hazard. We supply borax for professional, industrial and laboratory use only.
- Keep out of reach of children.
- Keep away from pets and animals, and away from food, drink and animal feed.
- Wear chemical-resistant gloves and eye protection when handling powder or paste; add face protection for forge work.
- Do not swallow. Do not eat, drink or smoke where borax is used, and wash your hands afterwards.
- Not for use on skin, in cosmetics or with food. Use flux pots, brushes and dishes for flux only, never kitchen items.
- Avoid breathing dust and the fume from heated flux. Work with good ventilation or local exhaust ventilation.
- Do not use if you are pregnant or trying to conceive.
- Store in the original, labelled packaging, sealed, cool and dry. Label any flux pot or paste you make up.
- Read the Safety Data Sheet before use and include borax in your COSHH assessment.
Buying borax for flux in the UK
Our Borax Decahydrate Powder 99.9% is a technical grade powder in 900g and 3kg resealable packs, supplied with a Safety Data Sheet for professional users. The 900g pack suits a jeweller’s bench; the 3kg pack is better value for forges and workshops that flux every day. Compare grades and pack sizes on our Borax Powder UK page.
Note that borax substitute is not a flux. It is a boron-free blend of washing soda and baking soda made for household laundry and cleaning, and it will not do the job of borax on hot metal.
Borax flux FAQ
What does borax do as a flux?
Once it melts, borax forms a glassy layer that dissolves metal oxides and keeps air off the hot metal, so molten solder or brazing alloy can wet the surface and flow into the joint.
Can I use borax flux for soft soldering?
No. Borax only becomes active once it has fused, at a far higher temperature than soft solders melt. It suits hard (silver) soldering, brazing and forge welding. Use a dedicated soft soldering flux for electronics or plumbing.
Why does borax bubble when I heat it?
Standard borax is the decahydrate and holds a lot of water of crystallisation. As it heats, the water boils off and the flux swells. Apply a thin coat, heat gently at first, or use a pre-fused anhydrous flux.
How do I remove borax flux after soldering?
Cooled borax flux is a hard glass that dissolves slowly. Soak the piece in hot water to soften it, then clean or pickle it by your usual method, following the Safety Data Sheet of any pickle you use.
Is borax substitute a flux?
No. Borax substitute contains no boron. It is a household laundry and cleaning product and will not flux metal. For soldering, brazing or forge welding you need sodium tetraborate.
This guide is general information for professional users. Always follow the label and Safety Data Sheet supplied with your product.