Anyone who melts scrap silver or gold for casting soon learns that the metal is only half the job. The other half is keeping the melt clean, and for generations jewellers, silversmiths and small foundries have reached for the same material to do it: borax powder. This guide looks at how borax is used when melting and casting precious and non-ferrous metals, from seasoning a new crucible to pouring a clean ingot, and at the safety controls that go with it.
If you are joining metal rather than melting it, our separate guide to borax flux for soldering, brazing and forge welding covers paste and solution fluxes for joints.
Why borax is used when melting metal
When borax is heated it first gives off its water of crystallisation, puffing up and bubbling as it does so. Once the water has gone, the remaining anhydrous borax fuses into a clear, glassy liquid at around 743 °C. In a crucible, that molten glass does three useful things:
- It dissolves oxides. Metal oxides and fine dirt that form on the surface of the melt are taken up into the glassy layer instead of ending up inside the casting.
- It shields the metal from air. A thin film of molten borax over the melt slows further oxidation while the metal is held at temperature.
- It collects dross. Impurities gather in the flux, which tends to stay behind in the crucible or float clear during the pour, so the metal runs cleaner into the mould.
Borax will not refine metal or remove alloying elements. It is a housekeeping aid that helps clean metal stay clean, so the quality of what goes into the crucible still matters.
Seasoning a new crucible with borax
Many casters glaze the inside of a new melting dish or crucible with borax before its first use. The glassy coating seals the surface, helps the metal roll out cleanly when poured and reduces the amount of metal that clings to the dish.
How crucible seasoning is usually done
- Start with a clean, dry crucible. Check it for cracks or chips first. A damaged crucible should not be used for molten metal.
- Warm the crucible gently with the torch so that any moisture is driven off.
- Sprinkle a small amount of borax powder into the warm dish. It will foam as the water of crystallisation comes off.
- Keep heating until the foam settles and the borax melts into a clear glaze.
- Spread the glaze by tilting the crucible with tongs, or by working the flame across it, until the inner surface where the metal will sit is evenly coated.
- Let it cool naturally on a heat-proof surface before storing it.
A seasoned crucible can be topped up with a pinch of borax before later melts. Over time the glaze builds up and picks up colour from the metals melted in it, which is one reason many workshops keep a separate, labelled crucible for each metal or alloy to avoid cross-contamination.
Using borax during the melt
Once the crucible is prepared, borax is normally added in small amounts as the metal melts. A typical sequence looks like this:
- Load clean metal. Remove solder, stones, plating residues, grease and any non-metal material from scrap before it goes in.
- Heat the charge evenly until it begins to slump and melt.
- Add a pinch of borax to the molten metal. Use dry powder, as added moisture can make the melt spit violently.
- Stir gently with a graphite or other suitable stirring rod so that the flux contacts the whole surface of the melt.
- Keep the torch on the metal right up to the pour, so the flux layer and the metal stay fluid.
Small additions work better than large ones. Too much flux can be carried into the mould and leave glassy inclusions or pits on the surface of the casting.
Hydrated or anhydrous borax?
Standard borax powder is the decahydrate form, which contains a lot of water and therefore foams vigorously on heating. Some casters prefer anhydrous (pre-fused) borax for additions to an already-molten melt because it bubbles far less. For a clear explanation of the differences, read our guide to borax decahydrate vs pentahydrate vs anhydrous.
Pouring clean ingots and castings
Good fluxing only pays off if the pour is handled well:
- Prepare moulds in advance. Ingot moulds should be clean, completely dry and warmed before the pour. Moisture in a mould can turn to steam instantly and throw molten metal out.
- Pour in one smooth movement. A steady pour helps the flux stay in the crucible rather than following the metal into the mould.
- Skim if needed. If a lot of dross has formed, it can be held back or skimmed before pouring.
- Clean up afterwards. Any fused borax on the ingot or casting can be removed in your usual pickle, followed by thorough rinsing.
Common problems and what they usually mean
| Problem | Likely cause | What to try |
|---|---|---|
| Metal sticks to the crucible | Unseasoned or worn crucible glaze | Re-season the crucible with a fresh borax glaze |
| Glassy spots or pits on the casting | Too much flux carried over in the pour | Use smaller additions and pour smoothly |
| Heavy foaming when flux is added | Hydrated borax added to a hot melt | Add smaller amounts or use anhydrous borax |
| Dull, dirty-looking melt | Contaminated scrap or not enough flux | Clean the charge first and add a small pinch of borax |
| Metal spits during the pour | Damp mould, flux or tools | Dry and preheat moulds and tools; keep borax dry |
Choosing borax for casting work
For melting and casting, workshops normally use technical grade borax, which is what our 99.9% pure borax powder is. It is supplied with a Safety Data Sheet for trade and professional users. You can browse the full borax powder range, and larger quantities for production work are covered on our industrial borax page.
Keep the flux you use for casting in its own labelled container. A small pot taken from the main pack for use at the bench keeps the bulk stock dry and avoids contaminating it with metal filings or dirt.
Safety: handling borax in a casting workshop
Borax (disodium tetraborate) is classified in Great Britain and the EU as Repr. 1B, H360FD: may damage fertility, may damage the unborn child. It is supplied for professional and trade use only. Melting metal adds its own serious hazards, so follow both the borax label and your normal hot-work procedures.
- Not for use by or near children. Keep borax and casting equipment out of reach of children at all times.
- Pregnancy: borax should not be used during pregnancy or by anyone trying to conceive. Employers must assess the risks for new and expectant mothers.
- Do not swallow. Never eat, drink or smoke while handling borax, and wash your hands thoroughly afterwards.
- Avoid inhaling dust and fumes. Measure out powder gently without raising dust, and melt metal only with good ventilation or local exhaust ventilation.
- Wear protective equipment: chemical-resistant gloves when handling the powder, heat-resistant gloves for hot work, eye protection or a face shield, and non-flammable clothing.
- Keep everything dry. Water, damp flux or wet tools near molten metal can cause violent spitting.
- Store safely: keep borax in its original or a clearly labelled, closed container, in a dry, locked or restricted store away from food and drink.
- Follow the label and Safety Data Sheet, and carry out a COSHH assessment for workplace use.
For PPE, spills, disposal and first aid, read our full borax safety guide.
Frequently asked questions
Why do jewellers put borax in a crucible?
Molten borax forms a glassy layer that dissolves oxides, shields the melt from air and collects dross. Glazing a crucible with borax also helps molten metal pour out cleanly instead of sticking to the dish.
How do you season a crucible with borax?
Warm a clean, dry, undamaged crucible, sprinkle in a little borax powder and keep heating until it stops foaming and melts into a clear glaze. Spread the glaze over the inside and let the crucible cool naturally before use.
How much borax should I add to a melt?
Only a small pinch at a time. Too much flux can be carried into the mould and leave glassy inclusions or pits on the casting. Follow your crucible and equipment instructions and adjust from experience.
Can I use the same crucible for silver and gold?
Many workshops keep a separate, labelled crucible for each metal or alloy, because the borax glaze holds traces of previous melts that could contaminate the next one.
Is borax safe to use for casting?
Borax is classified Repr. 1B and is supplied for professional use. With ventilation, gloves, eye protection, dust control and a COSHH assessment it can be used in a workshop, but it is not for use by or near children or during pregnancy.
Sources
- Borax: dehydration on heating and melting point of anhydrous borax (743 °C). Wikipedia
- ECHA, substance information for disodium tetraborate decahydrate (CAS 1303-96-4): harmonised classification Repr. 1B, H360FD. echa.europa.eu
- HSE, COSHH guidance for businesses. hse.gov.uk/coshh
- HSE, protecting new and expectant mothers at work. hse.gov.uk/mothers
Need borax for your casting bench? Our 99.9% pure borax powder is sodium tetraborate decahydrate supplied with a Safety Data Sheet for trade, workshop and laboratory users. See our delivery information for how orders are sent.