Borax is one of the oldest industrial chemicals still in everyday use. It is a white, salt-like boron mineral that has been traded for centuries and today goes into glass, ceramics, metalwork, laboratories and agriculture. This guide explains what borax is, where it comes from, the forms it is sold in, how it differs from boric acid and borax substitute, and how it is regulated in the UK.
Borax at a glance
| Property | Borax (standard grade) |
|---|---|
| Chemical name | Disodium tetraborate decahydrate, usually called sodium tetraborate |
| Formula | Na₂B₄O₇·10H₂O |
| CAS / EC number | 1303-96-4 / 215-540-4 |
| Appearance | White, odourless crystalline powder or granules |
| pH | About 9.2 (1% solution at 20 °C) |
| Solubility in water | Roughly 50 g per litre at 20 °C, and far more in hot water (see our borax solubility chart) |
| Boron content | 11.3% boron, equivalent to 36.5% B₂O₃ |
| On heating | Gives off its water of crystallisation, then fuses at about 740 °C into a clear glass |
| UK hazard classification | Repr. 1B, H360FD: may damage fertility or the unborn child |
What is borax, chemically?
Borax is a hydrated sodium borate: a salt of sodium, boron and oxygen that holds ten molecules of water for every formula unit, which is why the standard grade is called the decahydrate. In water it forms a mildly alkaline solution that resists changes in pH. That buffering behaviour is so reliable that a 0.01 mol/kg borax solution, with a pH of 9.18 at 25 °C, is used as a pH reference standard in laboratories.
Heat changes it in stages. Warm borax first gives off its water, swelling and bubbling as it does. Above about 740 °C the dry residue melts into a clear glass that dissolves metal oxides, which is the property that makes borax such a useful flux.
The name has travelled a long way. It reached English through medieval Latin and Arabic from the Persian būrah, and for centuries borax was traded from Asia as tincal, the crude crystals collected from dry lake beds.
Where does borax come from?
Borax forms naturally where lakes in dry, volcanic regions evaporate and leave boron-rich salt deposits behind. These deposits are mined, then dissolved and recrystallised to make refined borax. Some borates are also recovered from underground brines.
Turkey is the world’s largest producer by a wide margin and has the largest known reserves. Southern California is the historic centre of production in the United States, and borates are also produced in Bolivia, Chile, Argentina, Peru and China, according to the US Geological Survey.
Borates are also attracting more strategic attention. In 2025 boron was added to the US critical minerals list, and the Jadar project in Serbia, planned to produce boric acid alongside lithium, was named one of the EU’s strategic raw materials projects.
The three forms of borax
Borax is sold in three forms that differ only in how much water they contain. Less water means more boron per kilogram.
| Form | Formula | CAS number | B₂O₃ content | Typical use |
|---|---|---|---|---|
| Decahydrate (standard borax) | Na₂B₄O₇·10H₂O | 1303-96-4 | 36.5% | Fluxes, buffers, studio glazes and general technical use |
| Pentahydrate | Na₂B₄O₇·5H₂O | 12179-04-3 | 47.8% | Bulk industrial use, where more boron per tonne cuts freight costs |
| Anhydrous (fused) | Na₂B₄O₇ | 1330-43-4 | 69.2% | Glass and enamel melting, and pre-fused jewellery flux |
The B₂O₃ (boric oxide) figures are theoretical values for the pure compounds. Most borax sold in small packs in the UK is the decahydrate.
What is borax used for?
According to the USGS, more than three-quarters of the world’s borates go into ceramics, detergents, fertilisers and glass. The main uses are:
- Glass and glass fibre: boron is a key ingredient of heat-resistant borosilicate glass and of glass-fibre insulation.
- Ceramics and enamels: borax lowers the melting point of glazes and frits and helps them flow into a smooth surface.
- Metalwork: molten borax dissolves metal oxides, so jewellers, brazers and blacksmiths use it as a flux (see our guide to borax flux for brazing, hard soldering and forge welding).
- Laboratories: as a buffer and a pH reference standard.
- Agriculture: boron is an essential micronutrient for plants and is supplied in borate fertilisers.
- Industry: detergents, flame retardants, adhesives and timber preservatives.
Borax vs boric acid vs borax substitute
These three products are often confused, but they are quite different:
| Property | Borax | Boric acid | Borax substitute |
|---|---|---|---|
| Chemical | Sodium tetraborate | H₃BO₃ | Sodium carbonate and sodium bicarbonate |
| Contains boron | Yes | Yes | No |
| In water | Mildly alkaline | Weakly acidic | Alkaline |
| UK classification | Repr. 1B (H360FD) | Repr. 1B (H360FD) | Eye irritant |
| Who can buy it | Professional and trade users | Professional and trade users | Anyone |
Boric acid is made by treating borate minerals with acid. Borax substitute is a household laundry and cleaning product that replaced borax on UK shop shelves; it does many of the same cleaning jobs but none of the boron chemistry.
Is borax legal in the UK?
Yes, with conditions. In Great Britain borax is classified as toxic for reproduction, category 1B, with the hazard statement H360FD. Under UK REACH, substances with that classification may not be supplied to the general public, either on their own or in mixtures above set limits. That is why borax disappeared from supermarket shelves around 2010 and was replaced by borax substitute.
Borax remains legal to buy, own and use for professional, industrial and laboratory work, and specialist suppliers sell it with a Safety Data Sheet. Our guide to buying borax in the UK explains how to read the label, SDS and specification before you order. It should never be added to food or drink, home-made cosmetics or children’s toys such as slime.
Is borax safe?
Borax calls for sensible handling rather than alarm. It has two recognised hazards: it causes serious eye irritation, and it is classified as toxic for reproduction, mainly on the basis of high-dose animal studies. Professional users keep exposure low by avoiding dust, wearing nitrile gloves and eye protection, washing their hands after use, and storing borax sealed, labelled and away from children and food. Workplaces must also follow the COSHH regulations and the supplier’s Safety Data Sheet.
Frequently asked questions
Is borax the same as baking soda?
No. Baking soda is sodium bicarbonate and contains no boron. Borax is sodium tetraborate. The two white powders look alike but behave differently and carry very different safety classifications.
Is borax natural?
Yes. Borax occurs naturally as a mineral. Commercial borax is refined from mined ore or brines and recrystallised to a consistent purity.
Does borax go off?
No. Borax is chemically stable and keeps indefinitely if it is stored dry and sealed. Damp conditions make it cake into lumps.
Can I buy borax in the UK?
Yes, for professional, trade, studio and laboratory use, from specialist suppliers who provide a Safety Data Sheet. For laundry and cleaning at home, use borax substitute instead.
This guide is general information. Always follow the Safety Data Sheet supplied with your borax.
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