Borax is sold in more than one form, and the label does not always make the difference obvious. The white borax powder most people buy is borax decahydrate, but industry also uses borax pentahydrate and anhydrous borax. All three are sodium tetraborate. What separates them is how much water is locked into the crystal, and that changes the boron content per kilo, how the material behaves when heated and how it should be stored.
This guide compares the three forms side by side, shows how to convert a recipe from one to another, and explains which one suits flux, glaze, laboratory and other technical work.
Three forms of the same chemical
Every form of borax is built on the same tetraborate unit, Na2B4O7. The hydrates simply carry water of crystallisation as part of the solid.
Borax decahydrate (Na2B4O7·10H2O)
This is “ordinary” borax: the soft, white crystalline powder or granules sold as borax powder, CAS 1303-96-4. It is the form that crystallises from water at room temperature and the one most recipes, data sheets and older textbooks mean when they simply say “borax”. Our 99.9% pure borax powder is decahydrate.
Borax pentahydrate (Na2B4O7·5H2O)
Pentahydrate (CAS 12179-04-3) crystallises from solution above about 61 °C. With roughly half the water of the decahydrate, it carries more boron per tonne, which makes it attractive for bulk industrial users such as glass, fibreglass and detergent manufacturers who pay to transport and handle every kilo.
Anhydrous borax (Na2B4O7)
Anhydrous borax (CAS 1330-43-4), also called fused or dehydrated borax, contains no water of crystallisation. It is produced by heating a hydrate until all the water has been driven off, and is often supplied as a glassy, granular material. It melts at around 742 °C without the foaming stage that hydrated borax goes through.
Borax decahydrate, pentahydrate and anhydrous compared
| Property | Decahydrate | Pentahydrate | Anhydrous |
|---|---|---|---|
| Formula | Na2B4O7·10H2O | Na2B4O7·5H2O | Na2B4O7 |
| CAS number | 1303-96-4 | 12179-04-3 | 1330-43-4 |
| Molar mass | 381.4 g/mol | 291.3 g/mol | 201.2 g/mol |
| Water of crystallisation | about 47% | about 31% | 0% |
| Theoretical B2O3 content | about 36.5% | about 47.8% | about 69.2% |
| Behaviour on heating | Loses water from about 75 °C, swells and foams, then fuses | Loses less water, foams less | Melts directly at about 742 °C |
| GB CLP classification | Repr. 1B, H360FD for all three forms | ||
The B2O3 figures above are calculated from the formulas. The value on a real batch’s Certificate of Analysis can differ slightly, because commercial material may contain a little more or less water than the ideal crystal. Always work from the figure your supplier gives for the batch in front of you.
Converting a recipe between borax forms
Because each form contains a different amount of water, they cannot be swapped gram for gram. A recipe written for anhydrous borax needs almost twice the weight of decahydrate to deliver the same amount of boron. The conversion is a simple ratio of molar masses:
| You have a recipe for | To use decahydrate instead | To use pentahydrate instead | To use anhydrous instead |
|---|---|---|---|
| 100 g decahydrate | 100 g | 76 g | 53 g |
| 100 g pentahydrate | 131 g | 100 g | 69 g |
| 100 g anhydrous | 190 g | 145 g | 100 g |
Two practical notes:
- Concentrated solutions behave differently. The conversion gives the same amount of tetraborate, but the extra water in decahydrate also dilutes a solution slightly. For buffers and standard solutions, calculate from molar mass rather than adjusting by eye.
- Glaze software usually needs the exact material. If you mix glazes, select the correct borax entry in your calculation software, because the oxide analysis is very different between the hydrated and anhydrous forms. Our guide to borax for pottery and ceramic glazes covers boron in glazes in more detail.
Which form of borax do you need?
Soldering, brazing and forge welding
Decahydrate works well as a flux and is easy to mix into a paste or solution. Its drawback is that it bubbles and swells as the water boils off, which can push small solder pieces out of place. Many jewellers and smiths therefore pre-fuse a small amount of decahydrate into anhydrous borax for the bench, or use dry powder only on metal that is already hot. See our full guide to borax flux for soldering, brazing and forge welding.
Ceramic glazes, enamels and glass
Studio potters mostly use decahydrate for tests and small batches, or take their boron from a frit for production glazes. Large glass and fibreglass plants tend to buy pentahydrate or anhydrous borax, because shipping less water lowers the cost per unit of boron and reduces steam in the furnace.
Laboratory buffers and reagents
Borate buffers and pH reference solutions are traditionally prepared from decahydrate, which is the form named in most standard methods. Use the grade and form your method specifies, and record the batch number for traceability.
Timber treatment, cleaning and other technical uses
Where borax is dissolved in water before use, the form matters mainly for dosing: decahydrate dissolves readily and is the form most technical data sheets assume. For an overview of professional applications, read borax uses: professional applications of borax powder. For household laundry and cleaning, use a borax substitute rather than genuine borax.
Making anhydrous borax from decahydrate
Small workshops sometimes prepare their own anhydrous (fused) borax for flux. The usual method is to heat borax decahydrate in a clean ceramic crucible, gently at first, until it stops foaming, then raise the heat until it melts into a clear glass. Once cool, the glass is broken up and ground. Points to keep in mind:
- Heat a small amount at a time. Decahydrate can swell to several times its original volume as the water escapes.
- Work in a well-ventilated area, wear safety glasses and heat-resistant gloves, and keep the crucible well away from anything flammable.
- Grinding the cooled glass creates fine dust. Grind in a closed mortar or under extraction and wear a suitable dust mask (FFP2 or better).
- Label the container clearly as “anhydrous borax” with the hazard information, so it is not confused with the hydrated powder or with anything else.
Storage: why borax powder cakes
Borax decahydrate is sensitive to its surroundings. In damp conditions the powder absorbs moisture and particles bond together into lumps. In warm, dry air the crystals can slowly lose some of their water, so the surface turns chalky and the boron content per gram creeps up. Anhydrous borax, in turn, readily picks up moisture from the air and starts to rehydrate.
To keep any form of borax consistent:
- Keep it in the original, tightly closed and labelled container.
- Store it in a cool, dry place at a steady temperature, off the floor and away from damp walls or heat sources.
- Close the bag or tub straight after use. Broken-up lumps are usually still usable for technical work, but weigh carefully if dosing matters.
- Note the date of opening and use older stock first.
Our borax safety, handling and storage guide covers COSHH, PPE, spills and disposal in more depth.
Safety: the same rules for every form
All three forms of borax carry the same harmonised classification under GB and EU CLP: Repr. 1B, H360FD: may damage fertility, may damage the unborn child. Anhydrous and pentahydrate contain more borate per gram than decahydrate, so their dust deserves at least the same care. Borax is supplied for professional, industrial and laboratory use only, and should not be handled by anyone who is pregnant or trying to conceive.
- Keep out of reach of children and away from pets, food, drink and animal feed.
- Wear nitrile gloves and safety glasses when weighing, mixing, heating or grinding borax.
- Do not ingest. Do not eat, drink or smoke where borax is handled, and wash hands after use.
- Not for use on skin, in food, in cosmetics or in toys such as slime.
- Avoid breathing dust or fumes. Work with ventilation or extraction, scoop rather than pour, and clean up with a damp cloth instead of dry sweeping.
- Keep borax out of drains and dispose of waste according to section 13 of the Safety Data Sheet.
Buying borax decahydrate in the UK
Genuine borax is not sold to the general public in Great Britain, but remains available for professional and trade use. Our technical grade borax powder is sodium tetraborate decahydrate, supplied with a Safety Data Sheet, with Certificates of Analysis available on request. For larger quantities, see our industrial borax page.
Frequently asked questions
Is borax powder decahydrate or anhydrous?
Standard borax powder is sodium tetraborate decahydrate, Na2B4O7·10H2O, CAS 1303-96-4. Anhydrous borax is a different product made by driving off all the water, and is usually labelled as anhydrous or fused borax.
Can I replace anhydrous borax with borax decahydrate?
Yes, if you adjust the weight. Use about 1.9 g of decahydrate for every 1 g of anhydrous borax to supply the same amount of tetraborate. In flux work, expect more bubbling from the decahydrate as its water boils off.
What is the difference between borax pentahydrate and decahydrate?
Pentahydrate carries five molecules of water per formula unit instead of ten, so it contains about 48% B2O3 compared with about 36.5% for decahydrate. It is mainly used by large manufacturers who want more boron per tonne.
Why has my borax powder gone hard and lumpy?
Borax decahydrate cakes when it picks up moisture, and can turn chalky when it loses water in warm, dry storage. Keep it sealed in its original container in a cool, dry place. Lumps can usually be broken up for technical use.
Is anhydrous borax safer than borax decahydrate?
No. All forms of borax have the same classification, Repr. 1B (H360FD), and anhydrous borax contains more borate per gram. Use the same precautions: gloves, eye protection, dust control and storage away from children, pets and food.
Sources
- Borax: hydrate forms, dehydration on heating, crystallisation of the pentahydrate above about 61 °C and melting point of anhydrous borax. Wikipedia
- ECHA, substance information for disodium tetraborate (anhydrous, pentahydrate and decahydrate): harmonised classification Repr. 1B, H360FD. echa.europa.eu
- HSE, COSHH guidance for businesses. hse.gov.uk
Need borax decahydrate for professional use? Our 99.9% pure borax powder is supplied with an SDS for trade, workshop and laboratory users. For home laundry and cleaning, choose our borax substitute.