Borax is one of the classic sources of boron for potters, enamellers and glass workers. In a glaze, boron acts as both a flux and a glass former: it helps the glaze melt at lower temperatures and, used in the right amount, helps it fit the clay body without crazing. The catch is that raw borax dissolves in water, which makes it awkward in a glaze bucket. That is why most production glazes take their boron from a frit, and raw borax powder is mainly used for testing, enamelling and specialist studio work.
This guide explains what borax actually does in a glaze, where raw borax is useful in a ceramics studio, when a frit is the better choice, and how to handle borax powder safely at the bench.
What borax brings to a glaze
Borax (sodium tetraborate decahydrate, CAS 1303-96-4) supplies two oxides to a fired glaze: boric oxide (B2O3) and sodium oxide (Na2O). The boron is the main reason potters use it.
Boron as a flux and a glass former
Most glaze oxides do one job. Silica forms the glass, and fluxes such as sodium, potassium and calcium help it melt. Boric oxide does both. It forms glass in its own right and lowers the melting temperature of the silica around it. The ceramics reference site Digitalfire notes that below cone 10, additions of B2O3 are almost always needed to make glazes melt well.
How much boron different firing ranges need
The lower the firing temperature, the more boron a glaze generally needs. Digitalfire gives a rough guide of about 0.5 molar B2O3 at cone 06 (earthenware) but only 0.1 to 0.2 at cone 6 (mid-fire stoneware). Low-fire transparent and coloured glazes therefore lean heavily on boron, while stoneware glazes use just enough to get a good melt.
Glaze fit and crazing
Boric oxide has a low thermal expansion. Swapping some high-expansion flux, such as sodium or potassium, for boron is a common way to reduce crazing. The effect has a limit: past an optimum level, Digitalfire notes that crazing can increase again, so high-boron glazes need testing rather than guesswork.
Side effects to watch for
- Early melting: too much boron can seal the glaze surface before gases from the clay body have escaped, leading to pinholes and blisters.
- “Boron blue”: in some low-fire transparents with calcium, excess boron can cause a milky blue haze as crystals form on cooling.
- Runny glazes: boron increases melt fluidity, so a glaze that is fine at one temperature can run at a slightly higher one.
The problem with raw borax in a glaze bucket
A glaze is a suspension of fine, mostly insoluble particles in water. Borax does not stay a particle. It dissolves, and its solubility rises steeply with temperature:
| Water temperature | Borax in a saturated solution (by weight) |
|---|---|
| 0 °C | about 2% |
| 20 °C | about 4.7% |
| 40 °C | about 11% |
| 60 °C | about 30% |
In practice this causes several problems:
- Migration: dissolved borax soaks into porous bisque with the water, so the glaze layer ends up with less boron than the recipe intended.
- Surface crystals: as the glaze dries, borax recrystallises on the surface and can leave a crust or uneven melting.
- Batch drift: the amount dissolved changes with the temperature of the studio and the water, so the same bucket can behave differently from one week to the next.
- Changes in the slurry: soluble salts can change how a glaze suspends and flows, making it harder to apply evenly.
Digitalfire sums it up plainly: borax and boric acid are soluble and are not suitable as raw glaze materials, although they work well once melted into a frit.
Borax, frits and other boron sources compared
| Boron source | Soluble in water? | Typical studio role |
|---|---|---|
| Raw borax (decahydrate) | Yes | Tests, enamelling and glass flux, specialist recipes |
| Borax frit (pre-melted glass) | Very low | Everyday low-fire and mid-fire glazes |
| Calcium borate frits | Very low | Mid-fire glazes needing boron with calcium |
| Natural borate minerals (e.g. colemanite, ulexite) | Partly | Traditional recipes; can release gas and vary by batch |
A frit is made by melting raw materials, including a boron source such as borax, into a glass, quenching it and grinding it to powder. The boron is locked into the glass, so it no longer dissolves in the bucket. For glazes you mix and use over weeks, a frit is almost always the more reliable choice.
Where raw borax powder still earns a place in the studio
Glaze testing and line blends
Because it dissolves fully, borax is a quick way to add a known amount of boron and sodium to small test batches. Mixed fresh and applied the same day, it lets you see how a base glaze responds to more boron before you reformulate it with a frit for production.
Enamelling and glass work
Borax is a traditional flux in vitreous enamels and glass. It melts into a clear glass at around 743 °C once its water has been driven off, dissolving metal oxides and helping a coating flow. The same property makes it useful as a flux for metalwork; see our guide to borax flux for soldering and brazing.
Slips and glaze fit on greenware
Borax is also used in small amounts as an additive in some slips and glazes to help them adhere to greenware or bisque. As the water evaporates, the recrystallising borax helps bind the dried coating. Add it sparingly and test first, because the same solubility issues apply.
Practical tips for working with borax in ceramics
- Allow for the water. Borax decahydrate is roughly 47% water of crystallisation by weight. Recipes written for anhydrous borax or a frit cannot be swapped gram for gram. Use glaze calculation software and the B2O3 figure on your batch’s Certificate of Analysis.
- Dissolve before adding. Stir weighed borax into a little warm water until clear, then add it to the test batch so it is evenly distributed.
- Mix small and use fresh. Make test amounts you will apply the same day rather than storing a borax-containing glaze.
- Fire on a test tile or cookie. Boron-rich glazes can run. Fire new tests on a waste tile or a sacrificial cookie to protect your kiln shelves.
- Record everything. Note the batch, water temperature, application thickness and firing schedule, so a good result can be repeated.
- Test functional ware properly. If a glaze is intended for tableware, have the fired ware tested for durability and metal release by a suitable laboratory before use. Do not assume a glaze is suitable for food contact.
Safety: handling borax in a pottery studio
Genuine borax is classified under GB CLP as Repr. 1B, H360FD: may damage fertility, may damage the unborn child. It also causes serious eye irritation (H319). It is supplied for professional, industrial and laboratory use only, and should not be used by anyone who is pregnant or trying to conceive.
- Keep out of reach of children and away from food, drink and animal feed. Never use borax in food, cosmetics or toys such as slime.
- Wear nitrile gloves and safety glasses when weighing, mixing or applying borax, and when handling wet glaze that contains it.
- Do not swallow and do not eat, drink or vape in the glaze area. Wash your hands before breaks.
- Avoid breathing dust. Scoop rather than pour, work under extraction where you can, wipe benches with a damp cloth and never dry-sweep. Wear an FFP2 respirator if dust cannot be controlled.
- Ventilate the kiln room. Kilns release fumes during firing, so follow your kiln manufacturer’s ventilation guidance.
- Store borax in its original, labelled packaging, sealed, dry and locked away. Never decant it into food or drink containers.
- Dispose of waste properly. Keep borax and glaze waste out of drains; follow section 13 of the Safety Data Sheet.
Businesses and teaching studios should cover borax in their COSHH assessment. Our borax safety, handling and storage guide covers PPE, exposure limits, spills and first aid in detail.
Buying borax for a ceramics studio in the UK
Borax may not be sold to the general public in Great Britain, but it remains available for professional, industrial and educational use. Our 99.9% pure borax powder (technical grade) is supplied with a Safety Data Sheet, and Certificates of Analysis are available on request. For larger volumes and trade supply, see our industrial borax page. For an overview of other applications, read borax uses: professional applications.
Frequently asked questions
Can I use borax in pottery glazes?
Yes, but with care. Borax supplies boron, a flux and glass former that helps glazes melt at lower temperatures. Because raw borax dissolves in water, it is best used in fresh test batches, while glazes for regular use usually take their boron from a frit.
What is the difference between borax and borax frit?
Raw borax is a soluble crystalline powder. A borax frit is a glass made by pre-melting borax with other materials and grinding it, so the boron no longer dissolves in the glaze bucket. Frits give more consistent results in glazes that are stored and used over time.
Does borax stop crazing?
Boron has a low thermal expansion, so replacing some sodium or potassium with boron can help reduce crazing. Beyond an optimum level crazing can increase again, so always test changes on your own clay body and firing schedule.
At what temperature does borax melt?
Borax first loses its water of crystallisation as it is heated, from about 75 °C. The resulting anhydrous borax melts into a clear glass at around 743 °C.
Is borax safe to use in a pottery studio?
Borax is classified as Repr. 1B (H360FD) and is for professional use only. With gloves, eye protection, dust control, labelled storage away from food and children, and a COSHH assessment, it can be handled in a controlled way. It should not be used by anyone who is pregnant or trying to conceive.
Sources
- Digitalfire Reference Library, B2O3 (boric oxide) and boron frit entries: role of boron as flux and glass former, boron levels by firing range, crazing, and unsuitability of soluble borax as a raw glaze material. digitalfire.com
- Borax: solubility in water, 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. hse.gov.uk