Borax in Ceramic Glazes: What Potters Need to Know

Borax works in ceramic glazes because it brings boron oxide (B₂O₃) and sodium oxide into the melt, and both help a glaze melt at lower temperatures. The catch is that raw borax dissolves in water, so most potters get their boron from a borax frit or a borate mineral instead of mixing the powder straight into a glaze bucket. Raw borax still has a place in the studio: in home-made frits, small low-fire and raku tests, enamelling and glass work. This guide explains what borax does in a glaze, where it fits, and how to handle it safely.

What borax brings to a glaze

Borax is sodium tetraborate decahydrate, usually written Na₂B₄O₇·10H₂O. Worked out from that formula, the powder is roughly:

• B₂O₃ (boron oxide): about 36.5%
• Na₂O (sodium oxide): about 16.3%
• Water of crystallisation: about 47.2%, which is driven off on heating

So close to half the weight of a bag of borax decahydrate never reaches the fired glaze. That matters when you convert a recipe that calls for anhydrous borax or a borax frit: you cannot swap them gram for gram.

Both oxides do useful work. Boron oxide is unusual because it acts as a glass former, like silica, and as a strong flux at the same time. Sodium oxide is a powerful flux in its own right. Together they let a glaze melt and smooth out at temperatures where silica alone would barely soften. Once its water has gone, borax itself melts at about 743 °C into a clear glass, well below the temperature of a typical earthenware firing.

Why boron matters in low-fire and mid-fire glazes

Boron is the main melter in most glazes that fire below stoneware temperatures. Ceramic technician Tony Hansen’s Digitalfire reference notes that below cone 10 boron is almost always needed for a good melt, and that the lower the firing, the more boron a glaze tends to need. In practice it brings several benefits:

• Lower melting point: glazes mature at earthenware and mid-fire temperatures without large amounts of other, more troublesome fluxes.
• Better flow and gloss: boron loosens the glass network, so the melt flows and heals over pinholes more easily.
• Lower thermal expansion: boron has low expansion, so in moderate amounts it can help a glaze fit the clay body and reduce crazing.
• Brighter colour response: borosilicate glazes are the base for many bright low-fire colours, and boron can produce the milky blue opalescence some potters call “boron blue”.

More is not always better. Too much boron can make a glaze melt early and trap gas, leaving bubbles and surface defects, and past a certain level its effect on expansion becomes hard to predict. Test before you scale up.

The big problem: raw borax is soluble

A glaze is a suspension of fine, mostly insoluble particles in water. Borax does not stay a particle: it dissolves, a few per cent by weight in cold water and far more in hot water. That causes three well-known problems:

1. It moves with the water. When you dip or brush a bisque pot, the porous clay soaks up water from the glaze, and dissolved borax travels into the body with it. The glaze layer ends up with less boron than the recipe says.
2. It crystallises as it dries. Dissolved borax comes back out of solution at the surface of the drying glaze and in the bucket, giving crusts, uneven melting and a slurry that changes from week to week.
3. The recipe drifts. Because some of the borax is always in solution, a glaze made with raw borax rarely behaves the same twice.

Digitalfire sums it up plainly: borax and boric acid are both soluble and unsuitable as direct glaze sources, but fine for frits. That is why studio recipes usually list a “borax frit” rather than borax itself.

Borax frits and other boron sources

A frit is a glass that has already been melted, cooled quickly and ground to a powder. During fritting, soluble materials such as borax are locked into an insoluble glass, so the boron stays where you put it and melts cleanly without releasing gas. Around 80 to 90% of commercial frits contain some boron. The common studio “borax frits” are low-alumina, high-boron frits that most potters use as the boron source in their own recipes.

Other insoluble or slightly soluble boron sources include colemanite, ulexite and Gerstley borate. They are raw minerals, so they release gas as they break down and vary from batch to batch. They are useful, but they need more care than a frit.

Where raw borax powder still earns its place

Raw borax is not the right choice for a standard dipping glaze, but it is a useful studio material in other ways:

• Making your own frit. Ceramicists who experiment with glass chemistry melt borax with silica and other materials in a crucible, quench the melt and grind it to make their own small batches of borosilicate frit. This needs a kiln that can take a crucible, good ventilation and full protective kit.
• Small low-fire and raku tests. In small batches that are mixed, applied and fired quickly, a little borax can be used to push a glaze to melt lower or to explore boron effects before you switch to a frit for production.
• Slips and glaze fit. Small additions of borax are used in ceramic slips and glazes to help them hold on to greenware and bisque.
• Enamelling and glass. Borax is a traditional ingredient in vitreous enamels and in borosilicate glass, and its glassy melt dissolves metal oxides. Enamellers and glass workers use it in fluxes and test melts.

Borax is also the classic flux for joining and melting metal. If you also work in silver or copper, see our separate borax flux guide for soldering and brazing; for a wider overview of trade uses, read what borax powder is used for.

Practical tips for working with borax in the studio

• Weigh it dry and account for the water. Borax decahydrate is nearly half water. If a recipe gives anhydrous borax, you need roughly 1.9 times the weight in decahydrate to get the same oxides.
• Mix small batches and use them fresh. For tests that contain raw borax, mix only what you need and apply it the same day.
• Keep it dry and sealed. Decahydrate can lose water slowly if left open, which changes its effective weight.
• Record everything. Note batch, weight, firing temperature and kiln schedule so you can repeat a result or move it to a frit-based recipe.
• Keep studio and kitchen separate. Use dedicated scoops, scales and containers, and never use kitchen equipment for glaze materials.

Safety and legal status in the UK

Important safety information. Borax (disodium tetraborate decahydrate, CAS 1303-96-4) is classified under GB and EU CLP as toxic to reproduction, category 1B (H360FD: may damage fertility; may damage the unborn child). It was added to the REACH Candidate List of Substances of Very High Concern in 2010. Our borax powder is supplied for professional, studio, industrial and laboratory use only.

• Keep out of reach of children and store in the original, clearly labelled packaging.
• Wear gloves and eye protection when weighing and mixing.
• Avoid breathing dust: weigh and mix dry powder with good ventilation and wear a suitable dust mask or respirator, and clean up with a damp cloth or a vacuum fitted with a suitable filter, not a brush.
• Do not swallow. Do not eat, drink or smoke in the studio, and wash your hands after use.
• Not to be used by anyone who is pregnant or trying to conceive.
• Read the Safety Data Sheet supplied with your order before first use.

For more on what you can and cannot buy, see is borax legal in the UK.

Frequently asked questions

Can I add borax straight to a glaze?

You can, but it is not recommended for a glaze you plan to keep and reuse. Borax dissolves in water, migrates into bisque, crystallises as the glaze dries and makes results inconsistent. For reliable glazes, use a borax frit or another insoluble boron source, and keep raw borax for small tests, home-made frits, slips and enamel or glass work.

What does borax do in a ceramic glaze?

It adds boron oxide and sodium oxide, two fluxes that lower the melting temperature of the glaze. Boron also acts as a glass former, improves flow and gloss, and in moderate amounts lowers thermal expansion, which can help reduce crazing.

Is a borax frit the same as borax?

No. A borax frit is a pre-melted, insoluble glass that contains boron, usually together with other oxides such as silica and calcium. Borax is the raw, water-soluble mineral salt sodium tetraborate. They are not interchangeable gram for gram.

How much water does borax decahydrate contain?

About 47% of its weight is water of crystallisation, which is driven off on heating. The rest is roughly 36.5% boron oxide and 16.3% sodium oxide.

Is borax safe to use in pottery?

Borax is classified as toxic to reproduction (Repr. 1B, H360FD), so it must be handled with care: wear gloves and eye protection, avoid breathing dust, do not swallow it, keep it away from children and do not use it if you are pregnant. It is sold for professional and studio use only.

Buy borax powder for studio use

Our 99.9% pure borax powder is technical-grade sodium tetraborate decahydrate in 900g and 3kg bags, delivered across the UK with a Safety Data Sheet. Browse the full borax range.

Sources

• Digitalfire Reference Library (Tony Hansen), B₂O₃ (boric oxide) and Boron frit
• U.S. Borax, Boron for ceramic glazes
• Wikipedia, Borax (properties, melting point, solubility, REACH status)
• European Chemicals Agency (ECHA), harmonised CLP classification of disodium tetraborate decahydrate (Repr. 1B, H360FD)