Borax has been part of the potter’s and enameller’s materials shelf for a very long time. It is one of the most familiar sources of boron in ceramics, and boron is the oxide that lets a glaze or enamel melt at a lower temperature than silica could manage on its own. This guide explains what borax actually contributes to a glaze, why most studios use it through a frit rather than raw, how to account for its water content in a recipe, and how to handle it safely in a UK workshop. It is written for professional potters, ceramics studios, enamellers and technicians.
What borax adds to a glaze
Chemically, borax is sodium tetraborate. In the kiln the water and structure burn away and two oxides are left behind to join the melt:
- Boron oxide (B2O3): a glass former in its own right that also acts as a powerful flux. It melts at a much lower temperature than silica and helps the whole glaze become fluid earlier in the firing.
- Sodium oxide (Na2O): a strong alkaline flux that lowers the melting point further and tends to give bright colour responses, but also raises thermal expansion.
That combination is why borax is associated with low and mid temperature glazes, glossy surfaces, and vitreous enamels on metal, where the coating has to melt well below the temperature that would distort the metal underneath. In practice borax works alongside silica and alumina rather than replacing them: silica builds the glass, alumina stiffens it so it stays on the pot, and boron and sodium bring the melting point down.
How much oxide is in a kilo of borax?
Standard commercial borax, and the form we sell, is the decahydrate, Na2B4O7·10H2O. Nearly half of its weight is water of crystallisation that simply leaves during firing. Worked out from the molar masses, the theoretical composition is:
| Component | Share of decahydrate by weight | Per 100 g of borax |
|---|---|---|
| Boron oxide (B2O3) | approx. 36.5% | approx. 36.5 g |
| Sodium oxide (Na2O) | approx. 16.3% | approx. 16.3 g |
| Water (lost on firing) | approx. 47.2% | approx. 47.2 g |
This matters when you enter borax into glaze calculation software or adapt a recipe from another source. A recipe written for borax pentahydrate or anhydrous borax contains far less water per kilo, so swapping in decahydrate gram for gram will under-dose the boron and sodium. Always check which form a recipe means, and use the material entry in your software that matches the bag in front of you. Our guide to reading a borax label and spec sheet explains how to tell the forms apart by name and CAS number.
Raw borax or a borax frit?
The single most important thing to know about borax in glazes is that it dissolves in water. Most glaze materials sit in the bucket as insoluble particles; borax does not. That solubility causes a set of well known problems when raw borax goes into a wet glaze:
- Migration: dissolved borax travels with the water into the bisque as the glaze dries, so the boron is not evenly spread through the coating.
- Crystals on the surface: as the glaze dries, dissolved borax can recrystallise on the surface, giving a fragile, powdery skin.
- Changing slurry: soluble salts can alter how a glaze slurry thickens and settles, so a bucket may behave differently from week to week.
- Inconsistent batches: if you decant or sieve a glaze, some of the dissolved borax leaves with the water.
For these reasons, most commercial glaze work gets its boron from a frit: borax or other boron sources melted together with silica and other oxides by the frit manufacturer, cooled into glass and ground. The boron is then locked into an insoluble glass powder that behaves like any other glaze material in the bucket. If you are developing a production glaze, a borax-bearing frit is usually the more predictable route.
Where raw borax still earns its place
Raw borax powder is still used by professionals in situations where solubility is either irrelevant or useful:
- Making your own frits in a studio with a suitable kiln, crucibles and ventilation, so that the borax is fused into glass before it ever goes into a bucket.
- Glaze testing and research, where small batches are mixed, applied and fired the same day.
- Dry-applied mixes and fluxing additions in some enamelling, glass and metal-joining work, where no glaze slurry is involved.
- Small, measured additions to a glaze that is used promptly rather than stored for months.
If you do mix raw borax into water, it dissolves much more readily when warm. Our borax solubility chart shows how much will dissolve at temperatures from 0 to 100 °C, and why crystals form again as a warm solution cools.
What happens to borax in the kiln
Heat borax decahydrate and it first gives up its water. As that happens the powder puffs up and swells, then settles back down and fuses into a clear glass. Anyone who has used borax as a soldering flux will recognise the same behaviour, described in our guide to borax flux for brazing and hard soldering.
In a glaze or enamel, that early swelling and water loss is another reason to fire steadily through the low end of the schedule, especially with thicker applications. Once the boron and sodium are in the melt, they make the glaze flow sooner. A boron-rich glaze that is over-fired can therefore run more than expected, so test tiles and a sensible kiln shelf protection routine are worth the time.
Practical tips for the studio
- Weigh accurately and record the form. Write “borax decahydrate” in your recipe book, not just “borax”.
- Keep the bag sealed. Borax can cake if it picks up moisture, and the decahydrate can slowly lose water in warm, dry storage, which changes its effective strength. Store it cool, dry and closed.
- Test before you scale up. Fire test tiles on the clay body you actually use, at the temperature you actually fire, before committing to a full batch.
- Watch the fit. Sodium raises thermal expansion. If you add borax and start seeing crazing, rebalance the recipe rather than adding more flux.
- Label everything. Any bucket, test batch or frit you make that contains borax should be clearly labelled with its contents and hazard.
- Use dedicated equipment. Keep scoops, sieves and buckets for ceramics use only, never kitchen items.
Safety: handling borax in a ceramics studio
Borax (disodium tetraborate) is classified in Great Britain as toxic for reproduction, category 1B, with the hazard statement H360FD: may damage fertility; may damage the unborn child. Firing it into a glaze or frit does not make the raw powder any less hazardous to handle. We supply borax for professional, industrial and laboratory use only.
- Keep out of reach of children. Not for use by children or in children’s craft activities.
- Keep away from pets and animals, and away from food, drink and animal feed.
- Wear chemical-resistant gloves and eye protection when weighing, mixing or applying borax or borax-containing glazes.
- Do not swallow. Do not eat, drink or smoke in the studio, and wash your hands after use.
- Avoid contact with eyes. If powder or solution gets in the eyes, rinse with water and follow the Safety Data Sheet.
- Avoid raising dust: weigh carefully, clean up with a damp cloth or suitable vacuum rather than dry sweeping, and use good ventilation or local exhaust ventilation.
- Do not use if you are pregnant or trying to conceive.
- Store in the original labelled packaging, sealed, cool and dry.
- Follow the product label and read the Safety Data Sheet before use, and include borax in your COSHH assessment.
Buying borax for ceramics in the UK
Our Borax Decahydrate Powder 99.9% is a technical grade sodium tetraborate decahydrate in 900g and 3kg resealable packs, supplied with a Safety Data Sheet for professional users. The 900g pack suits glaze testing and small studios; the 3kg pack is better value for frit-making and regular production. You can compare grades and pack sizes on our Borax Powder UK page or browse the borax powder range.
Borax substitute is not a glaze material. It is a boron-free household laundry and cleaning blend and contributes no boron oxide to a melt, so it cannot stand in for borax in a ceramic recipe.
Borax for pottery: FAQ
What does borax do in a glaze?
Borax supplies boron oxide and sodium oxide. Both act as fluxes, lowering the temperature at which the glaze melts, and boron oxide also helps form the glass itself. That is why borax is linked with low and mid temperature glazes and enamels.
Why do potters use borax frit instead of raw borax?
Raw borax dissolves in water, so in a wet glaze it can migrate into the pot, recrystallise on the surface and change how the slurry behaves. A frit locks the boron into an insoluble glass powder, which makes the glaze more consistent.
How much boron oxide is in borax decahydrate?
Calculated from molar masses, borax decahydrate is roughly 36.5% boron oxide, 16.3% sodium oxide and 47.2% water by weight. The water is lost on firing, so use the decahydrate entry in your glaze software.
Can I swap borax pentahydrate for decahydrate in a recipe?
Not gram for gram. Pentahydrate contains much less water, so it delivers more boron and sodium per kilo. Recalculate the recipe using the correct material entry before swapping one form for the other.
Can borax substitute be used in glazes?
No. Borax substitute contains no boron. It is a household laundry and cleaning product and will not act as a boron flux in ceramics. For glazes, frits and enamels you need sodium tetraborate.
This guide is general information for professional users. Always follow the label and Safety Data Sheet supplied with your product, and test any glaze change before production use.