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Borax Decahydrate vs Pentahydrate vs Anhydrous: Which Grade Do You Need?

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Borax is sold in three main forms: borax decahydrate, borax pentahydrate and anhydrous borax. Chemically they are all sodium tetraborate. What sets them apart is how much water is locked inside the crystal, and that changes how much boron you get per gram, how the powder behaves when heated and how it should be stored. This guide explains the three grades side by side, shows how to convert a recipe from one to another, and covers safe storage and handling under UK rules.

Our own borax powder is the decahydrate form, so that is where most of the practical advice below is aimed.

What “decahydrate” actually means

“Deca” means ten. Borax decahydrate, Na2B4O7·10H2O, carries ten molecules of water of crystallisation for every unit of sodium tetraborate. This is the form found in nature as the mineral borax, and it is the white crystalline powder most people picture when they think of borax. It is sometimes called “10 mol borax” in the trade.

  • Pentahydrate (Na2B4O7·5H2O) has five molecules of water. It is sometimes called “5 mol borax” and occurs naturally as the mineral tinkhanite.
  • Anhydrous borax (Na2B4O7) has no water at all. It is made by heating a hydrate until the water is driven off, then melting and grinding the glassy result.

Chemists note that the traditional formulas are a simplification (the “decahydrate” is structurally closer to an octahydrate), but the names and formulas above are the ones used on labels, specification sheets and Safety Data Sheets, and they are the ones you need for recipe maths.

The three grades compared

The figures below are theoretical values worked out from each formula. Commercial material is very close to them; for example, U.S. Borax’s specification for technical granular decahydrate gives 36.9–38.2% B2O3, and its pentahydrate specification gives 48.6–49.3%.

Grade Formula Molar mass (g/mol) B2O3 Na2O Water
Decahydrate Na2B4O7·10H2O 381.4 36.5% 16.3% 47.2%
Pentahydrate Na2B4O7·5H2O 291.3 47.8% 21.3% 30.9%
Anhydrous Na2B4O7 201.2 69.2% 30.8% 0%

The key takeaway: almost half the weight of a bag of borax decahydrate is water. The boron and sodium content is identical in every form; it is simply more diluted in the hydrates.

Converting a recipe between grades

Because the active part of every grade is the same Na2B4O7, you can convert between them by molar mass. To get the same amount of boron as 100 g of borax decahydrate, use about:

  • 76 g of pentahydrate (291.3 ÷ 381.4 = 0.764)
  • 53 g of anhydrous borax (201.2 ÷ 381.4 = 0.528)

Going the other way, if a recipe calls for anhydrous borax and you only have decahydrate, multiply the weight by about 1.9. If it calls for pentahydrate, multiply by about 1.3.

Worked example

A glaze or flux recipe asks for 20 g of anhydrous borax. Using decahydrate instead, you need 20 × 1.895 ≈ 38 g. The extra 18 g is water that will boil off when the material is heated, so it does not end up in the finished melt.

When the recipe does not say which grade

Many older recipes simply say “borax”. In most craft and trade literature that means the decahydrate, because it is the common form. If the result seems too strong or too weak, check whether the author was working with a different grade before changing anything else. For glaze work specifically, see our guide to borax in ceramic glazes, which explains why most potters use a borax frit rather than raw powder.

How each grade behaves when heated

This is where the difference is most obvious at the bench.

  • Decahydrate starts to melt in its own water of crystallisation at about 61 °C. As you keep heating, it bubbles and swells while the water escapes, then settles into a clear glassy layer.
  • Pentahydrate carries less water, so it foams less and gets to the glassy stage a little faster.
  • Anhydrous borax has no water to lose. It goes straight to melting, at roughly 742 °C for the stable crystalline form.

For soldering and brazing, that swelling is the main practical reason people mix decahydrate into a paste with water and warm it gently before bringing the joint up to temperature. Our borax flux guide covers the full method.

Dissolving borax decahydrate in water

Borax is only moderately soluble in cold water and far more soluble in hot water. According to the solubility table on Wikipedia’s borax page, a saturated solution holds about:

  • 4.7% borax by weight at 20 °C
  • 11.2% at 40 °C
  • 30.3% at 60 °C

Two practical points follow from this. First, use warm water if you need a stronger solution, and expect crystals to come back out as it cools. Second, finer powder dissolves faster than coarse granules, because more surface is in contact with the water. The solution is mildly alkaline, which is why borax is used for borate buffers in laboratories.

Storing borax decahydrate: why it cakes or goes crusty

Hydrated borax is sensitive to its surroundings in two opposite ways.

  • Too dry or too warm: decahydrate slowly gives up some of its water (efflorescence). Crystals turn chalky and opaque and can crumble into a white powder; in mineral terms, borax gradually changes into tincalconite. This also means the water content, and so the boron per gram, can drift in old stock. Digitalfire’s materials reference recommends checking water content before precise work.
  • Too damp or with temperature swings: moisture dissolves the surface of the particles, and they bond together as it dries. U.S. Borax also notes that warm periods can release water of hydration inside a sealed bag, which condenses and causes caking even without outside damp.

Storage checklist

  • Keep borax in its original, labelled packaging, tightly closed after each use.
  • Store somewhere cool, dry and at a steady temperature. U.S. Borax recommends keeping hydrated borates below 29 °C.
  • Do not stack heavy items on bags. Weight compacts the powder.
  • Use the oldest stock first. The longer borax sits through temperature cycles, the harder any lumps become.
  • Keep it on a high shelf or locked cupboard, away from children, pets, food and drink.
  • If a bag has caked, break lumps up inside the closed bag rather than tipping it out and crushing it in the open, to avoid raising dust.

Safety and UK legal status

All three grades share the same hazard classification. In Great Britain, disodium tetraborate in its decahydrate, pentahydrate and anhydrous forms is classified as toxic for reproduction, category 1B (Repr. 1B), H360FD: may damage fertility; may damage the unborn child. Taking the water out does not make borax any safer; if anything, anhydrous borax is more concentrated per gram.

Because of that classification, borax can no longer be sold to the general public as a household product. It is supplied for professional, trade, industrial and laboratory use. Read more in is borax legal in the UK?

Handling rules

  • Wear gloves and eye protection when handling the powder.
  • Avoid breathing dust. Weigh and mix gently, in a well-ventilated area.
  • Do not swallow. Do not eat, drink or smoke while working, and wash your hands afterwards.
  • Do not use borax if you are pregnant or trying to conceive.
  • Never decant borax into food containers or unlabelled jars.
  • Read the Safety Data Sheet supplied with your order before first use, and follow its first-aid and disposal advice. Our guide on how to read a borax label and Safety Data Sheet explains each section.

Borax is not sold for laundry, home cleaning, slime or any use on people, animals or plants. For laundry and household jobs, use a borax substitute instead.

Which grade do you need?

  • Jewellers, metalworkers and restorers: decahydrate is the usual choice. It mixes easily into a flux paste and dissolves well for solutions.
  • Potters and enamellers: decahydrate works for tests, home-made frits and glass work; just convert recipes written for anhydrous borax.
  • Laboratories: decahydrate is the traditional form for borate buffers and as a primary standard, provided it has been stored properly so its water content is stable.
  • Large-scale industry: pentahydrate and anhydrous borax are mainly used where shipping and storing less water saves money. They are not usually needed for small-batch trade work.

Borax UK supplies 99.9% pure borax decahydrate in 900g and 3kg bags, with a Safety Data Sheet in every order. You can also browse the full borax range.

Frequently asked questions

What is borax decahydrate?

Borax decahydrate is sodium tetraborate with ten molecules of water of crystallisation, Na2B4O7·10H2O. It is the most common form of borax and contains about 36.5% boron oxide and 47% water by weight.

Is borax pentahydrate stronger than decahydrate?

Per gram, yes. Pentahydrate contains less water, so it carries about 47.8% boron oxide compared with 36.5% in decahydrate. About 76 g of pentahydrate supplies the same boron as 100 g of decahydrate.

Can I use borax decahydrate instead of anhydrous borax?

Yes, if you adjust the weight. Multiply the amount of anhydrous borax in the recipe by about 1.9. For example, 20 g of anhydrous borax becomes about 38 g of decahydrate. The extra weight is water that boils off on heating.

Why has my borax powder gone lumpy or crusty?

Lumps usually come from damp or temperature swings, which make particles dissolve slightly and bond together. A chalky white crust means the crystals have lost some water in warm, dry air. Store borax sealed, cool, dry and at a steady temperature, and use older stock first.

Is anhydrous borax safer than borax decahydrate?

No. Decahydrate, pentahydrate and anhydrous borax all carry the same classification, Repr. 1B (H360FD). Anhydrous borax is actually more concentrated per gram. Handle every grade with gloves and eye protection, avoid dust, never swallow it and keep it away from children.

Which form of borax does Borax UK sell?

We sell 99.9% pure borax decahydrate (sodium tetraborate decahydrate) in 900g and 3kg bags for professional, trade and laboratory use. We do not sell pentahydrate or anhydrous borax.

Sources

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