Long before spectrometers and XRF guns, chemists and mineralogists identified metals with little more than a loop of wire, a flame and a pinch of borax powder. The borax bead test is still taught in qualitative analysis and is still used by geologists, mineral collectors and teaching laboratories as a quick screening test for certain metals. This guide explains how the test works, how to make and read a bead, what the classic colours mean and how to carry it out safely.
If you want to know more about the material itself, start with our overview of borax uses or our guide to technical grade borax.
What is the borax bead test?
The borax bead test, sometimes called the blister test, is a traditional method of qualitative inorganic analysis. It was introduced by the Swedish chemist Jöns Jacob Berzelius in 1812 and is the oldest of the bead tests. Later variants use other fluxes, the best known being microcosmic salt, but borax remains the classic choice.
The idea is simple. A small glassy bead of fused borax is made on a wire loop, a tiny amount of the sample is picked up on it, and the bead is heated again in different parts of a flame. Several transition metals dissolve in the molten borax glass and give it a characteristic colour. Because the colour depends on whether the flame is oxidising or reducing, the two observations together help narrow down which metal is present.
How the test works
Standard borax powder is sodium tetraborate decahydrate, which contains a large amount of water of crystallisation. When it is heated in a flame it swells up as that water is driven off, then shrinks and fuses into a colourless, transparent, glass-like bead. Chemically, this bead is a mixture of sodium metaborate and boric anhydride.
Molten borax glass is a good solvent for metal oxides. When a trace of a metal compound is fused into the bead, the metal forms coloured metaborates in the glass. The same metal can give different colours depending on its oxidation state, and that is controlled by the flame:
- The oxidising flame (the outer, upper part of the flame, where there is plenty of air) tends to push the metal into a higher oxidation state.
- The reducing flame (the inner, lower part, where there is less oxygen and more unburnt fuel) tends to push it into a lower oxidation state or even reduce it towards the metal.
The different forms of borax behave slightly differently on heating. Anhydrous borax does not foam, while the decahydrate puffs up first. Our guide to borax decahydrate vs pentahydrate vs anhydrous explains the differences.
What you need
- A wire loop: traditionally a short piece of platinum wire with a small loop at the end, fixed in a holder. A magnesia (magnesium oxide) stick can be used instead.
- A flame: a Bunsen burner, or a blowpipe flame in traditional field mineralogy.
- Borax powder: a small amount in a clean, labelled dish, taken from the main pack so the bulk stock stays clean and dry.
- The sample: crushed to a fine powder. Sulphide minerals are traditionally roasted first.
- Basic lab kit: a heat-proof mat, tongs, a small beaker of water and somewhere safe to rest hot equipment.
- Protective equipment: eye protection, suitable gloves and a lab coat (see the safety section below).
How to do a borax bead test, step by step
1. Make a clear borax bead
- Heat the wire loop in the hottest part of the flame until it glows red, to burn off any contamination.
- Dip the hot loop into the borax powder so that a little sticks to it.
- Return the loop to the hottest part of the flame. The borax will swell and bubble as it loses water, then shrink and melt.
- Repeat until you have a small, round, colourless and transparent bead filling the loop. If the bead is coloured before you add any sample, the loop is contaminated: remove the bead and start again.
2. Pick up the sample
Let the bead cool slightly, moisten it and touch it to the powdered sample so that only a tiny amount adheres. This is the step most people get wrong. If too much material is used, the bead turns dark and opaque and the colour cannot be read.
3. Heat in the reducing and oxidising flames
- Heat the bead in the lower, reducing part of the flame until the sample has fused into it. Let it cool and note the colour.
- Then heat it in the upper, oxidising part of the flame. Let it cool and note the colour again.
- Where you can, note the colour both while the bead is hot and after it has cooled, because some metals change colour as the glass cools.
4. Remove the bead
The traditional way to clear the loop is to heat the bead until it is fully molten and then plunge it into a beaker of water, which shatters the glass away from the wire. Wear eye protection when you do this. Clean the loop in the flame before the next test so that one sample does not contaminate the next.
Borax bead colour chart
The table below summarises the classic results for the six metals best known for coloured borax beads. It is based on the borax bead table published by Webmineral, which in turn draws on Brush and Penfield’s Determinative Mineralogy and Blowpipe Analysis (1906). Exact shades depend on how much sample is used, so treat the chart as a guide rather than a precise standard.
| Metal | Oxidising flame (hot) | Oxidising flame (cold) | Reducing flame (hot) | Reducing flame (cold) |
|---|---|---|---|---|
| Copper | Green | Blue | Colourless to green | Opaque red with much oxide |
| Cobalt | Blue | Blue | Blue | Blue |
| Iron | Deep yellow to orange-red | Yellow | Bottle green | Pale bottle green |
| Manganese | Violet | Reddish violet | Colourless | Colourless |
| Nickel | Violet | Reddish brown | Opaque grey | Opaque grey |
| Chromium | Yellow | Yellowish green | Green | Green |
Many common elements, including silicon, aluminium, tin, calcium, magnesium and zinc, give colourless beads, or white and opaque ones when the bead is heavily loaded. A colourless bead is therefore a result in itself: it tells you the sample does not contain significant amounts of the strongly colouring metals.
Tips for reading the result
- Use a small amount of sample first. You can always add more; a saturated bead is hard to read.
- Look in good, neutral light and against a white background once the bead has cooled.
- Compare with a known reference where possible, such as a bead made from a labelled sample of the metal compound you suspect.
- Remember that mixtures confuse. A sample containing iron, copper, nickel and cobalt together can give yellow, green or blue beads of various shades.
Limitations of the bead test
The borax bead test is a screening tool, not a full analysis. It only responds well to a limited group of metals, it does not tell you how much of a metal is present, and different metals can give similar colours. Experienced users treat a bead result as a clue that points to which confirmatory tests or instrumental methods to use next. If an identification matters, for example for a valuable mineral specimen, confirm it by another method or with a qualified laboratory.
Choosing borax for bead tests
Only a few grams of borax are used in a session of bead tests, but purity matters because contaminants can colour the bead. Our 99.9% pure borax powder is technical grade sodium tetraborate decahydrate, supplied with a Safety Data Sheet for trade, workshop and laboratory users. Some laboratories specify an analytical reagent grade for formal work; our guide to borax grades explains the differences. You can also browse the full borax powder range.
Safety: borax bead tests in the lab
Borax (disodium tetraborate) is classified in Great Britain and the EU as Repr. 1B, H360FD: may damage fertility, may damage the unborn child. It is supplied for professional and trade use only. The bead test also involves an open flame, molten glass and samples that may be hazardous in their own right.
- Keep out of reach of children. Bead tests are for adults in a properly supervised laboratory or workshop setting, not for children.
- Avoid if pregnant. Borax should not be handled during pregnancy or by anyone trying to conceive. Employers must assess the risks for new and expectant mothers.
- Do not ingest. Never eat, drink or smoke where borax is handled, and wash your hands thoroughly afterwards.
- Wear gloves and eye protection. Use suitable chemical-resistant gloves, safety glasses or goggles and a lab coat.
- Control dust and fumes. Handle powder gently and work in a well-ventilated area or fume cupboard, especially when roasting or fusing samples.
- Check the sample hazards. Compounds of metals such as cobalt, nickel and chromium carry serious hazards of their own. Read the SDS for every reagent and sample, not just the borax.
- Take care with hot glass. Molten beads can drip, and quenched beads can shatter. Keep the work area clear and protect your eyes.
- Read the label and Safety Data Sheet before use, and carry out a COSHH assessment for workplace or teaching use.
For PPE, storage, spills, disposal and first aid, read our full borax safety guide.
Frequently asked questions
What is the borax bead test used for?
It is a traditional qualitative test for certain metals, especially copper, cobalt, iron, manganese, nickel and chromium. A sample fused into a borax glass bead gives characteristic colours in the oxidising and reducing flames, which helps identify the metal present.
Why does a borax bead change colour?
Molten borax dissolves metal oxides and forms coloured metaborates. The colour depends on the metal and its oxidation state, which is why the same sample can look different after heating in the oxidising flame and in the reducing flame.
What colour is a cobalt borax bead?
Cobalt gives a blue bead in both the oxidising and the reducing flame, hot and cold, which makes it one of the easiest results to recognise.
Why is my borax bead dark and opaque?
Usually too much sample has been picked up. Clear the loop, make a fresh colourless bead and touch it to the powder so that only a trace sticks to it.
Can I use any borax for a bead test?
Use a pure borax powder from a clean, labelled container so that contaminants do not colour the bead. Borax is classified Repr. 1B and is supplied for professional use, so follow the label, the Safety Data Sheet and a COSHH assessment.
Sources
- Bead test: history (Berzelius, 1812), procedure and bead composition. Wikipedia
- Webmineral, Borax Bead Tests: table of reactions based on Brush and Penfield, Determinative Mineralogy and Blowpipe Analysis (1906). webmineral.com
- ECHA, substance information for disodium tetraborate decahydrate (CAS 1303-96-4): harmonised classification Repr. 1B, H360FD. echa.europa.eu
- HSE, COSHH guidance for businesses. hse.gov.uk/coshh
- HSE, protecting new and expectant mothers at work. hse.gov.uk/mothers
Need borax for the lab or workshop? Our 99.9% pure borax powder is sodium tetraborate decahydrate supplied with a Safety Data Sheet for trade, workshop and laboratory users. Larger quantities are covered on our industrial borax page.