What Is Borate Fusion?
Borate fusion is the gold standard for preparing solid samples before X-ray fluorescence (XRF) analysis. The technique dissolves a powdered sample in a molten lithium borate flux at temperatures between 1000°C and 1100°C, producing a homogeneous glass disk that eliminates mineralogical and particle-size effects. The result is a sample that delivers highly accurate, repeatable XRF measurements — something pressed pellet methods often struggle to achieve.
Industries ranging from mining and cement production to environmental monitoring and geology rely on borate fusion every day. If your lab performs elemental analysis on rocks, ores, soils, slags, or ceramics, understanding this technique is essential to getting reliable data.
The Borate Fusion Process: Step by Step
1. Sample Preparation
The process begins with grinding your sample to a fine powder, typically below 75 μm. Consistent particle size ensures even dissolution in the flux. The sample is then weighed precisely — most methods call for a sample-to-flux ratio between 1:5 and 1:10, depending on the matrix and the elements of interest.
2. Mixing with Flux
The ground sample is mixed with a lithium borate flux. The two most common fluxes are:
- Lithium tetraborate (Li₂B₄O₇): Best for basic oxides (calcium, magnesium, iron)
- Lithium metaborate (LiBO₂): Better for acidic oxides (silica, alumina)
- Mixed fluxes (e.g., 66/34 or 50/50 blends): Versatile for a wide range of sample types
A non-wetting agent — typically lithium bromide (LiBr) or lithium iodide (LiI) — is often added at 1–2% to help the molten glass release cleanly from the platinum crucible and mold.
3. Fusion
The mixture is placed in a platinum crucible designed for XRF analysis and heated in a fusion instrument (fluxer) to approximately 1050°C. The crucible is agitated — either by rocking, swirling, or rotating — to ensure complete dissolution. Fusion typically takes 5 to 15 minutes depending on the instrument and sample type.
Leading fluxer brands include Claisse, Katanax, Phoenix, PANalytical, and Herzog — each requiring specific crucible and mold geometries for optimal performance.
4. Casting
Once the sample is fully dissolved, the molten glass is poured into a preheated platinum mold (round or square, depending on your XRF spectrometer). The mold is cooled in a controlled manner to produce a flat, transparent glass disk free of cracks or bubbles.
5. Analysis
The finished glass disk is placed directly into the XRF spectrometer for elemental analysis. Because the sample is now a homogeneous glass, matrix effects are minimized and calibration curves remain linear over wide concentration ranges.
Why Platinum Crucibles Are Essential for Borate Fusion
Borate fusion demands crucibles that can withstand extreme temperatures, resist chemical attack from aggressive fluxes, and release the molten glass without contamination. Platinum labware is the only practical choice for several reasons:
Chemical Inertness
Platinum does not react with lithium borate fluxes at fusion temperatures. This means zero crucible contamination in your analytical results — critical when you’re measuring trace elements at parts-per-million levels.
Non-Wetting Surface
Platinum’s natural non-wetting properties, enhanced by the addition of gold in Pt/Au alloys, allow the molten glass to release cleanly. This prevents sample loss and reduces the need for aggressive cleaning between fusions.
Thermal Stability
With a melting point of 1768°C, platinum operates well within its safe range at typical fusion temperatures of 1000–1100°C. Platinum crucibles maintain their shape and dimensional stability through thousands of fusion cycles.
The Role of Alloy Selection
Not all platinum crucibles are created equal. The alloy composition significantly impacts performance and longevity:
- Pt/Au 95/5 (95% platinum, 5% gold): The most common alloy for borate fusion. Gold improves non-wetting properties and hardness.
- Pt/Au 97/3: Slightly softer, used where maximum platinum content is desired for specific applications.
- Pt/Rh (platinum-rhodium): Higher hardness and better high-temperature strength, preferred for some thermal analysis applications.
- Custom alloy compositions: SIB Fusion manufactures crucibles in a wide range of alloy combinations to match specific laboratory requirements — a key advantage when standard alloys don’t meet your needs.
For a deeper dive into choosing the right alloy, see our guide on selecting platinum crucible alloys for XRF analysis.
Common Challenges in Borate Fusion (and How to Solve Them)
Cracked or Bubbled Glass Disks
Usually caused by incomplete dissolution or cooling too quickly. Ensure adequate fusion time and use the instrument’s programmed cooling cycle. Also check that your sample-to-flux ratio is appropriate for the matrix.
Glass Sticking to the Mold
Often a sign that your non-wetting agent is insufficient or that the platinum mold surface has become scratched or contaminated. Regular cleaning and inspection of your molds can prevent this issue. Severely damaged molds should be replaced or sent for refurbishment.
Poor Reproducibility
Check your weighing precision, grinding consistency, and flux purity. Even small variations in the sample-to-flux ratio can affect results. Using high-quality, certified-purity flux is essential.
Crucible Degradation
Platinum crucibles are durable but not indestructible. Avoid fusing samples containing metallic particles (iron filings, lead, tin) directly — these can alloy with platinum and cause permanent damage. Always pre-roast or oxidize samples that may contain reducible metals.
Borate Fusion vs. Other Sample Preparation Methods
How does borate fusion compare to alternatives?
- Pressed pellets: Faster and cheaper, but subject to particle-size and mineralogical effects. Best for routine screening rather than high-accuracy work.
- Acid digestion (ICP-OES/ICP-MS): Excellent for trace analysis but requires hazardous acids, generates liquid waste, and can’t easily handle refractory minerals like zircon or chromite.
- Loose powder analysis: Quick but least accurate. Useful only for very rough screening.
For laboratories that need accurate major and minor element analysis of geological, environmental, or industrial samples, borate fusion remains the method of choice.
Getting the Right Platinum Labware for Your Fusion Program
Whether you’re setting up a new fusion program or replacing worn crucibles and molds, choosing the right platinum labware is critical. Key considerations include:
- Fluxer compatibility: Each instrument brand requires specific crucible and mold dimensions. SIB Fusion supplies XrFuse, Vulcan, Linn, and many other fluxer-specific configurations.
- Alloy selection: Match the alloy to your application and flux chemistry.
- Buyback and recycling: Platinum is a precious metal. A reliable buyback program helps you recover value from worn-out labware.
SIB Fusion specializes in high-purity platinum crucibles and molds for all major fluxer brands, with custom alloy compositions available to match your specific analytical requirements. Contact us to discuss your fusion labware needs.