Understanding Platinum Crucible Alloys for Borate Fusion
Selecting the right platinum crucible alloy is one of the most important decisions in XRF sample preparation. The alloy composition directly affects fusion bead quality, crucible lifespan, and analytical accuracy. Yet many laboratories simply order whatever their instrument manufacturer recommends without understanding why.
In this guide, we break down the most common platinum alloy options and help you choose the right one for your application.
Pure Platinum (99.95% Pt)
Pure platinum crucibles offer excellent chemical inertness and the highest melting point (~1,768°C) of any standard crucible option. They work well for most oxide-based samples, including cement raw materials, geological silicates, and standard mineral analysis.
Best for: General-purpose fusion work, applications requiring maximum chemical purity, and laboratories running diverse sample types.
Considerations: Pure platinum is slightly softer than alloyed options, making it more prone to surface scratching over time. However, its excellent non-wetting properties with lithium borate fluxes make it a reliable workhorse.
Platinum-Gold Alloy (Pt/Au 95/5)
The 95% platinum / 5% gold alloy is the industry standard for routine borate fusion in XRF laboratories. The gold addition significantly improves the non-wetting behavior of the crucible surface, meaning fusion beads release more cleanly from molds with fewer micro-bubbles and surface defects.
Best for: High-throughput XRF labs, automated fluxer systems (Claisse, Katanax, Phoenix, Herzog), and applications where consistent bead quality is critical.
Considerations: Pt/Au alloys have a slightly lower maximum operating temperature (~1,200°C) compared to pure platinum. For standard borate fusion at 1,000-1,100°C, this is not a limiting factor.
Beyond Standard Compositions: Custom Alloys
What many laboratories don’t realize is that platinum crucible alloys are not limited to the standard compositions listed above. Depending on your specific application, sample matrix, and operating conditions, a custom alloy ratio may deliver significantly better performance.
For example, different platinum-gold ratios beyond the standard 95/5 can be optimized for specific flux compositions or sample types. Platinum-rhodium (Pt/Rh) and platinum-iridium (Pt/Ir) alloys offer enhanced mechanical strength and oxidation resistance for demanding high-temperature applications.
At SIB Fusion, we manufacture crucibles in a wide range of alloy compositions tailored to your exact specifications. Whether you need a non-standard Pt/Au ratio, a Pt/Rh alloy for extreme temperatures, or a completely custom formulation, our team can work with you to find the optimal solution.
Factors to Consider When Choosing
When selecting a crucible alloy, consider these key factors:
- Sample type: What materials are you fusing? High-iron samples, sulfide ores, and metallic materials each pose different challenges.
- Operating temperature: Standard borate fusion (1,000-1,100°C) vs. high-temperature applications above 1,200°C.
- Throughput requirements: High-volume labs benefit from the improved non-wetting of Pt/Au alloys.
- Fluxer compatibility: Automated systems may have specific crucible dimension and alloy requirements.
- Budget considerations: Factor in crucible lifespan, not just initial cost. A well-chosen alloy that lasts 3,000+ fusions is more economical than a cheaper option that fails at 1,000.
Extending Crucible Life Regardless of Alloy
No matter which alloy you choose, proper maintenance is essential:
- Pre-screen samples for metallic contaminants (Fe, Cu, Ni, Pb, Sn, Zn) that form low-melting alloys with platinum
- Clean with 10% HCl after each fusion session
- Run periodic flux blanks to dissolve accumulated contamination
- Inspect regularly for thinning, discoloration, or micro-cracks
- Store individually in soft cloth bags — never stack bare crucibles
With proper care, high-quality platinum crucibles can deliver 3,000-5,000 fusions before needing replacement.
Need Help Choosing?
Our team has decades of experience helping laboratories select the right crucible alloy for their specific applications. Whether you need standard Pt/Au crucibles for routine XRF work or a custom alloy for specialized analysis, we are here to help.
Contact us to discuss your requirements, or explore our full range of platinum crucibles and customized solutions.