What Are Non-Wetting Agents?
If you’ve ever struggled to remove a fused glass bead from your platinum crucible or mold, you’ve experienced the problem that non-wetting agents solve. These chemical additives — most commonly lithium bromide (LiBr) and lithium iodide (LiI) — reduce the surface tension between the molten flux and the platinum surface, allowing the bead to release cleanly without sticking.
For labs running borate fusion for XRF sample preparation, non-wetting agents are a small but critical part of the workflow. Getting them wrong can mean cracked beads, damaged labware, and wasted time.
How Non-Wetting Agents Work
During borate fusion, lithium tetraborate or metaborate flux melts at temperatures between 1000–1100°C, dissolving the sample into a homogeneous glass. Without a non-wetting agent, this molten glass has a natural tendency to adhere to platinum surfaces as it cools.
Non-wetting agents work by introducing halide ions into the melt. These ions migrate to the interface between the molten glass and the platinum surface, forming a thin release layer. The result: the cooled bead separates cleanly from the crucible or mold.
Lithium Bromide (LiBr) vs. Lithium Iodide (LiI)
- Lithium Bromide (LiBr) — The most widely used non-wetting agent. Typically added at 0.1–0.5% of total flux weight. Effective for most routine applications and compatible with all standard platinum crucible alloys.
- Lithium Iodide (LiI) — More aggressive release properties. Used for difficult matrices (high iron, high calcium) where LiBr alone isn’t sufficient. Requires slightly more attention to crucible care, as iodide can be more reactive at elevated temperatures.
How Much to Use
Dosing matters. Too little non-wetting agent and beads stick; too much and you risk:
- Interference with light elements — Bromine and iodine have their own XRF emission lines. Excess amounts can interfere with trace element analysis, particularly for elements like aluminum and silicon.
- Platinum surface damage — Over time, excessive halide concentrations can accelerate degradation of platinum labware, shortening its useful life. This is especially relevant when labs are weighing when to replace their crucibles.
- Bead quality issues — Bubbles, surface pitting, or cloudy beads can result from over-dosing.
The general guideline:
| Agent | Typical Dose | Maximum Recommended |
|---|---|---|
| LiBr (solid) | 0.1–0.3% of flux weight | 0.5% |
| LiI (solid) | 0.05–0.2% of flux weight | 0.3% |
| LiBr (solution, 250 g/L) | 2–3 drops | 5 drops |
Many pre-mixed fluxes from suppliers like Claisse and XRF Scientific already include a non-wetting agent at the correct concentration. If you’re using a Claisse fluxer with compatible platinum labware, check whether your flux is pre-dosed before adding more.
Practical Tips for Your Lab
1. Match the Agent to Your Matrix
Standard geological samples (silicates, carbonite rocks) work fine with LiBr at 0.2%. High-iron samples like iron ores or certain mining concentrates may need LiI or a higher LiBr dose. Cement and calcium-rich matrices often benefit from LiI as well.
2. Account for It in Your Flux Ratio
Non-wetting agents contribute to the overall flux-to-sample ratio. While the amounts are small, precision matters in quantitative XRF. Include the agent weight in your total flux calculation, especially for low-concentration trace analysis.
3. Keep It Dry
LiBr and LiI are hygroscopic — they absorb moisture from the air rapidly. Store them in sealed containers with desiccant. Wet non-wetting agents can cause spattering during fusion and introduce weighing errors. If using solution-form agents, ensure consistent drop size with a calibrated dropper.
4. Watch for Buildup
Halide residues can accumulate on platinum surfaces over repeated fusions. This contributes to the surface discoloration and roughening that eventually signals it’s time for cleaning or reconditioning. Regular acid cleaning (dilute HCl or HNO₃ soak) removes halide buildup effectively.
5. Don’t Skip the LOI Step
Non-wetting agents don’t compensate for incomplete loss on ignition. Organic matter and volatiles in your sample can cause foaming during fusion regardless of your release agent. Always complete LOI before fusion for organic-rich samples.
When Beads Still Stick
If you’re using the correct dose of non-wetting agent and beads are still sticking, the issue is likely elsewhere:
- Scratched or worn platinum surface — Micro-scratches create mechanical adhesion points. Consider refurbishment or buyback for heavily worn pieces.
- Incorrect cooling rate — Too-rapid cooling can cause differential contraction that locks the bead in place.
- Contamination — Residual sample material or flux from previous runs. This ties back to proper sample preparation practices.
- Wrong alloy for the application — Some alloys have better non-wetting properties than others. Pt/Au alloys generally release better than pure platinum.
The Bottom Line
Non-wetting agents are a minor consumable with an outsized impact on lab efficiency. The right agent at the right dose means clean bead release, longer crucible life, and fewer preparation errors. It’s one of those details that separates a lab running smoothly from one constantly troubleshooting.
Need platinum crucibles and molds that are optimized for borate fusion? SIB Fusion manufactures high-purity platinum labware — including custom alloy compositions — designed to work seamlessly with standard non-wetting agents. Contact us to discuss your lab’s specific needs.