Non-wetting agents (NWA) are iodides and bromides which are added in small quantities to a fusion so the molten flux will not stick to the crucible or mold. The visible effect of a non-wetting agent is to increase the surface tension of the melt. A fused disc with too little NWA will have a concave upper surface and may be difficult to remove from the mold, whereas a molten flux bead with excessive NWA will ball up when poured and not form a complete disc.

When glass discs for XRF are being made, non-wetting agent is mixed with the flux and sample before the fusion starts. Typically the amount of NWA is about 0.2% of the weight of the flux, e.g. 12 mg of NWA for 6 grams of flux. Certain samples such as iron ores, which greatly increase the “stickiness” of a melt, require additional NWA. As non-wetting agents gradually volatilize during a fusion, somewhat longer fusions (as for some technical ceramics) also need greater amounts. In the case of very long fusions the NWA may volatilize entirely, and it may be necessary to add a small amount to the crucible just before pouring the melt into the mold. The ideal amount of NWA for a specific procedure is usually determined by experiment.

When making solutions by pouring a bead of molten flux into dilute mineral acid, it is necessary to have complete transfer of the bead from the crucible to the beaker. This often requires a much higher proportion of non-wetting agent than is necessary to pour glass discs. A quantity of flux plus sample not exceeding 2 grams might require 50 to 100 mg of NWA.

Lithium iodide and lithium bromide are popular non-wetting agents because they do not add an impurity to the flux; however, lithium bromide is notable hygroscopic, so it is usually made into
a saturated solution and added to the flux from a dropper bottle. Lithium iodide, sodium iodide and cesium iodide are somewhat more air-stable, and easier to use as solids. While it is simpler to add a drop or two of NWA than it is to weigh out 10 or 20 mg of a solid, liquid NWA cannot be added to a hot crucible while a fusion is in progress. Note that non-wetting agents should be used with care when copper-bearing samples are being fused, as copper halides are
extremely volatile.

Lithium fluoride can be used as a fluidizer, lowering the melting point of a flux and making it flow far more easily. At 10% by weight, it lowers a flux’s melting point by about 100°C.

Oxidizers such as lithium nitrate and sodium nitrate can be valuable in eliminating unoxidized (and hence infusible) components from a sample. Graphite, often present in cement mix, is relatively harmless but can leave a black film on a glass disc or even cause it to crack: graphite can be oxidized to CO2. Other sample components such as phosphides and sulfides may be corrosive enough to damage or wreck a crucible in a single fusion; if they are oxidized to phosphates and
sulfates they will be comparatively harmless, and their cations will be present in the fused glass disc for analysis. As oxidizers have much lower melting points then borate fluxes, any fusion including them should proceed at a low temperature until oxidation is complete.

SPEX SamplePrep supplies a full line of non-wetting agents,
fluidizers, and oxidizers for borate fusions.