Step 1: Prepare the Sample.
A successful fusion relies on a well-prepared sample. The sample must be ground and, if necessary, treated for sulphides and other compounds that may be insoluble or can damage platinumware and prevent successful beads from being made. Grinding the sample reduces the particle size, reducing the dissolution time. The sample must be dry and the loss on ignition must be recorded to allow for any weight loss during preparation.
Step 2: Add Flux
A flux is added to bind the sample into a glass. The flux must be high purity and dry. Two important decisions need to be made when selecting a flux- which flux and what ratio of flux to sample will give the best results? Both of these decisions are based on the samples properties. Generally, we recommend the used of a blended lithium tetraborate and lithium metaborate flux because it provides the most flexibility and allows fusions at lower temperatures than a lithium tetraborate flux alone. The ratio of flux to sample can vary from 2:1 to 60:1 and is dependent on the solubility of the sample in the flux selected.
The sample and flux must be accurately weighed to within 0.0001g for quantitative analysis and the weights recorded. The sample and flux should be well mixed so there is no separation in the mixture. If the weighing is not performed in the crucible, care should be taken to ensure none of the mixture is left when transferring the mixture to the crucible.
Step 3: Melt the Mixture.
Once the mixture has been transferred to the heating vessel, the sample is ready to be dissolved. A mould release agent, such as ammonium iodide may be added to the dry powder just before melting. The halide mixes with the sample and helps the mixing, ensures the complete transfer of melt from the crucible to the mould and helps the finished bead release from the mould as it cools.
The melting is gradual and mixing takes place via the agitating cradle holding the crucibles. In an electric, radiant-heat machine, like our fusion machines, the fusion can easily be accomplished between 1050°C - 1100°C for any fusible sample, with minium loss of volatile elements and less aging of the platiumware.
A surface tension modifier (iodine gas) may be applied during the fusion process via the iodine injector system, available on the Beadmaster. The atmosphere in which the fusion takes place can be modified to be oxidising or inert using the automatic atmosphere control option. The average time taken for fusion is between 10 and 20 mins, depending on the sample.
Step 4: Cast the Melt.
We offer two different mould options- automatic on the Beadmaster or manual on the Fusilux series.
The Beadmaster automatic fusion machine uses mouldibles, combination mould and crucible receptacles, to fuse and create the beads. There is no casting step, just removing the mouldibles for cooling.
The manual pour moulding on the Fusliux machines offers more flexibility during casting. The moulds are heated in the hearth chamber while the samples are melted and agitated. The melt is carefully poured from the crucible into the mould while the mould is still in the hearth. The hot moulds are then removed from the hearth individually using the sliding arms and forced cooling starts automatically.
Step 5: Cooling.
Perhaps the most critical step of the fusion process, the cooling rate of the fusion bead must be carefully controlled to lock the fusion in an amorphous glass stage. If a sample is cooled too slowly, the fusion solution may crystallise, but if a sample is cooled too quickly, the bead can crack or burst because the internal stresses are too high.
As the moulds are individually withdrawn from the hearth, they are placed over a controlled cooling fan that dictates the cooling rate of the mould. A mould release agent can be applied at this stage using our unique iodine caps.
An optional annealing step can be added for difficult to fuse samples. This step reduces the internal stresses that can cause cracking of the bead. Once cool the beads can be handled and labelled on the non-analytical face and are ready for analysis.