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.