Borate fusion is an effective method for preparing samples including cement, ceramics, glass, ores, oxides, refractory materials and rocks. Normal prep methods are ineffective for these materials because inhomogeneity of particle size, density or composition makes it difficult to press them into homogeneous pellets for analysis by XRF or OES. Additionally, they are difficult to dissolve in acid for analysis by AA or ICP.
Virtually all fusions are performed with borate compounds. Samples are mixed with a flux, typically lithium tetraborate (Li2B4O7, m.p. ~920°C), lithium metaborate (LiBO2, m.p. ~845°C) or mixtures of the two. The sample/flux mixture is heated until the flux disintegrates or solubilizes the sample, yielding a melt that is homogeneous at the atomic level and can be cast as a glass disc for analysis by XRF or OES or dissolved in HNO3 or HCl for AA or ICP analysis.
Lithium tetraborate is better suited for dissolution of basic oxides while lithium metaborate is more suitable for acidic oxides. Between the two, you can dissolve almost anything with bonded oxygen. The addition of a high absorption diluent like lanthanum oxide has particular applications to geological and other similar types of samples exhibiting absorption-enhancement effects. The addition of non-wetting agents like fluorides can be added in small quantities so the molten flux will not stick to the crucible.