X-ray fluorescence is an analytical technique that returns information about the elemental composition of a sample. The sample is illuminated with an X-ray beam and the atoms which are struck by the beam emit X-rays in response, usually at several different energies. Different elements produce different distributions of emitted X-rays so the spectrum of emitted X-rays can be used to identify which elements are present in a sample.
Until quite recently XRF machines were sealed bench-top units which often required that samples be removed from an museum object for analysis. Portable XRF instruments can be taken to the object and used to analyze its surface without removing sample, but at a cost of less precise quantitative measurement.