Ashing is the process of removing all the organic material from a sample though heating in air. The proportional mass of residual ash may be the objective of the test, or may be an intermediate step in the preparation of the sample for other purposes such as characterisation of the inorganic components through XRF (X-ray fluorescence), or mass spectroscopy or for tests such as ash fusibility.

Ashing tests may also be defined within standard test methods typical examples of which are ISO 1171:2010, ASTM D3174-04(2010)