This phenomenon can be explained by metal excess defect due to anionic vacancies. It is a type of non-stoichiometric defect and commonly seen in alkali halides. When table salt crystals are heated, it forms an atmosphere of sodium vapour and the Na⁺ atoms are deposited on the surface of the crystal. The Cl^– ions diffuse to the surface of the crystal, combine with Na⁺ atoms and give NaCl. The released electrons diffuse into the crystal and occupy anionic sites. As a result the crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called F-centres (German word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl because of excitation of these electrons when they absorb energy from the visible light falling on the crystals. Hence, NaCl sometimes appears yellow at high temperatures.

Figure 2: Diagrammatic representation of "F-center.” The blue spheres are Cl^- ions and red spheres are Na⁺ ions.