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Two batches of analytical grade ZnO powder (ZnON) are heated to 920 °C and one, (ZnOA), quenched to room temperature, and the other, (ZnOB), cooled slowly. Samples of both ZnOA and ZnOB are afterwards heated for one hour at temperatures between 100 and 800 °C and quenched. From all these samples the amount of free zinc is determined and the reflection and luminescence spectra measured. The reflection spectra with ZnON as blank indicate the presence of a supplementary absorption band having an intensity which depends upon the previous treatment. From this variation and correlation with the amount of free zinc the yellow colour of the samples can be explained and new models for the centres deduced.

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