Azurite and malachite have been extensively used as pigments in ancient and medieval manuscripts, glasses and glazes. The thermal stability of naturally occurring azurite and malachite was determined using a combination of controlled rate thermal analysis (CRTA) combined with mass spectrometry and infrared emission spectroscopy. Both azurite and malachite thermally decompose in six overlapping stages but the behavior is different for the two minerals. These stages occur around 282, 328, 350, 369, 384 and 840 degreesC for azurite and 250, 321, 332, 345, 362 and 842 degreesC for malachite. The first two stages are associated with the loss of water, whereas stages 3 and 4 result from the simultaneous loss of water and carbon dioxide. The sixth stage is associated with reduction of cupric oxide to cuprous oxide and finally to copper. Infrared emission spectroscopy shows that dehydroxylation occurs before the loss of carbonate and that the thermal decomposition is complete by 375 degreesC. The implication of this research is that in the preparation of glass or glazes using these two hydroxy-carbonate minerals of copper the samples will decompose at low temperatures and any color formation in the glass is not due to azurite or malachite.