Color trajectories of natural pigments for model fresco samples under heating

The paper presents a concept of color trajectories as used to characterize fresco pigments and their color changes with increasing temperature. Color trajectories have shapes characteristic of certain colors or individual pigments. For 15 pigments, the minimum temperatures at which changes in their color coordinates become significant are determined. For several samples, heating can significantly change the color coordinates, but the integral color changes can be insignificant. The integral effect under heating of most samples to 740 degrees C is that the a* and b* color coordinates tend to zero and the lightness decreases. The exceptions are volkonskoite, the color of which becomes brighter after heating, and red lead, the lightness of which becomes greater due to the transformation of orange into light yellow. The results of this work can be used to construct an evolutionary line of real images under an increase in temperature.