I've been reading a great book about the sources and history of colors. One of the first 'gems' is about red ochre, or red iron oxide - a paint used in frescos.
" . . . scientists in Italy have found a new technique for dating frescos almost to the year they were painted, simply by examining the red paint. 'Red ochre contains iron, and the iron molecules act like compass needles,' explained Professor Giacomo Chiari of the Dept. of Minerological and Petrological Sciences at the University of Turin. He said that in the few minutes between daubing red ochre onto the wet clay, and the time it dries, the molecules realign themselves in the direction of magnetic north. 'And if you don't move the walls then that is how they stay.' Professor Chiari said. Magnetic north changes every year - it can fluctuate over a range of 18 degrees, so you can learn when the fresco was painted from the direction in which the red ochre is pointing."
- from Color; A Natural History of the Palette, by Victoria Finlay
More info about color to come.
xoxo
KP