5 BLUE PIGMENTS MIXED ON THE PALETTE BEFORE SPREADING IN A SINGLE LAYER OF PAINT

The presence of both azurite and smalt as a mixture in a single layer was determined by different methods. Chemical microscopy carried out on the cross section, using the tests described previously for smalt and for azurite, did not produce reliable results. SEM elemental analysis allowed the identification of only the larger crystals of each pigment, and the results were the same obtained for the individual components as described earlier. Polarized light microscopy of the sample mounted in Canada balsam resulted in an extremely useful technique for identifying the particles of each pigment. Between crossed polars under the microscope, the smalt particles appeared dark and quite different from the blue azurite particles.

Pacheco ([1649] 1866, 107) says of this mixture:

I am of the opinion that the blues should be worked up light, and the darker ones should be pure blue; at most, the darkest should be cut not with black but with purple, not with a�il, but with a bit of good smalt, thin and of a good color, mixed with the pure blue, which takes to it well.