In response to Joshua Cohen's "Thirty-Six Shades of Prussian Blue," some readers have wondered about the difference between synthetic colors and artificial ones. (Mauve was "the first aniline dyestuff prepared on a large scale"; that is, the first industrial, mass-produced color.) Clarification from the author follows:

Chemist: Though saying that Prussian Blue is the world's first synthetic color is correct, I should point out—and, of course, I'm no artist, just an interested man of science—that all colors nowadays are synthetic.

Egyptologist: Whatever do you mean, Doctor Farbstoff?

Chemist: I mean that all paint bought by painters today—very few painters manufacture their own paint—is artificial. Pigments that once were made from natural materials are today produced through the wonders of chemistry, Doctor Schaufel.

Egyptologist: How fascinating! My own issue with Joshua Cohen's essay is his utter disregard for the color called Egyptian Blue.

Chemist: Would you mean calcium copper silicate?

Egyptologist: Indeed, I would. Egyptian Blue predates Prussian Blue by thousands of years. It was used to decorate monumental sculpture, especially during the New Kingdom period (1570-1070 BCE).

Chemist: But why would Cohen leave that out of his essay?

Amen-Ra, AKA the Sun God: Forgive my intrusion, gentlemen. I believe Cohen consciously omitted mention of Egyptian Blue because, as a color, it wasn't stable. Due to the presence of copper, the "blue" veered wildly in hue, from a dark Prussian to a bright green. While certainly a synthetic color—painters could and did make it—Egyptian Blue's constituent chemistry is identical to that of the mineral cuprorivaite.

Chemist & Egyptologist: Meaning...?

Ra: Meaning even the gods can't tell whether the paint used in Egyptian artwork was made or found. Or, to be precise, we can't always tell.