#pigments
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Forbidden and allowed: what symmetry does to a spectrum
Dissolve cobalt chloride in water and the solution is pale pink; add hydrochloric acid and it turns an intense blue — same ion, same kind of transition, a hundredfold jump in intensity. The gap didn't change; the symmetry did. This post pays the pigment series' oldest promissory note and explains what "forbidden" and "allowed" actually mean — one integral, one parity argument, one character table — and why forbidden bands show up anyway.
The colors on the palette are energy-level gaps: engineering pigments for permanence
A tube of paint is an electronic-structure problem plus a scattering problem, perceived by an eye. This post builds the mechanism-first taxonomy of color — conjugated π-systems, ligand-field d–d transitions, charge transfer, and semiconductor band gaps — leading with the modern synthetic pigments engineered to fix the lightfastness failures of their historic ancestors, then puts absorption and scattering back together with Kubelka–Munk.