Its surreal: zinc-oxide degradation and misperceptions in Salvador Dalí’s Couple with Clouds in their Heads, 1936

The painting Couple with Clouds in their Heads by Salvador Dalí (1936) from the Museum Boijmans van Beuningen in Rotterdam was seriously disfigured by ring formations on the edges of the painting and a matt, white-looking semi-transparent material on the surface. Paint samples, taken from affected and unaffected paint areas, investigated with light, scanning electron and ATR-FTIR microscopy and X-ray diffraction showed that the zinc white-containing oil paint was converted into zinc soaps, zinc formate dihydrate, zinc acetate dihydrate and zinc sulfide. The conversion of the zinc white started at the surface of the painting. Zinc soaps are formed by a reaction of zinc white and fatty acids from the oil. Formate and acetate, most likely emitted from the wooden frame, reacted with the zinc white pigment. Finally, the zinc sulfide is suggested to derive from zinc white reacting with hydrogen sulfide.

The high degree of degradation of the zinc white-containing paint and the ring formation is postulated to be triggered by the exposures of the panels to high temperatures which occurred during a 1-day photo shoot in 1936 shortly after completion of the paintings by Dali. Furthermore, it seems likely that the glazed frame created a (semi-)closed system and subsequently acid gasses diffused into the system via "leakages" due to a poor match between the frame and the panel, and may have initiated a reaction with the surface of the painting. As the glazed and framed panels were a non-ventilated system, the acid gasses did not distribute evenly over the surface, but rather were most intense at the edges of the painting. The ring formation caused by the gasses is compared with the phenomena of Liesegang rings, a reaction-diffusion process.