An Encaustic without Wax: Microanalytical Binder Characterization of Diego Rivera's First Mural, Experimental Reproduction, and Identification of Degradation Products.

Diego Rivera (1886-1957) painted his first mural La Creación (Creation, 1922-1923) with an encaustic painting technique, which was traditionally utilized by the ancient Greeks and typically employed beeswax and other resins. Rivera further elaborated that he had innovated a Mexican encaustic technique by incorporating copal, a resin derived from the copal tree. This article presents the novel results of the characterization Diego Rivera's painting medium by nuclear magnetic resonance spectroscopy, gas chromatography-mass spectrometry, Fourier transform infrared spectroscopy, and optical and scanning electron microscopy of microsamples taken from Creation and experimental mock-ups following Rivera's formula. In the first place, reference standards of the main individual components of beeswax, Mexican copal resin, and elemi resin were analyzed to find the main biomarker molecules. The results obtained confirm the presence of selected biomarkers in the mural samples of copal and elemi resins such as α-amyrin, β-amyrin, and lupeol. Regarding the beeswax compounds, unexpectedly no traces were observed in any of the mural samples. Ethyl phthalate appeared in all mural samples, indicating environmental contamination or the possible application of consolidants with a plasticizer during conservation processes. This article also presents the study of zinc lactate as a degradation product.