While living in Los Angeles in political exile in 1932, Mexican artist David Alfaro Siqueiros painted Amirica Tropical. The mural, 24 meters long and five meters high, is painted on the exterior south-facing wall of the Italian Hall building on Olvera Street, part of the historic monument of El Pueblo de Los Angeles. For this mural, Siqueiros used an experimentalfresco technique in which he painted on fresh cement using an airbrush. Shortly after its execution, the mural was covered with white paint because of its controversial political message. The mural's exposure to harsh environmental conditions, under direct sunlight and heat as well as the painting technique used, resulted in its deterioration. Since the late 1980s, the Getty Conservation Institute has been involved in a project to conserve the only surviving public mural by Siqueiros in the United States. In order to develop an appropriate conservation treatment it was necessary to understand the causes of its deterioration through the study of the original materials and the painting technique used, and the history of the mural.

In this article, historical information and scientific evidences are combined to discuss Siqueiros' innovative technique -fresco on cement - and the mural's deterioration. Contemporary and historical sources provide information on the materials and the technique used in this mural; but give also contradictory evidences regarding the type of paint used. Scientific investigation was carried out on samples of the mural to identify the materials used to paint America Tropical. The pigments were identified using polarized light microscopy and x-ray fluorescence analysis. The identification of the binder was carried out using Fourier transform infra-red spectroscopy, organic elemental analysis and gas chromatography. Scientific investigation, in particular infrared analysis, seems to suggest that cellulose nitrate was the principal component of the binder. The deterioration of the mural is discussed in the light of these results.