Rivera carefully planned the area of the fresco he would paint each day because the pigments must be painted on the plaster before the plaster "sets" for the pigment to permanently bond to the plaster. The setting of the plaster limited the area of plaster he could paint. Rivera carefully concealed the boundaries between the giornata by weaving them between major compositional elements, often following the outlines of people, buildings, and machines. Assistants applied the thin wet plaster layer. These graphics are registered to the underlying shape and color layers and can be displayed and inspected as desired by the viewer.

The Giornata Color Key shows the boundary of each day’s work area as a black line. It also uses an orange line to indicate areas of pentimento.

Pentimento

Areas indicated by an orange line represent “pentimento.” Pentimento or “repentance” in Italian indicates a change made by the artist during the creation of the work. On Pan American Unity these areas are easy to find because Rivera added a new plaster layer over the top of the original. These elevated areas are easily distinguished in the Digital Elevation Model (DEM) layer. In fact, examination of the DEM layer directly led the conservators to identify a new pentimento region on the fresco.

Diego Rivera used an experimental technique on the fresco in an attempt to delay the setting of the plaster and extend the time he had to paint. The technique, developed by Ely de Vescovi, used the chemical Butanol, applied with a spray gun directly on the plaster. It was believed that a 50% Butanol 50% water solution could retard the setting of the plaster for up to 18 hours. It is clear from the surface relief of the DEM layer that, while there are many areas where Rivera painted into the plaster when it was soft and malleable, there are areas that show no imprint from the paintbrush at all. This suggests that the plaster setting was well-advanced at the time of painting.

The effect of the technique on the adherence of the pigment to the plaster surface is under investigation.

The special conservation team, sponsored by SFMOMA, provided a condition information map showing issues present in the mural. This layer contains multi-colored graphics with each color indicating a different condition type. The team worked to simplify and prioritize all of the conservation data into a meaningful presentation for the general public. In consultation with CHI, the conservation team decided to show 4 types of condition information: powdering, cracks, flaking, and delamination. Different levels identified to show the degree of powdering and flaking were collapsed into a single representation for each category to simplify the presentation of condition issues.