Diego Rivera: Frozen Assets, 1931-1932, fresco on reinforced cement in galvanized-steel framework, 94 and 1/8 by 74 and 1/4 inches. Museo Dolores Olmedo, Xochimilco, Mexico. 

PART OF THE POWER of fresco as a medium lies in its fusion of two distinct temporalities: the time of the artist—the rhythm of his workday as he moves across the wall—and the longue durée of the artwork after it has dried into architectural permanence. In the construction of a traditional fresco, wet plaster is laid down, pigments are applied, and, as the wall dries, a chemical reaction inseparably binds plaster and pigments so that the picture becomes part of the wall, not merely a thing upon it. Ironically, producing this singularly durable art form requires a race against the clock: since the plaster can be painted only while it's damp, muralists typically apply as much plaster as they can paint in a day, then hurry to complete that section of a fresco before it hardens into history. One can often calculate the number of sessions it took to paint a fresco by looking at the areas where two separate lengths of plaster have been joined; a length is known as a giornata, or "day's work."

Perhaps on some level the Mexican muralists (the major modern artists to use fresco) were attracted by how the medium itself can serve as a metaphor for historical process, for the integration of individual labor—a long string of giornate—into the collective, the intergenerational. There is a compelling, constitutive tension between the dialectic that fresco is and the dialectical history that painters like Diego Rivera often used it to depict, a kind of populist medium specificity.

I find no such tension, however, no dance of durations, internal to the "portable murals" Rivera constructed for an exhibition that ran from Dec. 23, 1931, to Jan. 27, 1932, at the Museum of Modern Art in New York—the museum's second show dedicated to a single artist. (Matisse was the subject of the first.) Five of Rivera's eight murals are currently on view at MoMA, reassembled for the first time in 80 years from the museum's and other public and private collections. In order to solve the problem of how to present an artist best known for his large-scale, immovable works, in 1931 MoMA provided Rivera with studio space within the museum, where he produced freestanding frescoes built on rigidly braced steel frames into which cement had been poured to form wall-like supports. The first five murals, completed in time for the opening, portray Mexican historical and revolutionary subjects. With the exception of The Uprising, which depicts protesting workers clashing with soldiers, they are all in fact details—or near-details—of Rivera's Mexican mural cycles.

After the show opened, Rivera added three more panels addressing Depression-era New York: laborers operating a pneumatic drill (the whereabouts of Pneumatic Drilling are now unknown); Electric Power, in which three laborers are seen through the cross section of a riverside power plant; and Frozen Assets, which seeks to depict the stratification of urban American life. Powerful skyscrapers loom at the top of the last composition; beneath them commuters are crammed into an elevated train. In the middle we see into a glass and steel shelter where rows of the homeless and unemployed are sleeping under the surveillance of a guard. At the bottom of the panel we find an underground bank vault in which a wealthy woman admires a gold necklace. In the room outside the vault three figures—one of whom resembles John D. Rockefeller, Jr.—await their turn to inspect their deposits.

As much reproductions as new works, the Mexico-themed frescoes are free (or doomed) to circulate like photographs of the originals. In an insightful catalogue essay, curator Leah Dickerman suggests that the artist was influenced by the widely viewed photos of his own work in architectural settings and used the metal frames of the portable frescoes to imply photographic cropping. She also contends that Rivera's arrangement of these details from various murals into a new syntax in the museum space owes something to the theory of montage developed by Rivera's friend Sergei Eisenstein; instead of a linear narrative embedded in a building, we now have quick jump cuts across scenes. This is an inspired curatorial strategy for recuperating the portable murals into a narrative of leftist experiment, and might well have been in Rivera's mind, but I can't reconcile it with the work: it's hard to think of these panels as forming enough of a sequence in the first place for transitions between them to be experienced as cuts. Our desire to integrate the frescoes into some horizontal relation is more notable than any particular strategy for doing so; it indicates how the medium of fresco feels unmotivated when rendered isolate, portable, no longer indistinguishable from the wall. We want these to be something other, something more, than really heavy paintings.

And they are something more—if only because they're metonyms for a series of interlocking contradictions that remain utterly contemporary: between the museum or gallery form, with its circulating and potentially salable pictures, and the site-specificity of the mural; between the kind of viewer posited by a public mural—which is not to say all of Rivera's in situ works were available to the general public—and the kind of viewer assumed by a private museum; between originality and reproducibility; between artworks critical of capitalism and an art world adept at co-opting those criticisms-the list goes on.

The story is far from as simple as a radical Rivera selling out to a Rockefeller-funded neutralization of his murals; after all, the Cuernavaca "originals" on which two of the panels are based were commissioned by the American diplomat and former J.P. Morgan partner Dwight Morrow as a gift to the state of Morelos. And the supposedly critical Frozen Assets panel was given a generous, full-color treatment in Fortune with the caption: "The industrial civilization of New York seen in the cross section of a Rivera fresco." In the context of a pan-American narrative of progress, one could insult capitalist leaders and acknowledge the suffering of the poor so long as one also posited a future beyond the Depression and celebrated the sheer force of U.S. industry. There were limits (painting Lenin's face at Rockefeller Center), but, as this show makes usefully clear, the relationship between political art and capitalism was never one of simple opposition.

The most interesting and unsettling aspect of seeing the portable murals now isn't mentioned in the exhibition materials: MoMA is revisiting its Depression-era project during the worst crisis of capitalism since the '30s. This time there is no massive industrial mobilization to temporarily suspend or occlude of Mexican liberation, return to a New York largely powered by an underclass of Spanish-speaking laborers locked in contemporary peonage. Truly portable murals—Rivera's weigh up to 1,000 pounds—might be just the thing for Zuccotti Park or wherever a public space is next declared through occupation. There's something compelling in the notion of murals awaiting their buildings—scattered fragments of an as yet unestablished public institution roaming the earth, a muralistic messianism. All of this to say: I can't look at these frescoes without entertaining a fantasy that puts them back in relation to architectural time and social space.

Ultimately, what's missing from Rivera's portable murals is a sense of the missing: there isn't enough of a negative charge, enough felt absence of the site and its bid for fixity, duration, collectivity. To me the panels feel fatally complete, not evocatively cropped, more like enlarged versions of Rivera's easel works than dramatically fragmented versions of his large-scale frescoes. Their failure, the impossible bargain they attempt to strike between incompatible modes, is precisely what makes the portable murals relevant now, during our current crisis of value, when later 20th-century narratives proclaiming, political antagonism, and these images, primarily in one way or another, the end of history, can feel more distant than the uneasy experiments of 1931.