[4][5][6][7] He was part of a multi-decade, 20th-century shift in American ceramics during which artists challenged clay's identification with function and craft, engaging fine-art domains such as emotional expression, social commentary, figuration and narrative.

[9][1] He drew on a wide range of art historical sources, from Tuscan Romanesque and Manueline Portuguese gothic architecture to humanistic artists such as Pieter Bruegel and Honoré Daumier to analytic cubism.

[28][12] Their dominant forms—influenced by Romanesque stone columns and pilasters—departed from the symmetrical, neck-shoulder-belly structure of conventional vessels, using exaggerated proportions and twisting, undulating stances that captured human movement in ways writers described as eerie, humorous and gendered.

[12][25][26] Zimmerman energized the work with rebus-like, rhythmic, carved abstract and geometric shapes, ridges, symbols and curvilinear motifs, as well as primal surfaces showing evident finger marks, scrapes and paddle lines.

[27][4][25][12] New York Times critic Helen Harrison wrote that the sculpture had "a mysterious and commanding presence … like huge ceremonial objects created by some unknown culture [whose] symbolic marks … seem to indicate hidden meanings that only the initiated may decipher.

[9][37] They employed subtle color, thick crawling layers of glaze, and tangles of manipulated clay conveying intimacy, individuality, and vulnerable, organic immediacy in contrast to the raw, herculean and communal qualities of his past work.

[32][22][7] These multi-figure works led to the 12-foot wide, unglazed terracotta piece, Fool's Congress Part 2 (1999), a macabre chorus of semi-abstract vertical forms capped by rudimentary heads and connected by tubular limbs and tendrils; the red clay characters "burning" with hypocrisy and shame were intended to convey senses of pretense, harmlessness and the bizarre.

[9][34][14] Subsequent multi-figure works took on a more representational and mythic quality, depicting small, vulnerable people busy building structures or monuments that seemed precarious or set to drift off into a void (e.g., Hell's Gate, 2002; Fool's Ship on Babel, 2003).

[40] Zimmerman's interests in human labor and ambition were perhaps most fully realized in Inner City (2005–9), a collaborative work involving more than 200 of his sculptures that architect Tiago Montepegado designed as site-specific installations at Museu da Electricidade (2007, Lisbon), Keramiekmuseum Princessehof (2008, The Netherlands), and the RISD Museum (2009).

[35][30][7] The handcrafted, closely observed clay pieces depicted a sprawl of Lilliputian craftspeople, objects (tools, i-beams, ladders), and architectural elements (chimneys, stairs, bridges, buildings, walkways) arranged in Manhattan-like grids and scenarios—accidents, setbacks, miscommunications, labor and recreation—that functioned like film stills or dream images.

[35][7] They likened the work's themes to Balzac's La Comédie Humaine, Fritz Lang's urban dystopia Metropolis, Depression-era Hollywood movies, and to art capturing the "heavy lyricism of human toil" by Bruegel, Bosch, James Ensor and Philip Guston.