Trade secrets

By Kirstin Ohrt

With an L-square, paper and pencil, archaeologist Samuel Holzman uncovered a 2,000-year-old trade secret held by the ancient builders on the northern Greek island of Samothrace. The finding, that the “flat arch” building technique was used in the island’s covered walkway, or stoa, alters the timeline of Greek and Roman architectural history.

Flat arches, Holzman discovered, appeared on Samothrace 150 years prior to what had previously been considered its debut in Rome. His discovery has architectural historians taking out their own L-squares and revisiting the drawing board on other ancient structures. “This new finding prompts the suspicion that more examples are also hiding in plain sight in other already excavated stoas, with the unexpected going overlooked,” said Holzman, assistant professor of art and archaeology and the Stanley J. Seeger ’52 Center for Hellenic Studies.

A wellspring of rivalry and innovation

Samothrace juts out of the northern Aegean Sea to a mile-high peak that scrapes moisture from the clouds and funnels it through waterfalls to crystalline pools around the island. Perhaps it’s the dramatic and uniquely verdant landscape that creates the special aura, described in writings by the ancient Greeks, that still draws visitors from around the globe today.

Since at least the seventh century B.C.E., Samothrace was the home of the “Sanctuary of the Great Gods,” a site for initiation into one of the ancient world’s most renowned mystery cults. Plato and Aristophanes mention it, and the fifth-century B.C.E. historian Herodotus was himself an initiate. Philip II of Macedon and Olympias met during their initiations here, connecting the site to their son, Alexander the Great.

The most prominent excavated structures date to the third and fourth centuries B.C.E. and reflect a panoply of architectural paragons erected by Alexander’s successors, each vying to outdo the other. This competitive spirit created an innovation incubator of sorts, brimming with architectural ingenuity: Samothrace boasts the largest rotunda of the Greek world, oldest known façade of freestanding Corinthian columns, first use of tapered beams, and Holzman’s latest discovery, the first known use of the flat relieving arch. The island is also the original home of the sublime Winged Victory of Samothrace statue, representing the goddess Nike, now exhibited at the Louvre in Paris.

“It was only in looking at the blocks all at once like jigsaw puzzle pieces that the pattern emerged.”

—Samuel Holzman

Of scholarly interest since 1444, Samothrace had been investigated by French, German, Austrian, Czech and Greek archaeologists before the American-led excavations began in 1938. The Institute of Fine Arts of New York University (NYU) and Emory University sponsor the American Excavations Samothrace project, which is funded by the National Endowment for the Humanities, National Geographic, and private donors. Since 2021, a grant from the Loeb Classical Library Foundation has supported work on the ancient city wall.

Holzman joined the Princeton University faculty in 2021 and began bringing students on the excavation. The team works under the auspices of the American School of Classical Studies at Athens and the supervision of the Greek Archaeological Service for the Evros region.

It’s all about the gap

The site of Holzman’s flat arch discovery is the Sanctuary’s largest building, the 104-meter-long stoa; “it just goes on for days,” Holzman said. Its rough outline was determined in 1866 by a French expedition, and NYU archaeologists revealed the building’s full plan in 1967 after a five-year excavation. Despite investigations in the decades since, the method by which ancient Greek builders had managed to construct this massive Doric structure from sub-optimal local limestone remained elusive.

The answer had been hiding among the blocks, waiting for a captive audience to detect it. “It was during the summer of 2020, when the COVID pandemic kept us from going into the field, that I sat down at home and tried to get to the bottom of the problem,” Holzman said. “Using our field measurements, I sketched each of the blocks schematically so that I could try fitting them together in different ways. It was only in looking at the blocks all at once like jigsaw puzzle pieces that the pattern emerged.” Holzman had to contain his excitement at solving the problem until 2021 when he could return to the site and recheck all the measurements. “That’s when everything clicked into place,” he said.

Holzman had pieced together evidence that the stoa’s classic Doric frieze — the decorative band above the building’s columns— concealed flat arches as a fail-safe against cracks in the beams below. Trapezoidal blocks in the shape of the keystone of an arch distributed the weight of the roof onto the columns. This gave the builders greater confidence that the limestone would not crack and unlocked a trade secret later used by the Romans to build larger and lighter structures.

The wedge-shaped blocks making up the flat arch had such subtle inclinations, just approximately three degrees, that it was easy to overlook their shape, until now. Placing an L-square on a stoa block, however, shows the critical difference: “You have this gap here,” Holzman said.

Holzman and his team meticulously re-measured many of the stoa’s 1,700 surviving blocks and found that the keystone blocks had space for a gap below, and each bore a cutting on top for lifting it with pulleys. Holzman deduced that blocks were placed in consecutive order in a continuous direction with keystones lifted into place. This allowed builders to maintain precise vertical control over the keystone while manipulating other blocks. The team published their findings in the September 2023 issue of the Journal of the Society of Architectural Historians.

With size as its wow factor, the stoa’s construction could not make use of the prohibitively expensive white marble imported from Athens and the islands of Thasos and Proconnesos (the latter is now called Marama Island) and employed in other Samothracian buildings. “To get such a big building, they had to use local limestone. And the local stone had limitations,” Holzman explained, noting its copious cavities and geological irregularities. Builders stretched poor-quality building material to cover maximum ground and summoned the flat arch technique to make it possible. In Holzman’s words, “They’ve innovated to do more with less.”

Holzman’s discovery is even more astounding given that the evidence was intentionally obscured by the ancient Greeks. “They liked the technology of the arch and they wanted to use it,” explained Holzman, “but god forbid you see it!” They veiled the flat arch blocks with a traditional Doric triglyph and metope frieze pattern, allowing triglyphs to slightly overlap metopes to conceal the angled joints. Rather than showcase their innovation, builders prioritized impressing their patrons with a classic-looking structure pushed to awe-inspiring proportions.

In their commitment to the illusion, Greeks used especially slight angles despite the increased risk of blocks slipping out of place. Later examples one can see around Rome diminish that risk and lay bare the technique, showing the zigzagging effect of angled blocks leaning against one another; to use Holzman’s description, for the Romans, “there’s no shyness about having cattywampus joints on the façade.”

Though technology plays an important role in much of Holzman’s research and teaching, it was decidedly absent from his investigative methods in this discovery. “I draw,” said Holzman, “and drawing is an essential skill for this kind of research. It’s hard to understand what a ruined building originally looked like unless you make a drawing of it. Our team includes architects, surveyors and digital modeling specialists, but I still draw many architectural finds by hand with pencil and paper. Drawing forces you to slow down, look very carefully, and think through what you are seeing.”

For Holzman, technology has an invaluable role to play in developing, confirming or illustrating an idea — post-epiphany.

Following breadcrumbs

The stoa’s innovative construction applies a tenet of engineering demonstrated in two other Samothracian structures built in the previous century: that a beam undergoes maximum stress at its midpoint. A different solution was conceived in two other structures, the Hall of Choral Dancers and the Hieron, which both have the columnar façades common to Greek temples but housed gathering spaces for initiation.

Ceiling beams that cross significant spans require maximum strength and size for which Greek builders deciphered a strategy that offered both fortitude and grace. They carved marble beams with a central spine that came to its highest peak at the midpoint, tapering down at the ends.

in 2023, Holzman collaborated with Branko Glišić, professor and chair of civil and environmental engineering, to explore the beam design’s efficiency. Using a static structures analysis, Glišić’s team evaluated the stress distribution and structural performance of the beam, employing both analytical expressions and numerical modeling. Jonathan Gagnon, a Class of 2024 student majoring in civil and environmental engineering, worked on the analytical expressions as part of his junior-year research project, and Professor Antonio Maria D’Altri of the University of Bologna assisted with numerical model verification.

To Glišić’s surprise, ancient builders had managed to optimize the shape of the beam based on the load. “Today’s structures are designed using scientific concepts, such as solid mechanics and beam theory, which enables structurally sound, reliable and safe-optimized design,” Glišić said, “whereas ancient Greeks optimized beams based on engineering intuition, experience and experimentation.” Of the two approaches, Glišić said, “the latter is the one that sparks curiosity, creativity and exploration, and this mixture of playfulness and discipline showed excellent results some 2,000 years before the scientific concepts even started to be developed.”

Glišić rates historical studies on structural engineering as extremely important. Though he had worked previously on heritage structures, his work with Holzman was Glišić’s first collaboration with an archaeologist. Modern engineering has resolved the challenges Greek builders faced, Glišić said, but we continue to learn from their approach. “New technologies certainly help in solving complex problems related to structural analysis and design,” he said, “but it is the understanding of the evolution of the engineering thoughts and experiences that helps build the intuition, creativity and confidence necessary to push the boundaries of the discipline.”

Holzman looks forward to collaborating with Glišić on the new flat arch discovery. He is also reaching across disciplines on another aspect of his research, working with the Parsons Conservation Lab at Emory University and Princeton’s newly formed interdisciplinary Art Conservation and Materials Science Working Group led by Craig Arnold, the Susan Dod Brown Professor of Mechanical and Aerospace Engineering and Vice Dean for Innovation, and Janna Israel, the Andrew W. Mellon Curator of Academic Engagement, Art Museum, to harness isotopic signatures to track the lead-sheathed iron staples that held the stone blocks of the stoa together.

Other nuts to crack

As long as there is a question, the Samothrace excavation team will dig for its answer. In the summer of 2023, they sought the gate in the fortification wall that allowed passage between the Sanctuary and the ancient town. Investigating three possible locations simultaneously, the team has zeroed in on a passageway they have dubbed the West Gate: “We got it — and it’s exciting!” Holzman declared.

Princeton alumna Hannah Smagh, who earned her Ph.D. in art and archaeology in 2023, along with Princeton graduate students Chiara Battisti, Eirini Spyropoulou and Robert Yancey, as well as undergraduate student Elena Evnin, Class of 2024, all engaged in this endeavor in 2023. Their efforts were supported by summer fieldwork grants from the Seeger Center for Hellenic Studies. Spyropoulou worked on the location once mistaken for the gate, which is now understood to have been a tower. Paralleling the important work on the stoa, she measured and catalogued blocks associated with this structure. The next step will be to draw the stones and try to piece them together, one of her favorite challenges.

Spyropoulou values the first-hand experience and intellectual exchanges with her fellow students, but above all, she is grateful for the insight she has gained from Holzman, her adviser. “The way he reconstructs things, it’s amazing!” she exclaimed. “He draws like Leonardo da Vinci.”

Holzman credits his first mentor, Tasos Tanoulas, the architect who oversaw the restoration of the Propylaea, or ceremonial gateway, on the Acropolis, with shaping his approach to archaeology. “As a college student, Tasos opened my eyes to how much there still is to discover,” said Holzman, “and he was one of the first people I told about our flat arch find. I had to get his read on it.”

When asked whether he thinks more architectural secrets may lie hidden on Samothrace, he responded: “Of course! We cracked this nut, but there’s a list of other tough ones that we’re still chipping away at.”

That said, next summer’s tasks are well defined. “We’re scratching the surface of something remarkable at the West Gate, and it will take several summers to uncover it completely,” he said, “and Princeton students who join our excavation in summer 2024 will be instrumental in getting to the bottom of it.”