The story behind the storytelling west facade of the abbey church of St. Gilles in Gard, Provence, begins in the 600s, when a wealthy Athenian named Giles gave away his family fortune and traveled to Rome to become a priest, and then into southern France as a missionary. When his reputation as a holy man drew too many fans there, he opted for a hermitic life in the forest.

One day Giles -- or maybe his pet deer; the story varies -- was wounded by an arrow shot by the local chieftain, who pledged to atone for the hunting accident by building a church at the site. Giles, who was known for miraculous healings, took in disciples and placed his monastery under Benedictine rule.

The tripartite west facade of the present church, built in the 1100s and holding his tomb, is an illustrated catalog of Christ's last days, interpreted in elaborate, expressive stone carvings for waves of illiterate pilgrims. The triple-arched entrance to Henry Hobson Richardson's Trinity Church in Boston is derived from the St. Gilles facade, long regarded as one of the finest examples of the southern French Romanesque.

And guess what? You don't have to cross an ocean to see it. Housed in Carnegie Museum of Art's Hall of Architecture, a life-size plaster cast of the west facade has been on view for the past 100 years.

Over the century, it has acquired its own patina of antiquity as the backdrop for countless dinners, parties and receptions. Thousands of students have sketched at its feet.

How this monumental work came to be created for the people of Pittsburgh is one of the stories told in "On a Grand Scale: The Hall of Architecture at 100," opening Saturday in the museum's Heinz Architectural Center. Digging into museum archives, exhibit organizer Mattie Schloetzer found photographs, correspondence, invoices and customs and railroad records that helped her piece together how founder Andrew Carnegie and the museum's first director, John Beatty, assembled more than 140 plaster casts for a display designed to bring Western culture to a working-class city.

"The few who travel much fail to remember that the masses of people travel but little," Carnegie wrote. So he would bring the Grand Tour to the masses, with plaster copies of sculptures and architectural fragments from ancient to Renaissance times.

That was what American museums did in their early days, when they were vast, empty and pressed for cash. But as they grew richer and could afford original artworks, Schloetzer said, some museums that had significant cast collections began selling them off. Today the Carnegie Museum collection, almost unchanged from its initial installation, is the largest in the Western hemisphere.

Most of the casts were ordered from catalogs, but the St. Gilles facade is thought to be a one-off unique to Carnegie Museum. Sprawling along an entire wall, it turns the Hall of Architecture into an Italian piazza and gives the cast collection a sense of place that makes it greater than the sum of its plaster parts.

The hall, whose high, colonnaded interior derives from the ancient Mausoleum at Halicarnassus in southern Turkey, was part of Carnegie Museum's first expansion, completed in 1907. Carnegie himself was personally involved in the hall's planning, visiting cast collections at the Metropolitan Museum of Art (now dispersed) and Slater Memorial Museum in Norwich, Conn. (still intact, and which Schloetzer visited during her research).

Carnegie and Beatty also had seen casts of the central portal of St. Gilles and the hall's other large architectural cast, a portal from the 13th-century Gothic cathedral at Bordeaux, at the 1893 World's Columbian Exposition in Chicago. And when they solicited recommendations for the hall from prominent architects, including Daniel Burnham, Cass Gilbert, Stanford White and the museum's own architects, Frank Alden and Alfred Harlow, St. Gilles was the write-in favorite with eight of the 15 who responded -- even though it wasn't on the museum's list, Schloetzer said.

Acquiring it was a complex process, from the negotiations to the casting. The facade, 75 feet long and 38 feet high, was cast in sections and shipped in 200 crates from Marseilles to New York, then by train to the Shadyside Station. Today it is the largest surviving cast in the world and one of the largest ever made.

Schloetzer also found four floor plans showing optional arrangements of the casts. The museum has early photographs of the hall from a scrapbook that Beatty kept.

Schloetzer, who has a master's degree in art history and museum studies from Case Western, is departmental assistant for the Heinz Architectural Center and has worked on several shows there. "On a Grand Scale" is her first major exhibit.

For proof that the casts have served the educational and inspirational purpose Carnegie envisioned, look no further than Schloetzer, who as a high school student in the early 1990s was one of those young people on the floor of the hall, sketching the cast of Nicola Pisano's 13th-century pulpit in the Siena cathedral.

"I'm sure it helped get me interested in studying art history," she said, "more than just studying in the library."