The soaring tile vaults of Rafael Guastavino and his son, Rafael Jr., grace some of America’s most iconic buildings. Yet for most of the past century, the architectural contributions of these two immigrants have gone unrecognized.

That’s changing thanks to the efforts of a few people—most notably John Ochsendorf, an MIT professor of architecture and civil and environmental engineering. He caught what he calls “Guastavinitis” two decades ago as a Fulbright scholar in Guastavino’s home country of Spain. Since then, Ochsendorf and others have identified more than 600 existing Guastavino projects in 30 states and six countries, as well as many that have been destroyed.

“If there’s one building that launched the career of the Guastavino family in America, it’s the Boston Public Library (pictured above),” says Ochsendorf, author of Guastavino Vaulting: The Art of Structural Tile. “It’s a seminal place. Without the Boston Public Library, we don’t have Guastavino coast to coast.”

When Rafael Guastavino Sr. emigrated from his home country of Spain, where he had begun to achieve success as an architect and master builder, he didn’t speak English. He also lacked connections in New York City, where he arrived in 1881. Yet by 1889 he was collaborating on the Boston Public Library with arguably the most dominant architectural firm of the day: McKim, Mead & White. Guastavino helped the firm achieve its vision for the first large, free municipal library in the United States—a monumental building with soaring, vaulted spaces that architect Charles McKim called a “palace for the people.”

Guastavino’s patented system was based on the principles of the tile vault, a Mediterranean technique dating to 1382 that uses thin clay tiles and plaster. His self-supporting tile arches were simultaneously lightweight, strong, and attractive—as well as fireproof, something McKim had demanded.

The University of Pennsylvania’s Penn Museum in Philadelphia contains a trove of Guastavino vaults. Moving from the museum’s original wing, with its conventional Neoclassical interior, into the first of the Guastavino sections—the Harrison Wing, completed in 1915—visitors enter a great rotunda housing a collection of Chinese art. “The Guastavino space is defined by revealed brick with almost no ornamentation,” says David Brownlee, professor of art history at the University of Pennsylvania, “and you have the feeling that you’ve suddenly come into the 20th century—that you’ve entered a modern world of large, simple forms and very constrained decoration.”

But that’s just the beginning. Beneath that space is an extraordinarily shallow-domed auditorium with trapezoidal coffered vaulting and Arts and Crafts tile from the Rookwood Pottery Company in Ohio. The Coxe Wing, opened in 1926, contains two more Guastavino vaults, one supporting the other. Even more austere, this wing serves as a solemn setting for the museum’s Egyptian art.

“For my money,” Brownlee says, “that pair of spaces together represent some of the greatest Guastavino ever.”

In 1908, as he was completing work on the Basilica of Saint Lawrence in Asheville, North Carolina, Rafael Sr. died. He was 65 years old and had a grand house in nearby Black Mountain, which he built after collaborating with architect Richard Morris Hunt on Biltmore House.

“The Basilica of Saint Lawrence is the only complete building in the country that Guastavino designed and supervised,” says Jane Rogers Vann, professor emerita of Union Presbyterian Seminary in Charlotte, North Carolina, who gives talks on Guastavino and the Asheville basilica. “Unlike other Guastavino buildings, the Spanish baroque design of the basilica reflects Guastavino’s own heritage.”

And what a design it was. Modeled after the 17th-century Basílica de la Virgen de los Desamparados in Guastavino’s native Valencia, the rectangular Catholic church is covered by a massive elliptical dome. Floors, ceiling, and stairs—from the foundation to the top of the twin towers—are built from tile vaulting. Guastavino donated all the tile, including decorative pieces brought from Spain, and paid for nearly half the total cost of the construction. He is buried in a ceramic tomb in the basilica’s chapel.

Like many historic churches, the Basilica of Saint Lawrence suffers from deferred maintenance issues. “It’s a local treasure that I would argue has national and international significance,” Ochsendorf says. “It’s the father’s crowning masterpiece.”

All Guastavino projects built after 1908, including some of the most iconic, were the work of Guastavino’s son, Rafael Jr., who was 8 or 9 years old when he immigrated to the United States. These include the towering Nebraska State Capitol in Lincoln, a revolutionary structure designed by architect Bertram Grosvenor Goodhue and built between 1922 and 1932. Rafael Jr. worked with mosaicist Hildreth Meière, another unsung hero of American design, to create a multicolor masterpiece of ceiling art integrated into the Guastavino structural tile system. Guastavino custom-fired tile to match Meière’s designs, including the vestibule dome’s Gifts of Nature to Man on the Plains, which features a central sun surrounded by depictions of four seasons of agricultural products. Making tile to fit the dome’s three-dimensional curves in a pre-computer age is a feat difficult to fathom.

Guastavino vaults connect to the immigrant experience at Cleveland’s West Side Market, a massive brick building completed in 1912 with an interior concourse providing room for 100 stalls. “The West Side Market is the Ellis Island of food,” says Kathleen H. Crowther, longtime president of the Cleveland Restoration Society. Not only do the food vendors reflect Cleveland’s international diversity, but the building’s soaring ceiling resembles the vaulted ceiling Rafael Jr. built for the Ellis Island Registry Room in New York. The West Side Market is still fulfilling its original function more than 100 years after it opened.

The Guastavino company’s work also appears in West Coast locations, including the Hearst Memorial Mining Building at the University of California, Berkeley, designed in the Beaux-Arts style by architect John Galen Howard and completed in 1907. Inside the golden oak doors, glass-capped domes tower more than 40 feet above the floor. The domes rest on steel arches and pendentives—the curved, tiled triangles of vaulting formed by the intersection of a dome with its supporting arches. The exposed beige tiles helped Howard achieve the brusque, industrial aesthetic he felt matched the character of a college of mining and engineering. “The profession of mining has to do with the very body and bone of the earth; its process is a ruthless assault upon the bowels of the world, a contest with the crudest and most rudimentary forces,” he wrote. “There is about it something essentially elementary, something primordial; and its expression in architecture must, to be true, have something of the rude, the Cyclopean.”

Kykuit, the 1913 Rockefeller property in Tarrytown, New York, that is also a National Trust Historic Site, contains its very own Guastavino ceilings. The vaults appear in two of the ground-floor art galleries, as well as the Grotto (shown), a fantastical circular space with sandstone walls, frosted-glass icicle-shaped lights, and floor tiles by Moravian Pottery and Tile Works. The rooms display art by the likes of Andy Warhol, Lee Bontecou, and Giacomo Balla.