NEW YORK, N.Y. - The domes and arched ceilings — each arrayed with tiles in herringbone and basket-weave patterns — are hidden in plain sight. Millions of people walk under them every day at the Oyster Bar at Grand Central Terminal, New York's St. John the Divine, the Boston Public Library, the San Diego Museum of Man and hundreds of other places.

But Rafael Guastavino and his son Rafael Jr., the makers of these sweeping tiled vaults that provide both decoration and structure, have mostly been forgotten, in part because credit went to the architects who commissioned them.

A new exhibition at the Museum of the City of New York tells the story of the master masons who, between the late 1800s and the time their company closed in 1962, created vaults, domes and other tile work in 1,000 buildings in 42 states.

"Palaces for the People: Guastavino and the Art of Structural Tile" focuses on their 250 commissions in New York — the most of any other city where their work is found.

The Guastavinos arrived in New York from Barcelona in the late 19th century and patented a thin-tile vaulting technique based on a 500-year-old Spanish building method that was lightweight, extremely strong, self-supporting and fireproof. Fires were a great concern in the late 19th century as many buildings were made of wood.

Their method of interlocking and layering thin clay tiles with quick-drying mortar in decorative patterns revolutionized architectural design. It created grand palatial interiors, such as the delightful Elephant Room at the Bronx Zoo and New York's City Hall subway station, an ornate subterranean cathedral of glazed green, ivory and brown tiles now shuttered and only viewable through the window of a passing train.

Instead of using heavy stone, they used a particularly thin ceramic tile that is similar to brick that can be glazed in different colours, said John Ochsendorf, the exhibition's co-curator and a professor of architecture at MIT and author of "Guastavino Vaulting: The Art of Structural Tile."

"I think of them as Gothic master masons in the great Gothic tradition where it took a century to build a cathedral," except they worked on 100 buildings simultaneously since the light tiles allowed them to work at a quick pace, Ochsendorf said.

"At the simplest level they're builders. But they're also acting as architects, as engineers and interior decorators choosing patterns of tiles, colour and schemes," added Ochsendorf. "The vault is ... their great contribution to American architecture. It is a fusion of art and technology that engineers still struggle to understand."

A replica in the show built by some of Ochsendorf's students "is our best guess at how they would have built a vault like this."

Kevin Faerkin, general manager of the famous Oyster Bar, called it "an inspiring, an incredible environment to work in." He said the first thing a lot of customers say even before they sit down "is how great the space is. ... They are struck by the grandeur of the ceiling."

The Guastavinos' big break — the one that launched their careers and allowed them to start their own company — came in 1889 when the architectural firm McKim, Mead and White hired them to build a set of vaults above the main entrance of Boston Public Library.

Today, 600 Guastavino-adorned buildings in 36 states still stand, magnificent structures such as the Ellis Island Registry Hall, the Boathouse in Brooklyn's Prospect Park, Riverside Church and the Washington National Cathedral in D.C.

Ochsendorf is convinced there are other Guastavino projects waiting to be discovered. To that end, the museum has created a crowdsourcing site — http://palacesforthepeople.com/ — to help uncover them.

News from © The Associated Press, 2014