A $177 million restoration of St. Patrick’s Cathedral in New York City—its first major overhaul in more than 70 years—was completed in October of last year. Designed by James Renwick Jr. and opened to the public in 1879, the landmark Gothic Revival building was in serious disrepair, with disintegrating interior plaster and falling chips of exterior stone.

The suburban New York quarry where the or­iginal buff-colored Tuckahoe marble was sourced had long since closed, but large pieces of the same type of marble were found in a Westchester, New York, backyard and used for repairs. Building Conservation Associates analyzed the original deep-set mortar and devised a new mortar recipe that matched the stone. “Renwick was looking for a monolithic look, almost as if he had used a sandcastle mold,” says Jeffrey Murphy of Murphy Burnham & Buttrick Architects. “Once we put back the right mortar, the building looked more like that.”

The interior vaulted plaster ceiling was returned to a lighter color palette based on paint forensics and archival research. This feature—along with the cleaning, repair, and UV-protective glazing of 3,200 stained glass panels—transformed the space. “Renwick was fascinated with the lightness of Gothic architecture,” Murphy says. “[That] lightness and the light coming in from above had a spiritual component that had been lost.”