Light sneaked into the derelict former powerhouse on the Gowanus Canal in Brooklyn through broken windows and time-torn holes in the roof as Joshua Rechnitz inspected layers of graffiti, squatters’ abandoned tents and moldering bikes made into an impromptu sculpture.

It was an overcast day in early May, just weeks after the intensely private Mr. Rechnitz burst onto the crowded stage of big-money New York philanthropy with a $40 million pledge to build a field house centered on a bicycling velodrome in Brooklyn Bridge Park.

He was about to lay down millions more.

Randy Polumbo, an artist and friend, had persuaded Mr. Rechnitz to tour the Gowanus space, known by locals as the Batcave, thinking that it would be a good home for Mr. Rechnitz’s growing art collection and for artists’ studios. A representative of the owner and a real estate broker had come along, Mr. Polumbo recalled in a recent interview.

Mr. Rechnitz asked how many studio spaces could be created in the building if his art were not shown there, Mr. Polumbo said. Perhaps 50 or 60, he remembered saying.