Marco Grassi, a connoisseur of old masters who brought a restorer’s hand to art criticism and a critic’s eye to restoration, died on March 30 in Manhattan. He was 90.

His death, in a hospital, was confirmed by his wife, Cristina, who is an artist.

As a third-generation scion of Florentine art restorers and dealers, Mr. Grassi, a rare freelance restorer, was the product of a world that was itself rarefied: the art of late medieval and early Renaissance Tuscany, and those who lived among it.

Like his grandfather, he had trained as a conservator at the centuries-old Uffizi Gallery in Florence. Moving between Europe and America, he was as comfortable in the gilded salons of the world’s richest private collectors as he was at his workbench.

In his sunlit restoration studio, first in downtown Manhattan, then on the Upper East Side, Mr. Grassi was in intimate touch with the paintings of long-dead masters, including Ugolino da Siena, Luca di Tommè, Giovanni di Paolo, Domenico Beccafumi and others. They gave him sustenance. He, in turn, gave them renewed life.

Mr. Grassi, who often wore a tweed suit underneath his blue apron, moved in “that murky backstage frequented by scholars, technicians and craftsmen where the pulleys, gears, curtains and props of the art world are manipulated,” he wrote in “In the Kitchen of Art: Selected Essays and Criticism, 2003–20” (2021), an anthology of his writings for The New Criterion magazine.

Mr. Grassi’s book, published in 2021, is an anthology of his writings for The New Criterion magazine.Credit…Criterion Books