For large parties, the company would park a truck with a Portovault in front of a client’s building and clear out the carpets and furniture for the night.

The company also had a seasonal business delivering to and from the grand summer mansions of Newport, R.I. Ms. Young remembers her father taking a team of men to that resort for weeks at a time in the early 1960s. Trucks loaded with Portovault units would travel back and forth between the summer “cottages” and the warehouse in New York.

OVER the years, however, more and more of those trips to Newport were intended to permanently empty houses that had been sold or broken up. A culture was disappearing. The great Newport summer mansion had become an extravagance that even the wealthiest families could no longer afford, or at least didn’t care to maintain.

Families that once had four or five sets of china and silver put them in storage or got rid of them. “People don’t want to take care of it anymore, or they don’t want to have people take care of it for them,” Ms. Young said. “I can’t think of anyone who uses silver these days. Maybe on Thanksgiving.”

The company emptied out its last grand house in 1996, a Georgian manor in the New Jersey woods with paintings by Cezanne, Picasso and Monet, and a menagerie of Meissen porcelain animals. “The house had been sold and they were going to divide the property up among the children,” Ms. Young said. “That was the end. I had heard about these things. I haven’t seen one since.”

If one segment of the company’s business was contracting, another had been growing, and for years. In 1962, The New York Herald Tribune reported that in “New York’s big warehouses, family furniture is losing space to works of art.” The previous year, Day & Meyer had stored a Rembrandt, “Aristotle Contemplating the Bust of Homer,” auctioned to the Metropolitan Museum of Art for $2.3 million, then thought to be a record price for a painting.

Today, art galleries, auction houses and interior decorators are the better part of the firm’s clients. The paintings vault has been expanded — at the expense of space for silver — to accommodate 400 canvases, and the private viewing room on the third floor, where clients can show works to prospective buyers, has been equipped with gallery lighting.