Mr. Beard said that Bank of America has about $6.5 billion of art-secured loans on its books and that clients have used much of this liquidity to capitalize their businesses.

Last year in the United States, the art-secured lending market grew 13.3 percent to an estimated $17 billion to $20 billion, according to the 2017 Deloitte Art & Finance Report. The market is dominated by the major investment banks, which charge lower interest rates for their loans than specialist lenders that securitize loans solely against art.

And then there is the money that savvy financier-collectors can make out of auction guarantees. Mr. Beard said that more than 10 of his wealthy clients routinely guarantee works at Sotheby’s, Christie’s and Phillips. They take the risk of being the only bidder and owning the artwork in return for a fee or a percentage of the “overage” if the bidding exceeds an agreed-upon price.

“They’ll guarantee three paintings in a season they’d like to buy and will be happy to own one of them if the overage on the others gives them a good discount,” Mr. Beard said.

At the latest biannual season of Impressionist, modern and contemporary art auctions in New York, Sotheby’s, Christie’s and Phillips took in about $2 billion, a 25 percent increase over the equivalent sales the previous May. Most of the more expensive lots had been certain to find buyers courtesy of these opaque arrangements with third-party guarantors, which blur the boundaries between private and public sales. Among the works sold were paintings by Amedeo Modigliani and Andy Warhol that fetched $157.2 million and $37 million, respectively.