FEW ART collectors are as liquid as Patrick Drahi, a French telecoms magnate, who purchased Sotheby’s, an auction house, for $3.7bn in cash last month. Selling art can take months, even years. The only way to unlock its value quickly is to borrow against it. And indeed the number of owners doing so is rising. Deloitte, an accounting firm, estimates that the outstanding value of loans against art in America reached $17bn-20bn in 2017, up 13% from the previous year. Industry insiders say such lending has continued to grow at double-digit rates since then.