“Railroads are the Rembrandts of investment,” said Henry Clay Frick, a 19th century US coal baron, because the value of railways, like Rembrandts, always went up. Today’s super-rich put similar faith in art, judging by the dizzying run-up in prices, especially of contemporary works. But Sotheby’s shares have come under pressure on jitters that its Damien Hirst mega-auction starting on Monday (centrepiece: a pickled 600kg bullock) could pull in less than the expected £65m. Is contemporary art a bubble about to burst, spattering investors (sorry, collectors) with formaldehyde?

Leave aside debates about whether art is an asset class or operates on some higher aesthetic plane. People buy and sell art – making it an asset – and it has peaks and troughs, loosely shadowing real estate prices.