When Piero Manzoni filled and famously relabelled 90 tin cans in 1961, the new wording on each included (in four languages) “with no added preservatives”. None was needed. According to one of Manzoni’s co-conspirators the contents were not excrement, as advertised on the tin, but plaster.

The whistle has been blown on one of the great coups of 20th-century contemporary art by one Agostino Bonalumi. Until this week he has stood by as the Manzoni tins, initially priced as if their contents were 24-carat gold, changed hands for up to 13 times that price. The Tate paid £22,300 for one tin five years ago.

Both men called themselves artists. They asserted that by doing so they rendered anything that they made “art”. To the delight of their dealers, three generations of collectors accepted this assertion. At the risk of taking them more seriously than they deserve, and at the even greater risk of being misconstrued as following the herd, the revelation that Manzoni violated the Trade Descriptions Act makes no difference at all to the value of his output. He produced it, ergo it was art. People queued up to buy it, ergo it was valuable art. If any duping was involved, it came long before the supposed pooping.

Manzoni’s coup was to mock the art world three times over for its gullibility – first to its face, again when collectors rose, wallets agape, to his bait, and now, 44 years after his death, with the powdery truth about his stool foolery. Not that this pulls the rug from under the great Ponzi scheme of commercial art. A diamond-studded skull? That work was itself a physical manifestation of the trend. It was brainless.