When the Italian artist Piero Manzoni put his excrement into tin cans in the early 1960s and offered it as art, he said that he was exposing “the gullibility of the art-buying public”.

Collectors and galleries that paid high prices for the tins — including the Tate — appeared even more gullible yesterday when it emerged that they contained not faeces, but plaster.

The tin at the Tate, for which the gallery paid £22,300 in 2000, is labelled Merda d’Artis-ta (Artist’s S***) 1961. Described by the Tate as a seminal work, it was No 4 of 90 cans made by Manzoni, each supposedly containing 30 grams of his excrement. A buyer paid €124,000 (£84,000) at an auction in Milan last month for tin No 18.

However, Agostino Bonalumi, who worked closely with Manzoni, recalled yesterday that he, Manzoni and a third young artist, Enrico Castellani, had rebelled against traditional art forms but had found no takers in Milan for their ideas.

“Piero said, ‘All these Milanese bourgeois bastards want is c***,’ ” Mr Bonalumi wrote in Corriere della Sera. He said that shortly afterwards Manzoni asked him and Castellani to his studio, where he showed them a can on which he had replaced the label with another on which he had written the words “Merda d’Artista”.

Mr Bonalumi said that “for decades since, many people have asked what was really inside the cans”. The answer was: “I can assure everyone that the contents were only plaster. If anyone wants to verify this, let them do so.”

Manzoni once said that he hoped that the cans would explode, and about half are reported to have done so. But none of the owners have revealed the contents. The cans, owned by the Tate, the Pompidou museum in Paris and the Museum of Modern Art in New York, are intact.

The Tate said yesterday that its can remained valid as a work of art. “Keeping the viewer in suspense is part of the subversive humour of the work,” a spokesperson told The Times.

In a letter to a friend before his death from drink and drugs at the age of 29, Manzoni said: “If collectors really want something intimate, really personal to the artist, there’s the artist’s own s***. That is really his.” Manzoni, born near Cremona in 1933, used phosphorescent paint and cobalt chloride for his paintings, so that the colours altered over time. He also made “pneumatic sculptures” containing his own breath.

Shock art

— The shock art pioneer Marcel Duchamp’s works included Fountain, a urinal exhibited in New York in 1917

—1966’s Destruction in Art Symposium in London featured Herman Nitsch’s Orgies of Mystery Theatre, a music and dance display amid dismembered animal corpses. Organisers were charged with indecency

— David Mach’s Polaris (1983), a protest against nuclear war, featured 6,000 used tyres in the shape of a submarine. One irate visitor to the Hayward Gallery died attempting to set it alight

— Rachel Whiteread displayed House, a full-size cast of an East London home, in a public park in 1993. It was demolished after an outcry

— Sarah Lucas installed a working toilet in the Institute of Contemporary Art as part of a 1997 show. Several visitors to the opening night used it

— Myra (1997), by Marcus Harvey, depicted the Moors murderer Myra Hindley with children's handprints, and was attacked by protesters

Source: Humanities Advanced Technology and Information Institute of Glasgow University; Minnesota State University

Editor's choice

• The super cool Vampire Weekend
• Streep and Baldwin get complicated
• Guy Ritchie on Sherlock Holmes