Sierra constantly sets up oppositional forces between work and worth, between poverty and the exclusivity of the international art world, between the people who are invisible in society, the junkies, the homeless, the recent immigrants, and the art gallery as a venue where viewing and viewer are centre stage.

Whether casting sculptures from human excrement or paying junkie prostitutes to have a line tattooed on their backs, Spanish artist Santiago Sierra is a provocateur whose art raise headlines. We filmed his latest performance piece at Tate Modern and asked him about this and other controversial works. (Please note: this video contains some strong language).