“Andy Warhol’s Oxidation paintings, perhaps more than any other series in his oeuvre, embody a striking paradox: they are at once aesthetically rich and gleefully transgressive,” reads the description of The Oxidations on the website of upscale auction house Christie’s. According to the site, one of the paintings, which features a coppery background interrupted by green splotches, sold for nearly $1.9 million back in 2008.

“Its surface offers up sensual delight in its coppery iridescence, and its composition is enlivened by elegant splashes that radiate outward with visceral energy,” the Christie’s description of the work continues. “Yet Warhol’s method for achieving such rich painterly abstraction was notoriously base, the product of Warhol and company urinating on the copper-coated canvas.”

In his 1989 book The Andy Warhol Diaries, Warhol explained that his assistant, Ronny Cutrone, was the ideal candidate to contribute the bodily fluid. “He takes a lot of vitamin B so the canvas turns a really pretty color when it’s his piss,” the artist wrote. 

Cutrone himself recalled — as quoted in Andy Warhol: The Late Work (via Christie’s) — that the artistic process for The Oxidations “became almost a sort of performance.” He added, “The studio would become like a toilet, a giant urinal.”