In the performance piece, Zone de Sensibilité Picturale Immatérielle (Zones of Immaterial Pictorial Sensibility) 1959-62, he offered empty spaces in the city in exchange for gold. He wanted his buyers to experience The Void by selling them empty space. In his view this experience could only be paid for in the purest material: gold. In exchange, he gave a certificate of ownership to the buyer. As the second part of the piece, performed on the Seine with an Art critic in attendance, if the buyer agreed to set fire to the certificate, Klein would throw half the gold into the river, in order to restore the "natural order" that he had unbalanced by selling the empty space (that was now not "empty" anymore). He used the other half of the gold to create a series of gold-leafed works, which, along with a series of pink monochromes, began to augment his blue monochromes toward the end of his life.