- He was one of the founding fathers of La Nouvelle Figuration in 1962, the new trend that brought the importance of figurative art into the spotlight. This direction developed almost at the same time as Pop Art picked up momentum in the US.

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- During the 1970s Klasen worked as assistant to Arman.

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- Clips from weeklies, bits of cardboard and parts of military planes and vehicles are but a few of the materials used in his art. Naked female bodies, breasts and parted lips are mingled with numbers and symmetric colour fields