I’ve seen these in person. They are ink on paper—then stretched (adhered) to canvas. I heard that he projects small old master drawings (and his own drawings) and then sort of kind of traces them and then makes up his own stuff/marks to “fill” the space. Really beautiful in person.
"Art experts from Nationale Suisse and the Fondation Beyeler in Riehen, near Basel, mastered in 2012 the three-year conservation project to restore Henri Matisse’s key work ‘Acanthes’" (art.nationalesuisse.com)