First, he used only ink, charcoal, and lithographic crayon on paper. (widewalls.ch)
Often large in scale, these drawings are typically made with a thick impasto of black paintstick (or, more recently, lithographic crayons melted into a brick), which is applied to a surface in broad, dense passages. (artbook.com)
In the early 1970s Serra drew primarily with ink, charcoal, and lithographic crayon on paper. (museumpublicity.com)