Of course media was carbon-based, since not much choice at the time. “Carbon-based” medium meaning charcoal. Unless da Vinci employed dry-brush oil technique. (He’s not known to have worked in tempera.)

No big deal here, and no amount of testing will ever be able to confirm whether da Vinci did the drawings. Apprentices abounded at the time and one didn’t get to be an apprentice by being a bad draftsman. Many were exceptional artists in their own right, if not so original. In the case of Rembrandt, there was one apprentice who was so good it’s difficult to determine whether a given painting is his work or Rembrandt’s. Which is interesting only under certain circumstances; namely, capitalist ones wherein and whereby art objects are regarded as so much bullion. See Wells’ “F for Fake,” for a fascinating take on the topic. Plus, the film is far more interesting than “Citizen Kane.” I

Assuming drawings are by da Vinci, which they appear at first glance to be, they mean nothing except that he got busy (as probably the busiest person who ever lived) drawing on the back of canvases, which were a pain in the butt to prepare at the time. Rabbit-skin (really)-glue primer, gesso or lead white (now banned) laid over top, after or before canvas was stretched over wood bars. Preparing canvases took time, and there was nothing aesthetic about having to do it. And pigments had to be ground down into flax-seed oil, which is a lotta work. Cellini reports, in his writing on painting about the same time, of how painters in general were the busiest people in the world. One couldn’t drive over to the art store to pick up supplies; one had to manufacture them. Apprentices were employed mainly for purposes of manufacturing supplies, though some indication of aesthetic sensibility was also required in order to become apprentice in the first place. They were also needed to paint in backgrounds. Rubens, long after da Vinci’s time, was able to produce stuff so monumental that by comparison the monuments to egos produced by the most the grandiose-minded of painters nowadays appear piddling. Rubens had a stable of apprentices who did most of the painting, according to Rubens’ vision. He then dropped in to paint in faces and hands. Then as now labor got little credit. (First came mercantile capitalism, which gave rise to the Renaissance, which among other things was when got started ideology of the individual, as represented by “cult of genius” that surrounded Michelangelo—da Vinci’s semi-free-market “competitor”—in particular. Root of Romanticism that later embodied apotheosis of a-historical individualism? Think so. Worship of celebrity remains to this day. Along with attendant belief that individuals make history, not the reverse; the latter the truth, not the former.)

Leonardo was just doing some drawing on bits of cloth—backside if front side was already prepared to receive oil; otherwise either or neither side. No mystery. Before or after painting on the reverse side he likely took his hand and dusted off charcoal to clean the thing up a little. Charcoal can’t be brushed away so easily, though enough that what remains appears as faintest drawing. Otherwise, charcoal outlining was standard method prior to application of paint. Still is for those who don’t wield house-painter’s brushes or throw buckets of interior or exterior latex onto four- to eight-hundred square feet of unprepared cloth from 10 to 20 feet away. Rigorously intellectual notes are either attached to same or printed in “Art Forum” for purposes of explication. Speaking of “mysterious”! No need in the case of da Vinci, whose astounding testimonials to what is possible remain as accessible today as they were then. Youngster capitalism allowed for as much and so wasn’t always evil.