Painting on Vellum ala Sergio Martinez

I've been a fan of Sergio's every since I discovered his illustrations for Max Lucado's "You Are Special". His composition and line work are wonderful and strong. He creates his luminous colors by drawing on both sides of vellum. I've been a fan of drawing on vellum for years now so I was very interested in his technique. In one of his blog posts he mentioned that his whole process was explained in an out-of-print edition of Step by Step Graphics. Fortunately, someone on ConceptArt who owned that edition scanned and posted the whole article. UNfortunately, the link is now down. If anyone would like to have a copy, let me know and I can email it to you. This is my experimental sketch & value study. Next I will cover the sketch with a sheet of heavy, 90gm vellum and redraw with a carbon charcoal pencil. Hopefully, I will be ready to post the next step tomorrow.

Edit: So I'm not hogging Mistress Sketchbook blog space, you can follow the rest of this experimental process on my art blog.