This is the "undersketch" for the figure of St. John. When you see the finished painting, you might think that I just copied the figure from the Karlsruhe Crucifix, but I didn't. I drew this figure first, and then draped the clothes on him. I tried to get my model to pose the same as the figure of St. John in the Karlsruhe Crucifix, but of course it is slightly different. After I put the clothes on him, I changed the position of one foot, but having drawn this, I could move his foot with confidence, knowing where they were under the robes. Here is a close-up of the hands in the sketch. Once again, it is slightly different from the Karlsruhe Crucifix: 
I wanted to know exactly where the fingers were before I started, and not just the fingers, but the wrist bones too. Here are the hands of St. John from the Karlsruhe Crucifix so you can compare. 
Here is the figure of the Virgin:
I'd show her hands too, but for some reason my scan is a bit blurry. Not enough pixels. OK, here is the first stage of the underpainting on the actual panel:
I took the final sketch and transferred it to the panel with graphite, cleaned it up, made minor changes, and sprayed it with a bit of fixitive. Then I went over the drawing with peach black oil paint, thinned with mineral spirits to the consistency of ink. When this dried throughly, I applied an imprimatura: a transparent wash of oil color over the entire panel. I thought Durer and Grunewald used a warmish ochre color; but Joseph Shepard's book How To Paint Like the Old Masters said to use burnt umber to create an imprimature like Durer would have used. I think I compromised and used burnt sienna mixed with a bit of umber but if I could do it over, I think I would use a red ochre. The dark area behind the head of St. John was a place where the gesso was too absorbent and took up too much of the imprimatura.
After I applied the imprimatura, and it dried, I worked up the figures in a grisaille: I used black and white paint to make a complete "grayscale" of the figures. You can see the completed grisaille of St. John on the right of the panel. After all the figures were painted in grisaille, I then began to glaze color over them.
I really should have made more photos as I worked, but I got into the painting and forgot. This weekend I'll photograph the finished painting and show here. I'll also say a lot more about how I worked and other sources for the painting.
