Copy of Sargent's "Lady Astor"

I just posted this picture in my "Master Copies" gallery, but I thought I'd say a bit more about it here. This is a copy of a small part of John Singer Sargent's "Lady Astor" (1909), which hangs in Cliveden, the former Astor estate in southern England.  As I noted in the Master Copies gallery, Lady Nancy Astor was, among other things, the first female member of the House of Commons. She argued politics frequently with Winston Churchill, once saying that if she were his wife, she'd put poison in his coffee. Churchill supposedly replied: "Nancy, if I were your husband, I'd drink it."

I embarked on this copy to learn more about how Sargent made his portraits look so three-dimensional. I painted this in several sessions, spending about 10-15 hours; I didn't do much glazing, more just fussing with proportions and color temperature. Did Sargent paint this alla prima, or did he glaze?

My likeness is pretty good but not perfect. My Nancy is a tad thinner, with a slightly smaller nose and mouth. It'd have taken me another 15 hours to get it just right, and the purpose of the exercise wasn't to make an exact replica. I also couldn't resist warming up her skin tone a tad; in Sargent's day, a pale face (with reddish ears!) was more in style. Anyway, at some point, I decided I'd learned what I was going to learn, and I declared victory. Of course, as soon as I posted it here, I saw imperfections that I'll have to fix: the shadows on her neck being a prime candidate.

Incidentally, when I was photographing this painting, I couldn't keep glare out of the right side of the black background, so I've photoshopped that bit to make it blacker. I'm going to try again with a better camera, but in the meantime, I thought I'd post what I have. Oil on canvas, 11" x 14"; comments and constructive critique welcome.