Re: Ivory or Lamp Black
I think Einion's got the answer with the gloss varnish. I read that Turner even varnished some of the darks in his watercolors to achieve that same depth. (He'd be banned from watercolor societies today!)
The darkest black I've got on my palette is my Rublev bone black; the lightest is Da Vinci's Magnetite Genuine. It seems like a pretty big difference on the palette; but after reading Einion's post, I'll have to test them directly, along with lamp, vine and Mars blacks, and compare them when dry. That would be a useful thing to know.