Place Blue Black and Bright Neutral Grey onto your palette in the following series of mixes of Blue:Grey-

4:0, 3:1, 2:2, 1:3, 0:4

1. Using the Blue Black , block in your shadows, and then wet-blend toward the highlights using the 3:1 mix.
2. Once your shadows are in place, wet-blend higher using your 2:2 mix, and once again with your 1:3 mix.
3. Layer on scratchy highlights using pure Bright Neutral Grey , using a small repeated brushstroke motion.