Melting.
He tries to process the front edges of the shadow to get a natural look from light to dark. After he has set up this variation, Bruce then adds a little more matt crescent-shaped substance to form deeper shadows under the back of the head, as in the last two photos on the left. It also adds deeper shadows to the lower cheek area as in the upper right photo. He will return and treat these areas with his scrubs until he gets the tonal qualities in the shadows he is trying to achieve. When he finishes changing the shadows, Bruce puts the glass on a hot plate to dry...