These paintings were done with oil on paper employing a “Dry Brush” technique. Oil dry brush could be a painting technique wherever a paint brush is comparatively dry . however still holds paint. a really skinny layer of color is applied to a paper.
Dry brush is layered layer after layer and reworked . with an eraser, at this point it . is possible to incorporate different lighting effects and details. Images painted . with a dry brush appear very similar to a photo. It is not a straight forward technique which needs lots of . time to . master.
Source : BoredPanda