Apparently making tonal studies is an important part of classical art training. In his popular book "The Practice and Science of Drawing" Harold Speed calls it "Mass Drawing". It just means painting something from life using only white and some dark color like Raw Umber or Black. I personally haven't spent much time making such studies but I have painted a few for my own advancement. What did I learn? I suppose painting in monochrome sharpens your eye and increases your ability to appreciate subtle value differences. The usual rule is to work from an actual plaster cast, but in this example, I worked from a black and white photograph. The subject is the well known Welsh painter, Augustus Edwin John 1878 -1961.