Drawing Techniques: a Guide
Liquid Media: Brown Inks  
Old Master drawings are popularly identified with the handsome brown color of their ink. Formerly these sheets were commonly described with the misnomer "sepia". It is usual today to describe all such drawings as "pen and brown ink" or "brush and brown wash", because, although a number of different inks were used, it is difficult to distinguish between them. Of the two primary kinds of ink, bistre presents more or less the original appearance at the time of use, and iron-gall ink turns from black to brown with time.  
Brush and Wash