In reading through older art books on painting technique, the word "pencil" often appears, but not with the denotation with which we are now accustomed. It is obvious by the syntax that when the authors use "pencil," they are speaking about a paintbrush, but then when they use the term "brush" in other parts of their books, that word obviously designates an item which they consider to be separate and different (eg. "outline the shadow area with a hog's hair pencil, then use a brush to mass-in the tone"). Why would someone use the word "pencil" when they mean paintbrush, and, in the mind of an earlier artist, what made these "pencils" different from "brushes"?
In actuality, the word "pencil" originally meant "paintbrush." Its usage can be traced to the late 14th century where it was derived from the Old French word pincel, which in turn, developed from the Latin term penicillus, or literally, "little tail." Specifically, it was a small paintbrush, as penicillus was a diminutive of peniculus, Latin for "brush."
Using the word "pencil" to refer to the graphite writing and drawing instrument was a development that came about in the 16th century. In 1565, a deposit of very pure and solid graphite discovered in the Cumbria region of England proved to be easy to saw into sticks, and these sticks, when wound with string or inserted into metal holders, were found to be extremely convenient for writing. "Pencil" brushes, which had been used for centuries as writing tools, were soon replaced. In many countries, the new implement - and its successive improvements - became known as "lead pens" (graphite was at first mistaken for lead, and was even called "black-lead," until Swedish chemist Karl Wilhelm Scheele identified the material as a crystalized form of carbon and named it graphite in the late 18th century). In France, the country which introduced the word pincel, the "lead pen" became known as the "crayon;" in England it appears that the word "pencil" remained in use, despite the fact that the original tool was replaced by an entirely new object.
To a painter during those earlier times, a "pencil" was always a paintbrush, while a "lead pencil" would have been the graphite tool.
Hair "pencils" were not sized with numbers as were other brushes, but in quill sizes. This reflects not only their history as writing implements, but also the origins of their construction. In the first "pencils," the quills of crow, duck, goose, or swan were boiled to soften and swell the ends, and the hairs were then inserted. As the quill cooled and dried, it shrank tightly around the hairs. Often the end was wrapped in wire to help secure the hairs, but could be removed once the quill had re-hardened.
Generally speaking, "pencils" were smaller, and made of soft filaments. These fibers could be mongoose, sable, swan's down, camel-hair, squirrel tail, or children's hair. Painting "brushes," on the other hand, were larger and almost always made of hog's hair bristle.