I searched for “letters” in the iPad Art Authority app and found 7 paintings with writers caught in the act. Interestingly, all from the 17th century, all using quills.

Some further observations:
All are using a very natural and relaxed “tripod” grip. I think ballpoint pens and other such awful instruments of satan have corrupted our grips to the kinked contortions seen today.

All write with the right hand. I think no one was allowed to write with the “sinister” hand even if one were naturally left handed.

Six of the seven have part of the right hand resting on the surface showing that the writing motion must have utilized wrist/fingers. I realize there is some controversy about this.

Five of the seven have the left arm resting on the surface, and two are sitting too far from the surface for it to serve as rest.

Documented writing from the period is overwhelmingly by male writers, and pen tutorials from the period suggest women when taught writing were taught simpler letterforms. Yet four of the seven paintings depict women writers. How I long to read their writing!

[Artist / date from left to right:

Borch 1655; Metsu 1665; Netscher 1665; Vermeer 1666; Vermeer 1670; Mieris 1680; Borch date unknown.

Hmm.  All seven were painted within 25 years of each other!

Update: A poster discussing this at the Fountain Pen Network says that the paintings all come from the Dutch/German era of portraiture which showed people doing every day stuff amid all their stuff.  The style then moved on to more formalized posed portraits.  I’ll be checking that out.  Doug]

Update of 6/16/2011: Added three more, which to take up where I left off would be: Erasmus by Hans Holbein the Younger 1523; A close up of same; Woman Writing a Letter, Gabriel Metsu 1662.  Interesting to note at least part of the plume remaining on the quills in all the paintings.

Update of 1/13/2011: Added the self portrait of Agnolo Bronzino (1503 – 1572) showing him writing. But look closely — he is writing with a brush!