Gianfrancesco Penni
Born in Florence to a family of weavers, Penni entered Raphael's workshop very early in his life, and collaborated with him on several works, including the famous Rooms of the Vatican Palace, as well as the frescoes of Villa Farnesina, both in Rome. Heinrich Wölfflin and some other writers credited him with the entire execution of the Raphael Cartoons, with Raphael only creating the initial design, though more recent writers believe Raphael did much of the work himself.
Eugen Kolb
• Raphael : the artist and his work in the perspective of the most famous photographs / edited and explained by a. Kolb, with a biographical introduction by Leopold Tzone, Tel Aviv: Massada, 5713