Answers to the Friday Afternoon Challege and THANKS to my DU art diehards!
1. Giotto, Baroncelli Altarpiece, Santa Croce, Florence.
2. Carracci Altarpiece, Cerasi Chapel, Santa Maria del Popolo, Rome.
3. Ghirlandaio Altarpiece, Sassetti Chapel, Basilica of Santa Trinita, Florence.
4. Durer, Jabach Altarpiece, Museo del Uffizi, Florence.
5. Filippino Lippi, Altarpiece, Carafa Chapel, Santa Maria Sopra Minerva, Rome.
6. Piero della Francesca, Misericordia Altarpiece, San Sepolcro.
Thanks to my hardy band of art lovers who were researching and posting into the wee hours of Saturday morning! Hats off particularly to Pinboy and BainsBane for going above and beyond the call of duty! 
The Challenge will now go into preparation for future Challenges. And even though you should have been warned that last Friday's quiz was titled "Part I," don't be afraid...Part II will not appear for a while. Rest up.
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I heard rumors that the next one will be on hats, so I've been studying up on the artsy ones for it...
BainsBane won't stand a chance next time! Muahahahahahaha! 
Many here love the Challenge even if they don't participate. Skinner's probably a secret Challenge admirer, himself.
Besides, your Challenges aren't really Lounge material. The last challenge posted in the Lounge was a word search puzzle. We had to find the word 'dog'. Really.
I don't think you have a thing to worry about. Really. 
It's on. 
Always look at them, rec them and almost NEVER know a single one of them. And yet I try to never miss it! Thanks for your creativity in bringing us such wonderful culture every week.
You're awesome. 
Julie--who doesn't know art but knows what she likes