Virgil,
This really has me mystified, as it's my understanding that pastels are made with exactly the same pigments as oils.
I can't imagine that oils or resins could protect this pigment from UV rays, any more than gum tragacanth would, so I imagine that this window test, if conducted fairly with both pastels and oils containing exactly the same pigment, would fade both of them equally.
Ambroise Vollard, in his 'Memories of an art merchant', describes Degas actually putting his pastels out on the window's edge to fade, as he found the colours too bright...what irony when we see his works now in specially darkened museum rooms.
Best wishes!
Gwenneth
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