Finally made the spell work! I was barking up the wrong tree as far as the shrouding ashes were concerned. Turns out they weren't necessary at all.
So that I don't forget, the formula for Merien's White Mask is as follows:
(Tamien will love this spell! I can't wait to show him.)
First, recite my incantation over the ritual table to begin the spell.
Then place the masking salve in the center of the table. I'll detail that recipe elsewhere, but I made quite a lot of it.
Third, light a candle and place it so it blends the scent of salve with the candle's smoke.
Finally, when the smoke and scent have blended, place the glass gems. They'll be infused with the aura created by the rest of the spell.