"Ratsch and M-E are historians of shamanism, witchcraft and psychotropic medicine"

Ratsch Phd. is an anthropologist and ethnopharmacologist. Muller-Ebling Phd. is an art historian and anthropologist. Their work as far as I can tell is pretty academically sound.

I've just scan-read the chapter on "Incense for the Holy Nights" in their Pagan Christmas (which is kicking around nearby coincidentally!), and I didn't spot any reference to smudging with ashes there. That's not to say they don't discuss this, I just haven't spotted anything on that.

The relationship between ceremonial shamanic 'smudging' "with the smoke that rises from certain woods, resins and leaves" and Catholic "Holy Smoke" is discussed. With special reference to the relationship between the German word for Christmas 'Weihnachten' and the related German 'Weihrauch' meaning "incense" or "Sacred Smoke." They also go on to offer "Incense Recipies for Smudging Nights."