I am not an aviator. In our Rus. media the volcanic ash is not described as "ash", for general public like me. But as "liquid glass", Eyjafjallajoekull peculiarity now is producing "liquid glass". Which then flies as "tiny glass particles, 2mm big in size only" and "if not coloured by additives - you wouldn't see them, by un-aided eye".
The BA 1982 example, of 4 engines failure (and then working again) (post 1016 here) in Russian media was described by the pilots seeing nothing in the air at all but good day and clear skies. And only on the ground it was noticed the front window is like peck ?-ed like was bombardded a bit by tiny glassy those particles, like as if someone scratched the window.
Otherwise here we also don't know nothing; volcanologists of all calibres are being terrorised re the exact composition of the "liquid glass" and how it behaves at var. temperatures, and all.