It's always a pleasure to learn here, thanks Jan! I would like to share an observation I made many years ago, mid 70's, I believe. An unamed gentleman used to take newly manufactured obsidian arrowheads and coat them with a mixture of relly fine clay/silt??? he would then "cook" them in a microwave oven ,(a fairly new invention back then), these points came out looking as if they had been buried for eons, you could not scrub or wash this dirt off, it was as permanent as it gets.