The same chemistry teacher who demonstrated the cool flame also showed us the ammonium dichromate volcano, except that instead of using a few grams of the stuff, he emptied out half a jar onto the bench top, stuck a bit of magnesium ribbon in the top of the pile, and lit it. Unlike for firework instructions, he didn't 'retire immediately', the heap went up with a whoosh, and he was covered with green chromium trioxide snow. With a rather surprised expression on his face he said 'Wham, it reacts!'. It has stuck in my mind ever since.
I think he was fresh out of teacher training college and hadn't yet got too much experience of lab demos of many things. I wonder if he survived the long-term toxicity.