Hello Wayne, we try to keep each thread on topic, so for your question about coal quality, take a look at this thread: Coal quality from various sources
Bad coal would be characterized as not burning well, or completely or not burning very hot.. or leaving hard ash that the shaker grates can't grind up or get to fall through into the ashpan.
For your question about the minor explosions, re-read this thread we are on. It has a very good description about how and why the explosions happen.
I'd just ask the Amish where they get their coal. They may be forthcoming with an answer. I will say that in my area, the N. Indiana, N Ohio and S, Michigan area, if the coal poor quality, the Amish will be quite vocal about it to the retailer!! So you should be OK buying from them, but this is by no means a guarantee. If they sell the coal to each other, it should be good quality.
Hope this helps..