That sounds like it was some of the lousy coal we had for several years in the early 1970's. It was reputed to come from a mine near Farmington, NM.
What you described, Earl, about the firebox becoming one big clinker is exactly what I recall, too. You could see layers of shale running all through the lumps of coal. Quite a few times, after cleaning the fire at the ashpit, I had to scrounge spilled lumps of what was probably old Monero coal scattered around the ground in the area, left over from the Rio Grande days, to get the fire to restart and spread out over the grates again.