I can't comment on the clinkers, but I have asked the coal people (mines) about the difference. The answer is always the same. Iron content. They tell me the the red ash has a higher iron then the white. Also, on the threads about the BTU's, our stoves (feeders) have been tested to run about 6.7 lbs an hr without dropping hot coals off of the grate. In all of our brochures and lit. we claim to input at 90,000. Our claims are about 72,000 BTU output. I don't see anyone else making the output claim. We have run 8.4 lbs across out feeder without dropping hot coals off of the grate, but the quality of the coal varied. We use the standard of 13,500 BTU's per lbs, input, at about 80-83% eff. at burn. I'm sure everyone realizes there are great differences in the coal. Next time you put gas into a container, look at it and tell me if it's low grade, medium grade or high test. I think its the same with coal. It all looks the same (within reason) but burns different. I think anyone with a coal stove has saved money on heating, and I believe you should get the most for your money.