Fig, I have that problem--sometimes. I think the science of it is complicated--burning too hot or too cool too long, the technique of shaking and riddling, general stove and chimney conditions (and model), and so forth can affect the quantity of clinkers, yes? I have read that anthracite can vary in carbon content (versus metals and other elements) somewhat, but this will occur naturally even within a given mine. Natural products have variations, whether animal, vegetable, or mineral. Also, human processing of anything natural will introduce variations--I assume a coal breaker for chestnut size will naturally pass diverse sizes and shapes within the limits of the grates (long and narrow will pass just as broken off bits). No mine inspects each nugget before bagging, though of course my mileage may differ in the event.
Last year was my first year, and the problems some people have with TSC coal are the very problems I had with the Lowe's coal I tried. (I think they no longer carry it.) My impression was that my Lowe's store handled things roughly--some of my bags were torn loading them for me (or before), spilling some of it. (I like Lowe's for many things, though.) My TSC store is so careful and accommodating with things that all my bags get home looking brand new/never touched.
Still, sometimes I get more clinkers than other times. Even, constant heat with regular, consistent shaking (and especially riddling) seems to help. The slicer can pull small clinkers through the grates before they grow. I can't get pea-sized coal, but those here who have my stove model seem to get better results with it. It burns slower and steadier and would probably be better in my climate (Zone 6 Dept. of Agriculture). Much of my winter continues to have (off and on) days in the 50's or 40's, so the stove is choked down as far as it will go for long periods. My theory is that this makes it an incubator for clinkers, apart from all the other issues. This has been a mild winter even in coal country, mostly.
