Your description of the ash sounds right. Even in my hand-load boiler's firebox, where the coal pile is usually 10-12" deep, and burns for 8-10 hours, some of the coal does not burn completely.
Most of the ash is just that, it will crumble to dust with at touch or slight pressure. But some pieces still have a black center and require two hands to break apart. Even under the pressure of the coal pile above, some pieces are hard enough to not break up by the shaking grate, and eventually get 'whittled away' by the grate and fall through into the ash pan, and look like a rock or piece of sandstone, with a black center.
I have taken some of these 'unburnt' or 'incompletely burnt' pieces and set them on top of a very hot fire, and..... nothing happens! The piece turns bright red, but when removed from the fire a few hours later, it looks the same, feels the same, and requires the same pressure to break. It still has a black center. Whatever was in that lump to burn had burnt, there was nothing left but hard ash.
Coal is not always consistant in how it burns. The coal comes from the top, middle, bottom and edges of a vein of coal. What we get to burn depends a lot on luck. I don't know which part burns best, but I'd assume near the center of a vein would be the purest coal. Some burns to a talcum-powder dust, some burns but hardly changes in appearance.
Hope this makes sense. Greg L
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- My ashpan with a typical load of ash, some looks like it is not burnt completely.