Minecraft wiki entry for Things_not_to_do, associated text: A coal block burns for 16 thousand ticks, which is equivalent to 800 seconds (13 minutes 20 seconds). A piece of coal burns 1600 ticks- that means a coal block is giving out an equal energy and burning time of 10 coal, despite the fact that a coal block is crafted with 9 coal. This may seem like a good deal, but however, one coal block is able to burn 80 items; that means if you do not have 80 items to smelt, you wind up wasting some of the coal. If you plan to smelt fewer items, coal blocks should be crafted back into coal first. You can get more info here.