Used to back the fire up overnight and to maintain longer burning. This is what is left after Bitumous Coal is sifted and separated into its different particle sizes. Coal is taken from the mine, shipped to its desitnation then separated by screens ( a bit like sieving flour ). The largest peice out is Coal which is around the size of a mans fist. Doubles is the next size down which is around the size of an 11 year old childs fist. The next size down is Singles, which is around the size of a mans thumb from the top of his nail down to the knuckle. ( Approx 20mm).
The next size down is Slack........basically what is left below the size of singles. Most people use slack to back up the fire to make your Coal and sticks last longer. Everyone remembers their Granny putting "Wet" slack on the fire to make the fire last all night. ( Maybe Granny was wiser than you thought ? ) Can be used on open fires but we would recommend that you use Singles in your stove rather than slack, as it is too fine, which tends to fall down through the grate.