What is hot coal?

Answer 1

The phrase "hot coals" (plural) can refer to hot embers (or coal) that keeps burning even after a fire dies down.

There are two types of coal: anthracte (hard coal) and bituminous (soft coal). Both burn, but bituminous coal burns hotter and more evenly than anthracite coal. Homes and businesses all used to be heated by coal furnances before furnaces were converted to gas. Now both use gas or electric to run furnaces. Coal also used to fuel steel mills and factories before those industries closed down.

When bituminous coal fully heats inside a coal lump, the coal piece turns a red-orange-purplish color depending on how much impurities/ore are still attached to the coal. The hottest coal at the bottom or inside a heap (built to make a fire) looks as if it is "red hot". You would feel heat radiating from the piece of coal from several feet away. Between radiation of heat from the coal and convection of heat, a bucket or two of coal in a coal furnace could heat a house for 4 to 8 hours, depending on the outside temperatures, winds, and size of house plus how well the house is insulated. When fire (made with wood, cardboard, paper) over coal burns away, leaving just the red hot coals, the coals might stay hot and still combusting from inside the coal for up to 12 hours or more. Generally, people tended the coal furnace every 2 to 4 hours, adding fresh coal and other materials to maximize the heat already being produced by the lowest layer of coal in the heap. As the coal was burned away, it left brittle pieces that most people called coal clinkers or clunkers. These could be raked off to the side to cool, then be shoveled out of the furnace. As well, coal fires with other materials made ash which fell through a grate or could be "shook" down with a lever ("shaker") that moved the grate slightly back and forth. The ash was deposited in a chamber near the floor-- that ash and smaller clunkers had to be shoveled out when the furnace cooled enough to permit opening the lower door. But even the ash could contain burning embers, so the wheelbarrow of ash / clunkers was wheeled outside, away from the house, and dumped on the ground. If not disposed of properly and spread out to cool, the refuse pile "could" catch fire.

So basically, a hot coal is any coal or ember that still contains enough heat-energy to be burning---from edges to the middle, becoming "red hot" at its peak of energy then slowly cooling.