Since charcoal is lighter and easier to transport than the wood from which it is made, charcoal burning was traditionally a craft carried out within the woodlands. Today, charcoal is produced and used by the chemical industry in the manufacture of products as varied as penicillin and plastics, although it is now rarely used as a fuel except in barbecuing of sausages and steaks! In the past, however, its use was vital in industry, particularly as the fuel for smelting iron.
Charcoal is almost the perfect fuel, yielding a high steady temperature with very little smoke or ash. Oak makes the best charcoal although charcoals from other trees were used for specific purposes. Charcoal from Willow, for instance, is still used by artists for drawing while Alder charcoal was used in the production of gunpowder. This accounts for the gunpowder mills to the west of cork city which obtained their charcoal from the Alder woods along the Lee Valley.
The principle of charcoal burning is to heat the wood in the absence of air thus preventing complete combustion. Water and volatile substances such as tar and creosote are driven off leaving a mass of solid carbon. Charcoal making required no special tools or equipment. The trees were usually felled and cut and split into 2-3 ft (0.6- m.) lengths in the late winter or spring and allowed to dry for several months.
Charcoal burning, therefore, took place in the summer months. A flat piece of ground, about 15ft (5m.) in diameter was chosen, and covered with earth or ashes. In the centre a pole about 6ft (2m.) in height was fixed and surrounded with small pieces of wood which would catch fire quickly. The 3ft (1m.) lengths of wood were piled upright, around the pole sloping inwards at the top. This formed a stack about 6ft (2m.) in height and 15ft (5m.) in diameter. The stack was covered with bracken and sods of grass and cemented down with damp earth and ashes. The central pole was then taken out and some cold charcoal, kindling wood, and burning charcoal dropped into the hole to fire the clamp. More cold charcoal was placed in to fill up the hole, and as soon as flames began to appear a sod of grass was placed over the hole to regulate the supply of air.
The clamp was checked at frequent intervals day and night and, if flames appeared, these were damped down with water. Fresh sods of grass were placed over any gaps that appeared on the outside of the clamp. To guard against changes in wind direction, the charcoal burner used a portable screen to shield the clamp from the prevailing wind. The screen was made up as a wooden frame, wattled with bracken and supported by firmer sticks.
The whole art of charcoal burning lay in the regulation of draught, so that the maximum quantity of wood was charred and the minimum burned. When a blue haze replaced the white smoke issuing from the clamp, the burning was complete. Once the fire was out, the clamp could be dismantled, and the charcoal put into sacks for transport. In general, one ton of oak (1000kg) would produce one cwt. (50kgs) of charcoal.