The History of Charcoal (Charcoal 1)

What is charcoal?

Charcoal is a natural wood derivative left behind after wildfires, structure fires or campfires. Humans learned early on to create charcoal deliberately as a superior fuel source that was concentrated, light, and clean to transport, and that made hotter, smokeless fires.

To create charcoal, dry wood has to be slowly carbonized to remove any moisture and many impurities. Hardwood converts more efficiently into charcoal than softwood (the denser, the better) and it must be seasoned (air-dried) for 6+ months in most climates before the charcoal burn, or else the moisture present in the wood will cause wasteful fuel loss during the burn. Ideally, oak wood that was cut in freezing weather (when the sap is underground) and dried until summer would make the best charcoal—although, other dense woods such as holly and birch were also used extensively in Europe, and any organic matter can be converted to charcoal under the right conditions.

Making the dry wood into charcoal requires heat above 350º and anaerobic conditions. If oxygen reaches the hot pile of charring material during the smothered charring process, it will ignite and burn up. Charring is not combustion, in the sense of oxidation, and excluding oxygen from the burn is the most critical part of creating charcoal.

This results in a light, compact carbon product that burns much hotter and more efficiently than wood. A regular campfire burning firewood only reaches, on average, between 400ºC to 900ºC. A regular charcoal fire typically exceeds 1,100ºC (2,010ºF). With proper air draft from below, as in a brazier or in a forge, charcoal fires exceed 1,260ºC (2,300ºF). This higher temperature allows the smelting of iron and the working of glass and higher-fire pottery in kilns. It is also a more concentrated fuel source; 100 kg of charcoal produces as much heat as 300 kg of seasoned, dry firewood (1). Because charcoal is only a fifth of the size of the wood that made it, and a third of the weight, it is much easier to transport into urban areas. It carries no bark, fungus, mud, or insect problems, and will not rot, unlike stacked firewood. When newly made, charcoal contains less than 1% moisture; in humid climate, it may absorb up to 15% moisture over time, and it must still be protected from rain. Still, the low moisture content means less water weight to transport and store.

Good quality charcoal (75% carbon or more) produces no smoke when it burns. This prevents ailments resulting from woodsmoke inhalation, and removes the need to direct smoke out of a house, as there is no soot or creosote build-up to deal with. Charcoal fires bank easily overnight, and generate fewer sparks, thus reducing the risk of structure fires (especially in an urban environment). Cooking with charcoal allows for direct-contact heating of cooking pots/vessels (more efficient than suspending cooking pots above flames). Sophisticated braziers can be designed to control airflow and heat output, but simple hearths on the ground can suffice.

Finally, charcoal has many uses as a pigment, tinder and medicine.

The tattoos on Ötzi the Iceman, a 5,300 year old mummy from Italy, were made of carbon black pigment, which was also the source of the earliest inks around the world, and early 'pencil' writings (this use of charcoal has never gone out of style). The artistic use of charcoal likely predates all others, in the form of cave paintings, long before the smelting of metals was ever imagined. Carbon black (paint and ink made of ground up charcoal) rose to special prominence when medieval artists began to char bones and dried grape vines specially in crucibles to make the finest-ground powdered pigments, bone black and vine black.

Charring organic material also creates an excellent tinder material, which is important when starting fires using iron sulfide (pyrite) and flint strikers. The tinder fungus Ötzi carried continued to be a popular charred tinder material in Europe through the medieval period, where the Vikings and the later Germanic cultures improved upon the charred fungus by simmering it in stale, evaporated urine to add nitrates (forming a primitive type of black-powder explosive, although they were trying to get a tinder that would ignite quickly and then smolder to light other kindling, not something that would burn up abruptly).

Smelting and smithing

Deliberate production of charcoal may have begun as early as the late Mesolithic or early Neolithic Periods, based upon charred pits and ditches from those periods (2). However, it is difficult to identify evidence in the archaeological record, since it typically appears only as a “charcoal spread” or a pit containing charcoal. Determining whether this charcoal remnant indicates production as opposed to a cooking/roasting pit, or a bonfire/funeral pyre, or other fire use, is sometimes impossible.

This means that it is impossible to say whether Neolithic settlements made deliberate efforts to produce charcoal, or not. It is certain that the earliest metal workers made use of charcoal in smelting copper, but there is no clear evidence of large-scale charcoal production during the Copper Age or the Bronze Age. The small quantities of lower-quality charcoal generated by a campfire would have sufficed for simple copper working.

The earliest definite evidence of intentional charcoal production dates from the transition period from the Late Bronze Age to the Early Iron Age—roughly the 12th century BCE in the Near East and Mediterranean. For example, there are many small pit kilns in Ireland (3), and charcoal clamps in the Netherlands dating from this period (4). Bulk charcoal production was a necessary invention for the Iron Age to happen, since smelting iron requires the hotter burn produced by charcoal. Iron begins to melt at 1,200ºC, which means that firewood cannot typically generate sufficient heat to smelt iron. A smith requires half a ton of charcoal to smelt a ton of iron ore, and another ton of charcoal (at least) to work that ton of iron in the forge. Thus, 1 ton of wrought iron requires 1.5 tons of charcoal (5)—which meant that the Iron Age could not occur without an efficient method for producing a constant supply of charcoal.

Cooking and urban heating

Once adopted for bloomeries and forges, charcoal soon became popular in domestic contexts. By the Archaic period of Greece (8th century BCE), small portable terracotta stoves/ovens/braziers had been invented for the use of charcoal for cooking and heating (6). These became so dominant that most homes excavated from the Greek and Roman Classical periods do not contain a fixed, permanent hearth; shards from these portable braziers replaced them.



These braziers can be as simple as a wide, fire-proof bowl that holds the charcoal and supports the additional weight of cooking pots. Frequently, however, they include air vents to the sides and beneath the charcoal. These air-intake holes allow greater draft of oxygen into the charcoal fire, making it hotter and more efficient. They often were designed to have a wide circular opening at the top where a cooking pot could fit very precisely. This reduced waste heat emission and increased heat transfer from the coals directly into the cooking pot (which could be put in contact with the coals, since the air-intake below would prevent smothering, and the greatest heat transfer occurs directly from the coals, not from being suspended higher above the flames).

Urban need for charcoal

It is worth remembering that at this point in history, chimneys had not been invented. Smoke holes had to be left in rooftops to allow woodsmoke an exit—which presented problems in inclement weather, unless the home had a special terracotta vent cap for the smoke hole—or the smoke was simply allowed to drift out through the structure's thatch and wattle. Note: in developing nations today, burning wood indoors without a chimney is still a cause of widespread lung ailments from smoke inhalation. Thus the switch from burning wood to burning charcoal represented a vast improvement in quality of life for Archaic and Classical Greeks and Romans.

As a side note, this switch was also important for the growth of nomadic desert dwellers, who lived in tents and had no wagons. Transportation of a much lighter, more compact fuel and the ability to burn it safely indoors transformed tent-dweller life. Charcoal braziers are therefore mentioned in the Hebrew Bible, circa 8th century BCE.

Charcoal was also a requisite invention for the development of urbanization. Urban dwellers in Rome sometimes had tubes or windows to allow smoke out from their urban apartments, but they did not have chimneys, which would generate a draft and draw smoke and sparks safely out from a building and release it above the rooftops. Without chimneys, burning firewood increased the risk of structure fires (which were frequent and deadly in the cities anyway). Smoke inhalation illness and airborn pollution would further have contributed to the misery of city life, and transporting bulk quantities of firewood (and storing it in crowded urban environments) was expensive (7). Only with the greater safety and efficiency of charcoal could cities form.

Unfortunately, the constant urban demand for imported fuel, and the resultant high consumption of wood to make charcoal, eventually doomed the woodlands of the Italian peninsula. By the 1st century BCE, deforestation had driven the Roman empire to importing wood from its colonies (see Coppicing Part 1). Both the Greeks and the Romans seem to have used the mound method (Charcoal Clamps) to produce charcoal in bulk, a method commonly associated with deforestation, and their perspective toward natural resources was largely extractive and exploitative.

The chimney, with a partially-enclosed hearth, was finally invented in the Medieval Period. This fireproof, vertical channel (with a smokestack reaching above the level of the roofline) created draft, drawing smoke safely up from an indoor fire and preventing hot sparks from drifting into the dry, flammable thatch. This invention vastly improved the lives and health of rural peasants, and many keeps, castles and manors made use of chimneys so they could burn the cheaper, readily available coppice wood instead of expensive charcoal.

In cities, however, charcoal remained a crucial fuel, particularly because of the lower transportation and storage costs. Braziers could be placed in any room, and provide heat, using charcoal, whereas smoky firewood required the expensive construction of a chimney. Retrofitting cities to include chimneys took centuries to catch on.

Likewise, charcoal remained a necessity for in smelting, in forges, in kilns for pottery, slaking lime, glass work and other high-temperature industries.

Note that charcoal fires do produce carbon monoxide and must therefore only be used in well-ventilated spaces. Drafty castles and wattle huts allowed the poisonous gas to escape safely, but modern readers should take precautions if planning to experiment with indoor charcoal braziers.

To read about the methods for making charcoal, see the four follow-up posts about charcoal: Charcoal Clamps, Pit Kilns, Charring Fence Posts, and Micro-charring.


Sources:

1) “Simple Technologies for Charcoal Making.” Food and Agriculture Organization of the United Nations, FAO Forestry Paper 41, 1983.

2) “Undated pits and ditches and Mesolithic/Neolithic, Iron Age and post-Roman finds.” Norfolk Heritage Explorer, Monument Record 37396, 2006.

3) Dolan, Brian. “The Social and Technological Context of Iron Production in Iron Age and Early Medieval Ireland c. 600 BC – AD 900.” University College Dublin Thesis, 2012.

4) Groenewoudt, Bert and Spek, Theo. “Woodland Dynamics as a Result of Settlement Relocation on Pleistocene Sandy Soils in the Netherlands (200 BC-AD 1400).” Rural Landscapes: Society, Environment, History, Vol. 3(1), p.1, 2016.

5) Warren, Graeme; McDermott, Conor; O'Donnell, Lorna; Sands, Rob. “Recent Excavations of Charcoal Production Platforms in the Glendalough Valley, Co. Wicklow.” The Journal of Irish Archaeology, 21 : 85-112, 2012.

6) Tsakirgis, Barbara. “Fire and smoke: hearths, braziers and chimneys in the Greek house.” British School at Athens Studies, Vol. 15, p. 225-231, 2007.

7) Veal, R. “The Wood Fuel Supply to Pompeii Third Century BC to AD79: An Environmental, Historical and Economic Study Based on Charcoal Analysis.” University of Sydney, 2009.