Environmental Engineering Reference
In-Depth Information
metals (copper, tin, and lead) and later, and more important, iron: charcoal was the
only source of carbon used to reduce ores in preindustrial societies (Biringuccio
1540). Metallurgical charcoal is made from wood (reeds and cotton stalks were also
used) by pyrolysis, that is, heating in the absence of oxygen; its energy density is
twice that of air-dried wood or dry straw and 20%-35% higher than that of
common bituminous coals. Traditional charcoal production in simple earthen kilns
(wood piles covered with mud or turf) was extremely wasteful. Typical yields were
only between 15% and 25% of the dry wood charge by weight, and in volume
terms, as much as 24 m
3
, and rarely less than 10 m
3
, of wood were required to make
a ton of charcoal.
The 1:4 ratio is perhaps the best conservative approximation of large-scale
charcoal:wood yields, and with 15 MJ/kg of air-dried wood, traditional charcoal-
making thus entailed an energy loss of about 60%. But the burning charcoal could
reach 900
0
C, and the use of a forced-air supply (using hollow bamboo or wooden
tubes, later bellows) could raise the temperature close to 2,000
°
C, well above the
melting point of all common metals. Copper was the i rst metal that was produced
in relatively large amounts; it melts at 1,083°C and lead melts at 232°C, while iron
melts at 1,535°C; consequently, the substitution of bronze by iron brought a major
increase in charcoal demand.
Forbes (1966) estimated that some 90 kg of wood were needed to smelt 1 kg of
copper; assuming 900 kg of wood per tree in 40 years and 300 trees/ha (or about
6.75 t/ha a year), a smelter producing 1 t Cu/day would have required nearly 5,000
ha a year, and if such operations persisted in a region for a century, the cumula-
tive demand would have reached 500,000 ha of forest. An estimate for Cyprus
(Constantinou 1982) showed forest demand equal to 16 times the island total area,
a calculation that shows a “tendency toward exaggeration” (Williams 2006, 78).
But even if we consider that the demand extended over a period of many centuries,
or even two millennia (and hence a large part of it would have come from regener-
ated growth), there is no doubt that copper smelting caused extensive deforestation
around the Mediterranean.
Iron smelting eventually became much more efi cient, but the metal was produced
in much larger quantities. The smelting was done i rst in only partially enclosed
hearths and yielded small (around 50 kg) masses (blooms) or slag-contaminated
metal that could be forged but not cast. Iron production in bloomery hearths and
forges needed as much as 20 kg of charcoal (up to 80 kg of wood) per kilogram of
hot metal (Johannsen 1953). These specii c needs declined with the adoption of
simple shaft furnaces that were operated in Europe, India, and China (Chakrabarti
