Geology Reference
In-Depth Information
Given Colonel La Rosa
s explanation of human cremation at the Tschudi burn, then calcium and other elements
found in the reconnaissance geochemical traverse were of interest. This is because human bone, or hydroxapatite,
contains mainly calcium and lesser amounts of phosphorus, potassium, and boron. Concentrations of these
elements are elevated within the burned area as compared to background concentrations from the soil outside
the burned area (Table 4.1.1). And, of these elements, phosphorus is especially significant because, as shown in a
court case, chemical analyses of disputed human cremains contained calcium, which is consistent with human
bone, but did not contain phosphorus, a definitive element in the bone (Warren and Van Rinsvelt, 2000). However,
as previously discussed, there were several possible sources for the above-background concentrations of calcium
and phosphorus.
'
Sulfur in the Soil
S ulfur, which is not a normal component of bone or adobe, was also higher in the soil samples from the burned area
(Table 4.1.1.); its presence suggested that coal may have been used as a fuel. In a modern example of the
significance of sulfur as a component of coal, acid rain is produced when sulfur, contained in pyrite (FeS 2 )in
coal, is volatized from coal burned in a coal-fired power plant. The volatized sulfur then combines with atmo-
spheric water to produce sulfuric acid (H 2 SO 4 ) which slowly attacks marble or other carbonate dimension stone. In
order to reduce acid rain and the pollution caused by the sulfur released during coal burning at power plants, it is a
common industry practice to add calcium, as limestone, to the burning coal so that the sulfur combines with the
calcium to precipitate synthetic gypsum (CaSO 4 ·
2H 2 O), which thereby reduces the release of sulfur to the
atmosphere. Therefore, a coal-fueled fire with calcium from an as yet unknown source becomes a reasonable
explanation for the elevated sulfur content of the Tschudi burn soil samples.
In studies of fuels used for metallurgy, coal is not a preferred fuel for smelting because of its sulfur content, whereas
wood charcoal was preferred because of its low sulfur content. In China, coal was used for smelting because of
limited timber for charcoal production combined with the regional availability of low-sulfur coal (Tylecote, 1980,
p. 204; Craddock, 1995, p. 196). Chemical analysis of some primitive European iron artifacts contained sulfur,
demonstrating that coal had been used, though rarely, as a smelting fuel (Tylecote, 1962, p. 191). The presence of
sulfur in the soil samples from the Tschudi burn pointed to coal as the fuel.
Fuel Ash Chemistry
C arbonized plant material at the Tschudi burn and subsequent compaction calculations led Lechtman and Moseley
(1975, pp. 140, 156) to conclude that a local plant,
, was the fuel for the Tschudi burn. However, the lab
temperature of 1320°C inferred for the Tschudi burn suggested a fuel that burned much hotter than plant material.
T. latifolia
Charcoal was the preferred fuel for ancient metallurgy and, in ancient Peru, charcoal from the algorrobo tree was
the most commonly used fuel for smelting (Shimada et al., 1983, p. 38; Shimada and Merkel, 1991, p. 81).
However, whatever is locally available, whether it is wood, coal, or plant material, may also have been used as a
fuel (Needham, 1980, p. 521).
The use of coal, wood, charcoal, or plant material as a fuel is inherently a destructive process, and therefore,
only chemical analysis of the remaining ash or burned residue provides direct evidence of the fuel that was burned.
Ash from several commonly used ancient fuels such as coal, charcoal, and peat was analyzed by Tylecote (1980,
p. 205), but locally used plant fuels, such as
Tillandsia
, were not included. By using SiO 2 and other oxides,
Tylecote easily differentiated coal ash (25
-
53 wt% SiO 2 ) from plant/charcoal ash (8.0 wt% SiO 2 ) and other fuels
(Table 4.1.2).
Major oxide and trace element data from 14 coal ash samples, collected as part of a regional coal quality inventory
(Finkelman et al., 2001; Brooks and Willett, 2004; Brooks et al., 2006) served as a database (Table 4.1.3) for
comparison of known Peruvian coal ash composition with the unknown ash from the Tschudi burn. Three samples
from the 10
-
20 cm ash exposed in the research trench were analyzed (Table 4.1.4 ). An average of the data for the
14 samples in Table 4.1.3 and the data from samples of algorrobo-wood ash are also included in Table 4.1.4.
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