Why charcoal is burnt in a hearth

Thus the carbon is left to turn into charcoal. This is why when charcoal is burned, only carbon dioxide is released and there is neither smoke nor smell. What does organic mean? What is the white powder strewn onto the streets when snow falls?

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Dupin et al. Heat-induced reddening is caused by the dehydroxylation of iron hydroxides, which induces their direct recrystallization to hematite, although lepidocrocite transitions to maghemite in an intermediate phase Cornell and Schwertmann, Although the heat from the charring varies during the operation of charcoal hearths Hollingdale et al.

However, the post-burn formation of maghemite or magnetite in our studied profiles by microbial processes cannot be excluded Maher, In addition, during the charring process, the iron-bearing minerals within charcoal hearths experience a reducing environment in the presence of carbon monoxide.

Hematite can be transformed to metallic iron in this reducing environment. Hence, the chemical environment in the charcoal hearth favors the formation of metallic iron from iron oxides and iron hydroxides.

This finding is supported by comparisons between pre- historic hearth remains and reconstructed experimental fires in which a change in soil color directly below the hearth was detected and associated with post-fire pedogenic processes Liedgren et al. Hence, the dating of the RCH operation by OSL might be a possibility in the absence of large enough charcoal pieces for dendrochronology or if radiocarbon dating is limited due to the 14 C-plateau.

Figure 6. Heat-induced transformation of iron hydr- oxides in soil environments, magnetism and saturation magnetization compiled from Schwertmann , and Cornell and Schwertmann Figure 7. Pedostratigraphy and influence of pyrolysis on the topmost few cm of the buried topsoil at RCH in. The photo on the left is a detail of the profile shown in Figure 2c. However, these thermally induced changes in soil properties are restricted to a 2-cm-thick contact zone at a profile depth of 28—30 cm.

Below this zone, in the underlying few centimeters of the profile and at depths of 30—33 cm, the temperature was too low to transform the iron hydr- oxides; however, it was sufficiently high to cause the combustion of SOM, as indicated by the lighter color. No indication of the combustion of SOM was found below a depth of 33 cm.

In a sandy-loamy topsoil beneath a fireplace described by Fenn et al. Experimental studies of heat transfer from fires into the underlying soils have shown that pore spaces act as a heat shield Aldeias et al. Furthermore, ash and organic matter can also reduce the transfer of heat from fires into soils. Moreover, the presence of soil moisture below an active charcoal hearth hampers the transfer of heat into the soil Powell et al.

We attribute the lack of SOM in the II fAh horizon between 30 and 33 cm depth to in situ combustion related to the heat of the pyrolysis occurring in the charcoal hearth above. Our micromorphological analysis shows no signs of the translocation of charred SOM from the charcoal hearths to the II fAh horizons like that reported for forest fires or fireplaces Fenn et al.

The II fAh horizons contain mainly amorphous SOM, as well as some charred plant fragments, but the SOM and pyrogenic organic matter are evenly distributed in the fine soil. Additionally, no enrichment in SOM or pyrogenic organic matter is detectable in previously identified accumulation zones, such as those near the contact points between sand grains.

The architecture of the RCH, the soil stratigraphy outside of the RCH, and our laboratory analysis suggest that the colliers placed the wood stock atop an undisturbed soil profile without mechanical preparation of the site with spades or hoes. During the operation of the charcoal hearth, the topsoil below was affected by the heat from pyrolysis, causing an alteration in the minerals and a combustion of the SOM.

The magnetic susceptibility of the RCH is higher than that in the non-fire affected substrate below, and the reddish color of the substrate suggests a thermal alteration on the contact zone between the RCH and the soil below. Because the RCHs in our study area were in use between the mid-sixteenth and the mid-nineteenth centuries, the dendrochronological dating of the charcoal suggested that was the year of operation for RCH Raab A. However, the anthropogenic origins and the inherited and new chemical and physical characteristics of these soil sediments jilC are ignored in this classification.

The implementation of a jM horizon would consider the anthropogenic raised charcoal content and indicate the anthropogenic nature non-erosive translocated soil substrate of this sediment. Hence, the presence of industrial-scale produced charcoal classifies the charcoal hearth soils as Technosols.

The architecture of our studied RCH sites indicates that due to the flatland topography together with a sandy soil, the operation of the RCH clearly differs from that in the mid-mountain ranges, where on slopes remarkable platform preparation and material movements has taken place. Although the stratigraphy and the coloration of the RCH soils suggest the development of pedogenic horizons, we found no indication of pedogenic translocation or transformation processes besides the enrichment of PyOM in the substrate of the RCHs and decomposition of SOM below the RCHs by combustion.

However, as we found no indications of the formation of pedogenic horizons, we suggest that, due to the young age of the RCHs, together with the sandy texture and the warm and temperate climate, soil development on the RCHs is still incipient. Thus, the technic character of the substrate is dominant. Due to the heating of the substrate, it might be possible to date the RCH operation by OSL in the absence of large enough charcoal pieces for dendrochronology or if radiocarbon dating is limited due to the 14 C-plateau.

The studied charcoal hearths contain The charcoal that is present in the RCH substrates occurs in a wide range of grain sizes, from large fragments up to two decimeters in size to particles that are finer than sand.

Because the specific surface area of these charcoal pieces affects soil chemistry and soil processes, further studies are required to examine the grain-size distribution of the charred organic matter and its chemical characteristics.

Even though charcoal hearths are common and widespread in Germany, the soils at these sites are rarely considered in terms of pedology, especially on soil maps and in soil taxonomic systems. Although the German Guidelines for Soil Mapping are designed to classify soils according to pedogenetic processes, the lack of a soil class for technic substrates and the minimum horizon thickness of 40 cm required to classify a soil as a Kolluvisol considerably hinder the classification of soils developed from anthropogenic sediments, such as charcoal hearths.

The classification of soils in charcoal hearths as Spolic Technosols Arenic according to the WRB takes into account the anthropogenic genesis and the technic character of the substrate, which clearly distinguish these soils from the surrounding soils. Therefore, we suggest restating the diagnostic criteria of the M horizon in the German Guidelines for Soil Mapping to allow the classification of SOM-rich soil sediments as jM to Kolluvisol.

FH conceived the presented idea and carried out the sampling and the analyses. All authors discussed the results and contributed to the final manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We're grateful to the editor Balwant Singh for handling and to the referees for their suggestions significantly improving the manuscript.

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