Heat impact and soil colors beneath hearths in northern Sweden

Publication date: March 2017Source:Journal of Archaeological Science, Volume 79
Author(s): Lars Liedgren, Greger Hörnberg, Tord Magnusson, Lars Östlund
The archaeological remains of Sami hearths in boreal and subarctic areas of northern Sweden are common finds. Greater understanding of the effects of heat on soil coloration will facilitate interpretation of the main purpose of these hearths, for example as heat sources or for cooking or other processing, and whether they were used seasonally for long or short periods of time. We therefore studied effects of heat on the coloration of natural B-horizons beneath traditional Sami hearths using three approaches: firing with dry pine wood in experimental hearths and measuring the temperature at various levels beneath the hearths; laboratory heating of B-horizon soils at different temperatures (200–900 °C) in a muffle oven and; measuring soil color changes in terms of RGB-values, and comparing the experimental results with soil profiles beneath real hearths used by nomadic Sami reindeer herders in northern Sweden. The study shows that the temperature reached beneath hearths strongly depends on the type of fuel used and the length of firing. The temperature can rise rapidly in upper layers of the soil but it takes considerable time for heat to penetrate 20 cm below the hearth surface. Our experimental firings, for 10 h on three consecutive days and for 72 consecutive hours, resulted in bowl-shaped areas of discoloration, with strong red coloration (rubification) towards the edges and dark grey/brown discoloration in the middle of the hearths in both tests. After laboratory heating, soil samples darkened during temperatures of 200–300 °C, and rubification at 250–350 °C depending on the amount of humus in the soil. The RGB analysis showed a steady increase in rubification from 300 °C, peaking at 750–800 °C. We believe that the rubification is caused mainly by transformation of iron compounds to maghemite and hematite and that the quantity of hematite is determined by temperature and not by time.Excavations of ancient hearths also revealed examples of bowl-shaped discoloration in B-horizons deeper than 20 cm. These discolorations had a rather uniform red tone with no dark areas. This suggests that the darker areas, probably colored by reduced iron and not by charred particles, could have been altered over time. The main conclusion is that rubification in B-horizons beneath hearths can arise after a relatively short period of firing but-bowl shaped areas of deep coloration can only arise, in boreal and subarctic areas, when hearths have been fired heavily and continuously for long periods of time, indicating winter use.