Depositional processes

Depositional processes of plant macroremains can be rather complex. Only charred material is usually recovered from dry mineral sites, for which charring becomes the only factor for the preservation or destruction of this material. The effects of charring on seeds and fruits have been approached in a relatively

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large number of publications. Most of them aimed to establish the necessary charring conditions for one seed or fruit to get carbonized and keep a recognizable form, as well as under which conditions some fruit or plant parts (for instance, chaff remains) turn into ashes (Wilson 1984, Boardman & Jones 1990, Gustafsson 2000, Jacomet et al. 2002, Jacomet 2003, Wright 2003, Wright 2005, Märkle & Rosch 2008, Sievers &

Wadley 2008). Thus, understanding the heating treatment that generated past assemblages was not always the aim of these experiments.

The work of F. Braadbaart set a milestone on this issue (see, for instance, (Braadbaart et al. 2004, Braadbaart 2004, Braadbaart 2008). It was mainly devoted to the testing of reflectance and digital image analysis as methods to get a reliable approach to the temperature and heating rates that were experimentally applied to single grains using a muffle furnace. The precise descriptions of the morphological changes of the grains that were observed by Braadbaart can easily be recorded during everyday archaeobotanical work. If one could demonstrate that the proportions in which those traits appear correlate with a particular heating treatment, such work could be carried out by any archaeobotanist (thus, avoiding the expensive reflectance analyses). With this idea in mind, I decided to carry out some experiments. I must here advance that I could not reach the ideal target during this work. The parameters that I will propose to guess the heating treatment that took place in the past are based on an extremely low number of experiments and I will largely rely on the results of the experiments carried out by F. Braadbaart. Despite this, it is still worth considering the results of my experiments because, to my knowledge, these are among the very few that were carried out with relatively large assemblages of grain, which make them more directly comparable to our archaeobotanical samples.

Six different sets of grain were put under the same heating treatment: the grains were put between two layers of sand (to create a relatively anoxic environment) in two aluminium trays and they were heated at 150ºC for 20 minutes, then at 180ºC for 60 minutes, at 200ºC for 40 minutes and finally at 250ºC for 45 minutes (which could be described as a low heating rate in Braadbaart’s terms). These sets consisted of a mixture of naked wheat (Triticum aestivum s.l.) grain (and some chaff), a small number of barley grains (contaminants in the wheat fields), and lentil seeds. The total amount of items ranged from 875 to 5111 and the proportion of lentils within the total went from 21,09 up to 45,95%. Consequently, naked wheat was always the principal component of the assemblages. The overall results obtained can be observed in Fig. 3.35 and 3.36 (for more detailed results see Annex II).

The obtained results are, to some extent, diverse (Fig. 3.35). After the heating treatment, the proportion of charred naked wheat went from 66% to 100%, similar for lentils. The proportion of charred material seems to depend on more variables besides time and temperature. These would include the total size of the assemblage and the composition of the assemblage. The first one could be considered rather obvious, while the second one was not so much foreseen. On the one hand, the two smallest assemblages showed the lowest proportions of charred grain, which was rather surprising. On the other hand, these are the two samples with a higher percentage of lentils. It is possible, then, that the presence of lentils affects the heating treatment of the assemblage and makes it more difficult for the cereal grains to become charred. It is interesting, though, to see that there is always some proportion of uncharred material, which brings us back to the classic statement by Wilson: “(…) Any single heating episode in antiquity is thus likely to have left some seeds uncarbonized, whilst carbonizing others, while others again will have been burned to destruction” (Wilson 1984).

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Despite these observed variations, though, the morphological modifications of the grains produced by the heating treatment were rather homogeneous. Between 13 and 17% of the cereal grains were popped, while between 2 and 7% had produced protrusions (see Fig. 3.36). The proportion of grain aggregates was also low, less than 7%. I could not find an official terminology to describe the effects observed in lentils.

Consequently, I had to create two new concepts (Antolín 2012): seeds with a cracked testa and seeds with opening cotyledons (the edges of the cotyledons seem to fold outwards or open as a result of charring) (Fig.

3.38). These evidences were rare under the present heating treatment and one should expect more cases at higher temperatures.

Fig. 3.35. Proportion of charred and uncharred grain and chaff from the 6 experimental assemblages exposed to the same heating treatment.

Based on these results and those published by F. Braadbaart, it was decided to put forward a preliminary table of equivalences (see Fig. 3.37). At the moment, it seems possible to distinguish several stages. At 250ºC one should get low percentages of popped grains and grains with protrusions. Between 300 and 350ºC, the first grains with concave flanks appear in low proportions at a low heating rate and they become more frequent at a high heating rate. Above 350ºC the preservation of some seed and fruit remains can become problematic. Determining the heating treatment above this temperature seems only possible through the use of reflectance analysis. This method could actually be complementary to the use of grain morphology modification, since it seems that better results are obtained at above 370º C (Braadbaart 2008).

NUMBER OF

Fig. 3.36. Effects on grain general morphology or surface originated by the experimental heating treatment.

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Fig. 3.37. Summary of the criteria that have been followed to make a guess on charring conditions (LHR:

low heating rate; HHR: high heating rate).

Furthermore, some additional insight into heating treatment was intended by trying to determine the space where the assemblage was located when the heating treatment was applied. When grain assemblages are located within a confined space (e.g. a pot), they usually adopt abnormal shapes (see Antolín & Buxó 2011c). At the same time, these charring conditions can also end up generating large lumps of aggregated grains. Grain aggregates are also produced when grains are charred in a more open space but these aggregates are weaker because they are produced by the explosion of the endosperm of the grain, which sticks to the nearby material before solidifying when contacting with air (see Fig. 3.38). Nevertheless, these aggregates are weaker and they would probably not survive in an archaeological context.

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Fig. 3.38. Pictures of the different effects produced by charring of naked wheat and lentil grains: a. Naked wheat grains with different degrees of charring; b. Barley grains with different degrees of charring and distorsion; c. Popped grain of naked wheat; d. Naked wheat grains with protrusion; e. Aggregates of naked

wheat grain; f. Lentil seeds showing a cracked surface and open cotyledons after charring.