Controlled muffle experiments—visual and structural changes
A summary of the visual and mineral transformations from the controlled muffle heated shells is shown in Table 1. Results of thin section analyses of selected heated shells are shown in Fig. 2. For both species, obvious alterations are visible at 550 °C, even after very short heating episodes of 5 min. These microscopic alterations are seen in the change from high birefringence (3rd order interference) colors associated with aragonite to duller interference colors, which are mineralogically associated with the complete transformation to calcite (see below). At higher temperatures of 700 °C, the shells tend to present several macro and microscopic vertical cracks as well as parallel fissures. The latter tend to be well developed for the thinner peppery furrow specimens (Fig. 2).
Since no differences were observed between mineral transformations related to heating durations of 5 versus 20 min, only the FTIR spectra associated with 5-min durations are presented here. Figure 3 also provides a comparison of spectra from FTIR transmission and FTIR-ATR, showing that these two collection modes produce similar results. Only a peak distortion and shift are observed for the FTIR-ATR spectra when compared with transmission spectra.
In respect to mineralogical transformations, results from unheated shells present FTIR spectra of aragonitic calcium carbonate (CaCO3) similar to published data (Llave et al. 2006; Loftus et al. 2015; Weiner 2010). Despite some inconsistency in the literature on the exact positioning of aragonite and calcite peaks obtained by conventional FTIR methods (see, for instance, Andersen and Brecevic 1991), both FTIR spectra of the unheated samples present the typical vibrational modes of the C-O band of carbonate ion (CO32−) for aragonite: the ν3 asymmetric stretch at 1475 cm−1, the ν1 symmetric stretch at 1082 cm−1, the ν2 out of plane bending at 860 cm−1 and the ν4 in plane bending doublet at 714 and 700 cm−1 (respectively, at 1465 cm−1, 1082 cm−1, 856 and 712 and 700 cm−1 in ATR). With heating, the transformation of aragonite to calcite is visible by the appearance of the peak at 876 cm−1 (the ν2 out of plane bending for calcite) and the broadness of the peak at 1475 cm−1—a stage designated here as “aragonite transforming to calcite”. This transformation starts at different temperatures in the analyzed shells, specifically around 250 °C for cockle and around 350 °C for peppery furrow. Complete transformation to calcite occurs at 500 °C for cockle and 400 °C for peppery furrow, identified by the present of only the typical vibrational modes of the C-O band of carbonate ion (CO32−) for calcite: the ν3 asymmetric stretch at 1427 cm−1, the ν2 out of plane bending at 876 cm−1, and the ν4 in plane bending at 714 cm−1 (respectively, at 1405 cm−1, 874 cm−1, and 712 cm−1 in ATR).
It is interesting, therefore, to note that for cockles the transformation of aragonite into calcite starts at a lower temperature, but the complete transformation to pure calcite also takes longer to occur when compared to peppery furrow. Cockles heated to 450 °C present all the characteristics calcite peaks, but traces of aragonite are still visible by the presence of the additional peak at 860 cm−1 (in a stage designated here as “calcite with traces of aragonite”), whereas peppery furrow shows already a complete transformation to calcite at the same temperature (Fig. 3). At 900 °C, a peak at 3640 cm−1 for both shellfish is also visible (see supplementary information). This peak can be attributed to the O-H stretching of calcium hydroxide (Ca(OH)2), produced by the heating of calcite that transformed into quicklime (CaO) and CO2, which afterwards reacts with atmospheric moisture to form calcite and calcium hydroxide, Ca(OH)2 (Toffolo and Boaretto 2014).
Fire cooking experiments
Roasting shellfish: temperatures and charcoal analyses
Table 2 summarizes temperature values, durations of heating, and results of shell mineralogy from our field cooking experiments with roasting shellfish. Temperature changes associated with the three main types of experiments are illustrated in Fig. 4—all other temperature readings and charcoal analyses summary are available in the Supplementary Information.
Since our experimentation protocol was more actualistic in nature than highly controlled experimentation (see discussion in Lin et al. 2016), there is considerable variation in temperatures, even in repetitions of the same experimental type. It should be made clear that our goal with these repetitions was not to exactly replicate the same experimental setting but instead provide some degree of variation for each type of cooking events. Consequently, the range of experimental durations and temperatures achieved result from dependent variables not being maintained constant (such as input temperature variations within each wood fire, or amount of time the fuel was left to burn, etc.).
Overall, and despite fire temperature fluctuations, readings of the thermocouples positioned at the base of the shellfish being cooked never reach values above 170 °C, with average maximum temperatures around the 95 °C (Table 2). The experiments with pebble cuvette systematically reached the highest and stable cooking temperatures, whereas the fast fires kindled with pine needles and embers had the lowest registered cooking temperatures at the base of the shellfish. Interestingly, it is in the fire below and fire above experiments that we observed macroscopic alterations of some shells, with brown to black colored domains and occasional fractures, namely in the uppermost exposed area of the shells’ umbos section. Such modification seems to result from localized hot spots where the shellfish was in direct contact with flames. For instance, particularly in the experiments where pine wood fuel was used (Fa1 and Fa2), it was hard to maintain homogeneous temperatures throughout the entire shellfish surface, and in some areas, the direct contact with embers and burning wood was more accentuated than in others. It is important to note that temperature of the fires, which was in contact with the shells, was high enough to reach the threshold for aragonite to calcite transformation, and this has important implications for shell mineralogical transformations as it will be discussed below. In the pebble cuvette fires, the shellfish were not in direct contact with fire, rather they were roasted on top of hot quartzite and quartz pebbles. Macroscopic color alterations were not observed in these shells, whereas the underlying pebbles did show significant color alterations, with a few pebbles fracturing with each experimental run (Fig. 5). In experiment C2 (Ck), we can observe that the pebble’s surface reached a maximum temperature of 647 °C in response to fire temperatures commonly above 700 °C (Fig. 4a).
In terms of the amount of thermal alteration expected in the subsurface, our temperature readings just below the pebbles show that the amount of heat transfer to ∼2 cm depth is directly dependent on fire duration, with higher temperatures accumulating during longer events. We obtained maximum temperatures of 307, 158, and 182 °C for experiments lasting for 124, 31, and 40 min, respectively (Table 2). This range of temperatures shows that higher substrate temperatures can easily be reached in events lasting longer than the fires performed here (Aldeias et al. 2016). Consequently, thermal alteration can occur in the sediments underlying a pebble cuvette cooking structure (March et al. 2014) and of course with fire below type of experiments. In contrast, substrate temperatures were constantly at values below 100 °C in fire above experiments (Fig. 4c, d), which entails little or no alteration of underlying deposits for similar fire types and durations.
In terms of the fires themselves, the combustion of P. pinaster fuels recorded substantial variance in maximum temperatures. These ranged from 922 to 488 °C (an average maximum temperatures of 797 °C) in experiments with pebble cuvette, fire below, and fire above with pine wood and lower maximum average temperatures of 417 °C in the fire above with pine embers (Table 2). This combustion process resulted in wood charcoal alterations in the form of vitrification and cracks (see Supplementary Information). In high levels of vitrification, the intense alteration observed lead to an almost complete elimination of the original cell morphology. Such levels of vitrification can significantly bias the observation of the cellular structure (e.g., Gale and Cutler 2000; Prior and Alvin 1983; Py and Ancel 2006). Vitrification tends to be more visible and intense in radial and tangential sections. Low to high levels of vitrification were observed on samples from all experiments, with low vitrification levels meaning that vitrification is identified but there was not a substantial damage to the cellular structure. Low levels of vitrification were particularly observed in charcoal fragments from one of the pebble cuvette runs (first repetition of C2 (Ck)), though this was not the case for all the pebble cuvette experiments, and therefore probably linked with probability sampling and intrinsic experimental variation. What this data does highlight is that repetitions of the same experimental procedures are very important in actualistic experiments such as the ones commonly performed in experimental archaeology. Finally, the presence of cracks on charcoal is evident in at least one fragment from each experiment. Although this alteration is not as frequent as vitrification, it is still abundant and cracks tend to co-occur in fragments where vitrification is also present.
Roasting shellfish: mineralogy-associated transformations
The results of the FTIR analysis are summarized in Table 2. Shells analyzed from the pebble cuvette experiments (C1-C3), where the shells did not come into direct contact with the fire, were still composed solely of aragonite. Similar results were consistently obtained from the experiments with fire above using pine embers (Fa5 and Fa6) where the maximum fire temperature was lower than in all other experiments (average of 417 °C, see Table 2), although since this temperature should allow at least partial mineralogical transformation, this result might be due to small sample size.
A more complex picture emerges from the experiments using fire below and fire above with pine wood or needles as fuel, with the mineralogical compositions of the shells varying in a number of these experiments. Several of the analyzed shells retain an aragonite composition, while others are already transformed to calcite. It is interesting to note that in experiments Fa2 and Fb, a few of the analyzed cockle shells have spectra associated with aragonite transforming to calcite, showing that such specimens had to be subject to temperature ranges roughly between 250 and 350 °C, as shown by our controlled muffle heating results discussed above.
Roasting shellfish: what are the microstratigraphic traces?
In Table 3, we synthesize the macro- and microscopic observations concerning the sedimentary and taphonomic signatures produced during our shellfish roasting experiments.
Of the three types of cooking tested, only the fire below experiment produced a microstratigraphic arrangement typical of in situ hearths, that is, a thermally altered substrate overlain by fire residues with abundant charcoals and ashes. For the most part, however, the employed cooking methods involved the partial or complete removal of the majority of fire residues prior to cooking (see Fig. 1). Even in the fire below experiment, part of the embers and ashes were pushed aside in order to create a central area where the shellfish was placed. This action resulted in an external rim with a relatively thicker accumulation of chaotically organized, cm-sized pieces of carbonized wood tissues, ashes, and charcoals. Locally, these deposits may or may not overly altered substrate, depending on how far they were dispersed from the original location. At the center of the hearth, that is, where the shellfish were cooked, the accumulation of fire residues is thinner (1–2 cm thick), though with some degree of structural organization as seen microscopically by the presence of articulated ashes overlying a thin rubefied substrate (Fig. 6).
In all other experiments, the cooking procedure entailed the removal, spreading, and dumping of fire residues outside the actual cooking area. In the case of the pebble cuvette hearths, the large pieces of carbonized wood and charcoals were removed and the remaining finer fraction blown out in order to exposed the surfaces of the hot pebbles. We could observe that smaller size fractions of ashes and charcoals had accumulated and rested in the spaces between pebbles, namely at the deepest part of the basin-shaped depression (Fig. 5). The actual fire residues were quickly dumped nearby this feature and presented a chaotic organization of large-sized components in a loose, open structure, overlaying a non-altered substrate. The “path” of deposits removal could be traced by the spreading of finer ashes from the pebble feature to the main dumping area. This type of fragile deposits and organization, however, are unlikely to survive archaeologically. The most conspicuous element of this type of cooking is obviously the structuring of the cooking area with the sedimentary depression and arrangement of the pebble cover. During our several repetitions, only a few of the pebbles were thermally fractured, whereas the central pebbles typically showed some type of color modifications, from reddish to black hues (Fig. 5). Although we could not sample these deposits for micromorphology, the temperature readings of the substrate directly underneath the cuvette do show that these deposits reached temperatures between 200 and 300 °C. This type of cooking has, therefore, the potential to produce some thermal alteration of underlying artifacts and sediments (e.g., resetting of thermoluminescence signal, charring of organic matter, rubefication), and such modifications may be identifiable in archaeological contexts (Aldeias et al. 2016; Canti and Linford 2000; Linford and Platzman 2004; March et al. 2014).
One of the most striking aspects of the fire above cooking experiments is the lack of association between the fire, its residues, and the surrounding deposits. These fast fires kindled on top of shellfish were fairly short-lived—otherwise, the mollusk was not cooked but rather burned. Subsequently, all the ashes and charcoals were quickly swept and blown out from the top of the shells and these were removed from their cooking location and consumed. Therefore, in all of our experiments, there is little or no visible macro- or microscopic alteration of the substrate underlying the cooking location. The swept out fire residues tended to be fairly dispersed, namely in the case of the fires fueled with the fragile pine needles. Some thicker accumulation of charcoals and partially carbonized wood pieces overlying unaltered substrates were produced in the pine wood-fueled experiments, namely in those were the fires were not left to burn to completion.
In the thin sections from fire above experiments, there are few diagnostic features, especially if we consider that the shells themselves would have been removed from their cooking arrangement and probably tossed somewhere else after the mollusk was eaten. Therefore, only the discrete presence of charcoals, charred pine needles, and rare domains with calcitic wood ashes indicate the presence of fire activities. In contrast, however, it was during these fire above experiments that significant physical and color modifications were produced in the shells themselves, since these were directly exposed to heat from the superimposing fire. Macroscopically, we could see that quite a few shells tended to have burned black to brown domains, with some of the more fragile peppery furrow shells also showing cracking and fissuring (Fig. 5). These cracks and burning is preferentially located in the uppermost umbos sections, which in our cooking arrangement were positioned upwards and in direct contact with the fires.