image/svg+xml129 X/2/2019 INTERDISCIPLINARIA ARCHAEOLOGICA NATURAL SCIENCES IN ARCHAEOLOGY homepage: http://www.iansa.eu Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) Freek Braadbaart a † , Anaya Sarpaki b* , Harry Veld c , Bertil van Os d a Faculty of Archaeology, Leiden University, Einsteinweg 2, 2333CC Leiden, Netherlands b Independent scholar, 137 Tsikalarion Rd., Tsikalaria, Souda,73200 Crete, Greece c Deltares, Daltonlaan 600, 3584 BK Utrecht, Netherlands d Cultural Heritage Agency of the Netherlands, Conservation of Built Monuments and Archaeology, Smallepad 5, 3811 MG Amersfoort, Netherlands 1. Introduction The Bronze Age settlement of Akrotiri is situated on the southern coast of Thera, a volcanic island, belonging to the Cyclades, in the Aegean Sea (Greece). From an archaeological point of view the Bronze Age is in this region divided into three major periods i.e. Early Cycladic (EC) (3000–2100 BC), Middle Cycladic (MC) (2100–1600 BC) and Late Cycladic (LC) (1600–1100 BC). The huge Plinian (Minoan) eruption buried and at the same time preserved the settlement by layers of hot tephra ejected from the volcano. The date of the eruption has been much debated in the literature and has led to a dispute between the approaches of conventional archaeology and scientifc archaeology. Based on recent information a late seventeenth BC date seems now to be accepted (see among others: Manning et al. , 2014, p.1176; MacGillivray, 2014). The ejected tephra layers are divided into a precursory phase followed by four main phases of the eruption each with its characteristic composition (Figure 1). Only the precursory and the two following phases are present in Akrotiri due to their erosion. Many studies deal with these phenomena and the reader is referred to these studies and the references cited therein (Bond and Sparks, 1976; Heiken and McCoy, 1984; Druitt et al., 1989; Heiken et al., 1990; Sparks and Wilson, 1990; Friedrich et al., 1990; McCoy and Heiken, 2000; Friedrich and Heinemeier, 2009; Friedrich, 2013).Systematic archaeological excavations at Akrotiri started in 1967 by Marinatos and are continued to this day by Christos Doumas. The data shows that already since, at least, the Late Volume X ● Issue 2/2019 ● Pages 129–141 *Corresponding author. E-mail: a.sarpaki@gmail.com ARTICLE INFO Article history: Received: 24 th July 2018Accepted: 16 th December 2019 DOI: http://dx.doi.org/ 10.24916/iansa.2019.2.3 Key words: Thera – AkrotiriCycladic PeriodBronze Agecharcoal analyses refectance analyses burning temperaturetephra ABSTRACT The Bronze Age settlement site of Akrotiri, situated on the island of Thera in the Aegean Sea (Greece), developed during a period of over 1500 years into a fourishing city. This process started from the Late Neolithic through the Early and Middle Cycladic periods to the beginning of the Late Cycladic period when at a date in the late seventeenth century BC the city was buried and at the same time preserved by four phases of hot tephra released from the Minoan eruption. The investigations covering the archaeological excavations showed the remains of the fuels used for the fres lighted by the occupants of the city i.e. charred organic materials (COM) and ash. The volcanological part of the investigations studied the infuence of the heat generated by the hot tephra on the wooden construction material, incorporated in the buildings, when they were covered and heated by the tephra. By measuring the refectance on the charred organic material, the temperatures at which they were heated in the past were determined by applying the existing calibration curves. The results provided very interesting information about the function of the fres and the type of fuel resource selected by the occupants. The elemental analyses and the opal phytoliths from the ash provided additional information. The emplacement temperatures measured for the various phases of the hot tephra ranged from 310–340 ºC for phase one, from 370–410 ºC for phase two and is around 500 ºC for phase four. It is interesting to note that the black charred material appeared not always to be charcoal.
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 130 Neolithic, and continuously in the EC and MC periods the settlement was occupied and gradually developed into a fourishing city at the end of the MC (Doumas, 2012). At the time of the eruption and its destruction, in the beginning of the LCI period, the city was inhabited by a very afuent society that built multi-story buildings with magnifcent wall-paintings, drainage systems, paved street and even toilets.The archaeological excavations showed that during all the periods of occupation charred organic material (COM) was present and recovered from the settlements. During the EC and MC periods the heat necessary to char the organic material was generated by fres induced by the occupants for heating, cooking or other pyro-technological purposes, such as metal-working. The fres and heating continued in the LC I period before the city was buried by the volcanic hot tephra of the Minoan eruption. However, around 50 years preceding the Minoan eruption seismic activities in the form of earthquakes had ravaged the city several times in the MC period causing severe destructions to the buildings (Palyvou, 2015; McCoy and Heiken, 2000). As a result, the then present streets and ground foors were covered with a layer of debris consisting of building material: the volcanic destructing level (VDL), consisting of building material, with a thickness in the order of 1 to 2 metres. (Figure 2), and the city underwent major architectural changes as they Figure 1 . Stratigraphy of the Minoan eruption deposits. The wall shown in the lower part of the section represents the human settlements of Akrotiri archaeological site that interacted only with the frst and second phases of the eruption (fgure redrawn from McCoy, Heiken, 2000). Figure 2 . West House after the Minoan eruption with underneath the Volcanic Destruction Level (VDL). 0 5 m
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 131 fuel resource used. The results may further provide insights into the workings of a particular society and the landscape it operated in and will provide information pertaining to the control and application of heat (pyro-technology) as used in past societies.The objectives for volcanology are the emplacement temperatures measured on the charred material that will be compared with investigations performed on lithics and pottery by palaeo-magnetic methods. It is believed that the results of the investigations for these two disciplines e.g. volcanology and archaeology, will provide additional information about the site of Akrotiri in general. 2. Material The material for this study consists for the greater part of samples of charred organic material (COM), i.e. charred wood (charcoal) and charred olive stones. In addition, two samples of fuel ash and two samples of pumice were investigated. Regarding the charred material, a distinction has to be made between samples heated by human fres and those heated by hot volcanic material, the tephra. Samples heated by human fres were selected from the EC and MC periods as being representative of the whole collection of Figure 3. Mean refectance measurements (%Ro) taken from modern olive stones (open circles) compared to modern oak (solid squares) and pine wood (crosses) samples experimentally heated under reducing conditions. Plotted as a function of the fnal temperature (ºC). incorporated the rubble and the streets became higher than the ground foors. This made ground foors of some buildings, especially along Telchines Road, become basements or semi-basements, after the architectural restructuring of the town in MC III. Subsequently, the thin layer of precursory material from the Minoan eruption, consisting of hot pumice, covered and penetrated the VDL. Moreover, the buildings and their many wooden construction parts (Palyvou, 2005) became covered by the hot tephra. The result was that the wooden construction parts became heated and since they were shut of from air became charred as well. There is no indication that the city would have been on fre by the combustion of the wooden material, which confrms the charring. Thus, in addition to the heat sources of the fres made by the occupants of the city for diferent pyro-technological purposes, now a second heat source, in the form of hot pumice, aficted them. The important point is that charred organic material (COM) heated by two heat sources provides a unique opportunity to investigate questions that may arise from archaeological as well as volcanological origin. Archaeological information about the types of fres and fuel type, used by the occupants, could be obtained. Emplacement temperatures could be measured from the diferent phases of tephra ejected from the volcano.For this purpose, the heated and subsequently charred materials are studied under refected light, which makes it possible to determine the type of material and by measuring the refectance, the temperature at which it had been exposed to, in the past. As fuel for the fres various organic materials could have been used including wood, charcoal, olive-pressing residue and, probably, animal dung (Braadbaart and Poole, 2008; Sarpaki and Asouti, 2008). The material excavated from the older periods, i.e. EC and MC, was not afected by the heat of the hot volcanic material, so here fuel type and fre type can be investigated, per se, in those levels. However, the charred material found on the occupation/destruction foor, in the LCI period, could have been heated by both heat sources. The temperatures measured though on the charred wood used as building material may provide the possibility to measure the emplacement temperature of the relevant tephra.To facilitate this type of research, reference materials were used of experimentally-charred, modern samples (Braadbaart, 2004) all heated at a range of temperatures, with and without air, under controlled conditions in the laboratory. Hereby are the calibration curves of angiosperm and conifer wood prepared by Braadbaart and Poole (2008) and those prepared for olive stones (Braadbaart et al. , 2016) showing the refectance (%Ro) as a function of the temperature (Figure 3). To get more information related to the type of fuel ash from diferent modern fuel resources this was investigated (Braadbaart et al. , 2016). For this purpose, the elemental composition was analysed and the presence of silica phytoliths was further investigated.The archaeological objective of this study is to investigate if more information can be obtained about the fre structures lit by the inhabitants: such as their function and the type of
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 132 Table 1. Charred wood (charcoal) and charred olive stones from EC and MC periods heated by anthropogenic fres. The temperatures are based on calibration curves by Braadbaart, and Poole, 2008. Sample No.Taxonomic statusExcavation sample code Mean refect (%Ro)SDNTemp. after 60 min heating (°C)Period 178CCCharcoal M4/64N005 1.0000.179100380EC667CCCharcoal M4/64(AEN015 1.2660.233100390MC860CCCharcoal M12/64NN015 0.8500.050100370MC1183CCCharcoal M22/64N033 1.0600.176100380EC580CCCharcoal M13/61N025 0.9700.180100380EC746CCCharcoal M26/61N048 0.9570.102100380EC178OSOlive stone M4/64N005 0.9820.169100390EC667OSOlive stone M4/64(AE)N015 0.9370.076100390MC806OSOlive stone M5/64N006 1.3660.09080470MC1183OSOlive stone M22/64N033 1.4810.112100490EC580OSOlive stone M13/61N025 0.9190.124100390EC745OSOlive stone M25/61N047 1.0930.104100390EC Table 2. Charred organic material heated in LC period by anthropogenic fres and occasionally heated again by hot material ejected from the Minoan eruption. The temperatures are based on calibration curves by Braadbaart, and Poole, 2008. 923Olive stone M14/65N024 0.9770.079100400LC715Olive stone M8/65N007 0.6960.090100380LC7-10Olive stoneRm#5 WH 0.2840.084100330LC8-12Charcoal M7/54N003 0.5830.13188340LC5-10Charcoal M11/6NO43 0.6850.085100340LC1-11Charcoal M12/65N87 0.2500.086100300LC18-12Charcoal M6/68N034 2.4850.372100500LC2-11Barley seed M11/1BN 0.2790.077100300LC6-10SeedsRm#6 WH0.3680.115100300LC4-10Dung M35/43NO49 0.3500.600100310LC Table 3. Refectance measurements on charcoal samples recovered from imprints in walls of buildings above the ground level. Samples have been afected by the hot tephra of the Minoan eruption. For situation see Figure 4. Taxonomic status of wood: angiosperm. Dates of excavation 25-5-2010 and 05-10-2012. Location of sample in wallMean %RoSDNTemp. (ºC) 1-10Sector gamma Rm #1 or 20.8500.1221004102-10House of beautiful pottery (gamma)0.8000.1021004003-10Sector gamma Rm #2a0.7600.0981003901-12Triangle square0.9060.1261003702-12West House Rm #3a0.3270.2171003003-12 West House stairs to 1st foor 0.4570.0971003204-12Arvanites square (Sector Alpha) Rm #20.4730.1301003205-12House of the Ladies1.2060.1891003906-12House of the Ladies south0.9410.1581003707-12House of the Ladies Rm #80.5830.1311003309-12Delta Rm # 161.0380.14910038010-12Beta Rm#10.5590.16810033011-12Beta Rm#20.3680.11510030013-12Delta Rm#1a0.4790.086100290
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 133 Figure 4. Situation of the investigated imprints plotted on a map of Akrotiri. For the samples 1–12, see Table 3. Sample nr.12 is not shown on the map. Black arrows indicating the situation ( 2–10).
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 134 excavated samples that is kept in the store room at the site (Table 1). For the LC period, samples were also selected from the store room that could have been heated by human fres (charred olive stones) as well as by hot tephra (seeds in pots) (Table 2). The ash samples from the store room are from the LCI period. Samples of wooden construction parts heated by the hot tephra were located and collected by the authors from imprints in the walls of the buildings (Table 3, Figure 4), holes that originally kept the wooden beams or other wooden construction material. For the situation of these buildings see Figure 5. These imprints and the charcoal samples found in these hollows were situated at a height of around 1.50 m above the actual foor, but it is not clear if this was the VDL or the original living foor (Figure 5). Pumice samples were taken from sector gamma by the authors.In addition, samples were obtained from outside the city of Akrotiri: two charred olive wood samples from the Caldera wall (see Friedrich, 2013 for exact location) collected by the authors and two charcoal samples from the site of Megalochori donated by Mr. Lefteris Zorzos. 3. Methods 3.1 Refectance on charred organic material The charring process and its governing variables, as well as the application of refectance measurements, have been described extensively in earlier studies and the reader is referred to the relevant publications (Braadbaart, Poole, 2008; Braadbaart, Wright, 2007). However, all vitrinite refections for establishing the maximum heating temperature of the Figure 5. Example of two imprints (arrows) in the wall of a building, being the original place of the wooden construction beams.
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 135 samples were measured according to ISO 7404-5 (Methods for the petrographic analysis of coals – Part 5: Method of determining microscopically the refectance of vitrinite). All measurements were individually calibrated against the proper refectance standards. (N.E.N. 2009). Temperature and time of exposure determine the charring process. For wood and olive stones the efect of the temperature has been studied in earlier studies and therein a time of exposure of 60 minutes was applied (Braadbaart et al. , 2016). For this study the efect of time was studied on charred angiosperm wood (Figure 7). It shows that until around 450 ºC the time of exposure does not have an important efect on the refectance and thus the temperature. And it was therefore decided to continue the application of an exposure time of 60 minutes for this study, as was used in earlier studies. The accuracy of the refectance measurements is determined by the standard deviation of (at least) 50 measurements per sample. The given refectance value is the average of these 50 measurements. The standard deviation generally ranges between 0.05%R. The accuracy of the temperature is determined by the accuracy and stability of the oven in which the original material was heated (Laura et al. , 2009; Pensa et al. , 2015; Veal et al. , 2016). It is noted that the refectance measured on the samples represents the highest temperature at which they were ever exposed to in the past. This means that reheating at a lower temperature does not afect the refectance. 3.2 Elemental analyses From each ash residue, fve samples were measured, and the mean of the normalized (corrected for organic compounds) results was calculated. The equipment used was the HH x-ray fuorescence (XRF) Thermo Scientifc Niton XL3t device with GOLDD detector equipped with a silver anode operating at a maximum of 50 kV and 40 μA. This handheld device was used in the laboratory, and the samples were measured on a stable fat surface. This device is well suited for measuring up to 25 elements simultaneously in the analytical range between sulphur (atomic number 16) and Figure 6. Microphotographs under refected light microscopy of charred organic material (COM) heated under reducing conditions. Cell walls and other charred material: A=178CC angiosperm wood; B=178CC, no cells homogeneous material; C=806OS olive stone; D=M35/43NO49, animal dung.
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 136 uranium (atomic number 92). Light elements (magnesium, aluminium, silica and phosphorous) can also be measured with the same detector using a helium purge. The samples were measured in bulk mode. The device was factory calibrated. 3.3 Opal phytoliths For the microscopic study, 4 mg of each ash sample was thoroughly mixed with 2 mL of deionized water. From each mixture, 0.05 mL was mounted on a glass slide. In this way, each slide contained an equal amount of ash, that is, 0.1 mg, which made comparison between the diferent ashes possible. The slides were examined using trans-illumination under a Leica DM6000 M microscope. 3.4 pH measurement on pumice The pH measured on the two pumice samples followed the specifcations outlined in NEN 5750, 1989 and for the measurements a Consort D514 digital pH meter was used. 4. Data4.1 Material excavated from the EC and MC periods Samples of charcoal and charred olive stones from the new trenches N61 and N64 were analysed under refective light.For the charred wood, the characteristic cell walls of angiosperm wood were observed (Figure 6A). It is noted that in some samples, the cell-wall material from numerous cells seemed to have been transformed into a homogeneous mass showing no cells and clearly diferent from the characteristic wood-cell structure (Figure 6B). The mean values of the refectance readings for these fragments ranged from 0.850 to 1.266 %Ro (SD = 0.05–0.18 %Ro). These values correspond to temperatures in the range of 370–390 °C (Table 1).The charred olive stones show their characteristic cells (Figure 6C). The mean values of the refectance readings for this material range from 0.919 to 1.481 %Ro (SD = 0.076–0.169 %Ro). These values correspond to temperatures ranging from 390 to 490 °C (Table 2). Figure 7. Mean refectance (%Ro) of modern charred organic material (COM) as function of time. Table 4. Refectance measurements on charcoal samples from locations outside the city of Akrotiri. Samples have been afected by the hot tephra of the Minoan eruption. The temperature is based on calibration curves by Braadbaart, and Poole, 2008. Sample No.Location of sampleMean %RoSDNTemp. (ºC) OT 1Olive tree in Caldera wall0.4300.600100290OT 2Olive tree in Caldera wall0.2550.087100280O1 Megalochori DR/ER/3 1.6260.309100420O2 Megalochori DR/ER/5 3.1490.354100580 Table 5. Mean normalized elemental analyses of fve specimens of 2 samples of chemically untreated ashes in wt%. Sample excavation codeM7/65N009M6/68N034 SiO2 34.733.5CaO9.62.2P2O53.20.7K2O2.32.0Al2O35.66.0TiO20.30.3Fe2O33.33.6Balance39.851.3 SiO2/CaO ratio 3.518
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 137 4.2 Material from the LC period 4.2.1 Refectance Diferent samples of COM, including wood, olive-pressing residue, seed and dung were analysed (Table 2). These samples showed their particular characteristic cell walls under refective light. For charred wood the variation in refectance is large and varies from 0.250 to 2.485 %Ro (SD = 0.085–0.372 %Ro) resulting in temperatures ranging from 300 to 500 °C. The samples of charred olive stones also show this variety and the temperatures vary from 330 to 400 °C. The two seed samples were heated at around 300 °C. Among the available samples only one sample of possible charred dung was found and analysed, whereby a temperature of 310 °C was measured (Figure 6D). 4.2.2 Elemental analyses The only two ash samples we analysed contained 34.7 and 33.5 wt% SiO2, while the calcium content was 9.6 and 2.2 wt% CaO, respectively (Table 5). This resulted in a silica/ calcium ratio of 3.5 and 18. 4.2.3 Opal phytoliths In both samples the dendritic long cells strongly dominate each slide (Figure 8). These can be considered as the Figure 8. Microphotographs of examples of dendritic morphotypes of opal phytoliths of cereals recovered from archaeological ash. (See also Ball et al. , 2009, Figure 1). characteristic morphotypes of cereals (Ball et al. , 2009). Further, the presence of phytoliths originating from olive stones is possible, but were not observed (Braadbaart et al. , 2016). No typical grass phytoliths were observed. Detailed analyses about the number of phytoliths or the taxa of cereals were not performed. 4.2.4 Material recovered from wood imprints in the walls of buildings These samples show the characteristic cell walls of angiosperm wood as well as conifer wood under refective light. Based on the measured temperatures two separate groups can be distinguished. For one group of samples the refectance was around 1.000 %Ro with temperatures ranging from 370to 410 °C. A second group showed a refectance of around 0.500 and the corresponding temperatures range from 310 to 340 °C (Table 4). 4.2.5 Samples from the island of Thera, but beyond the site of Akrotiri The two samples of charred olive wood from the Caldera wall and deposited in pumice showed refectances of 0.430 and 0.255 %Ro (SD=0.600 and 0.087 %Ro), which correspond to a temperature of around 300 ºC (Table 4). The two charred samples from the excavation of Megalochori
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 138 show the characteristic cell walls from angiosperm wood. Here the refectances were rather high with values of 1.626 and 3.149 %Ro (SD=0.309 and 0.354 %Ro), giving rather high temperatures of 420 to 580 ºC, respectively. 4.2.6 pH of the pumice recovered from sector gamma The results of the two samples showed values of 8.5 and 9.05. 5. Discussion This study is mainly focused on the charred organic materials (COM) excavated from the site of Akrotiri and although it is a rather small data set, it reveals interesting information which would need to be further explored and enriched in the future. For example, at Akrotiri, to char organic material like wood or olive stones, it has to be exposed to heat under reducing conditions. On this site, two heat sources, generating the required heat, made this possible. To begin with, small fres ignited by the occupants of the site, such as hearths, which they used for their daily need to prepare food, to light up, or induced by other pyro-technological sources, we name: the anthropogenic heat . A second source of heat was the hot tephra ejected from the volcano during the Minoan eruption, which buried the site: the volcanic heat . The charred wooden construction material within the architecture makes it possible to measure the refectance of this charred material and accordingly the emplacement temperatures of the various volcanic materials from the relevant phases, as ejected by the volcano. 5.1 Anthropogenic heat source In the EC, MC and LC periods, charred wood or charcoal and charred olive stones were excavated (Sarpaki and Asouti, 2008). In such archaeological sites it seems that these materials were often charred when used as fuel in freplaces. To initiate a fre, an interaction has to take place between the fuel, air and the heat generated by an external heat source– the three basic elements of the fre triangle (Emmons and Atreya, 1982). Fuel is composed of water, an inorganic ash fraction and an organic fraction, the latter providing the potential energy that can be transformed into the required heat energy. Once a heat source has been introduced to the fuel, heat is absorbed and the fuel begins to increase in temperature. When a temperature of 280–300 ºC is reached with no air, a complex chemical reaction starts that leads to the thermal degradation of the organic constituents of the fuel, producing volatile gases and a carbon rich or charred residue (Rein, 2009; Braadbaart et al. , 2012). When enough air is available at this temperature and higher, both new products will oxidize, a highly exothermic process, meaning that heat will be generated. As long as the temperature remains above 300 ºC and with enough air, the volatile gases will produce fames and the carbon-rich char will be converted into carbon dioxide (CO 2 ) and a residue of ash, both reactions releasing heat. When enough air is not made available, the temperature will decrease and the fre will extinguish. Under these conditions not only the char, but also the ash provide indications of the original type of fuel and possibly the heating conditions of the fre, which in turn may give an indication of the function of the fre. The results show that wood, olive stones and animal dung must have been available to meet the fuel demands of the occupants. The question now arises as to whether there were enough of these fuels available on the island. Based on the available literature it is not clear how the pre-eruption surface of the island looked like. In Assouti (2003, p.472), a wooded landscape and early olive cultivation has been described. However, in the work of Sarpaki and Asouti (2008, p.370) and Bottema-MacGillivray (2005), a certain level of thrift, or rather of economizing, in wood has been indicated. On the other hand, wood was used extensively as a building material, although it only survived in very few places in Akrotiri (Palyvou, 2005, p.111). It is also possible that some timber was imported, in turn, perhaps, suggesting that not enough suitable wood was available on the island (Palyvou, 2005, p.112). Therefore, the fuel issue, as further discussed, is only based on the investigations on the material and their samples used in this paper.In an earlier paper it was shown that the olive-pressing residue (OPR) is an excellent fuel (Braadbaart et al. , 2016). After pressing the olives, two types of fuel can be prepared: (1) after air drying OPR can be used directly as a fuel and (2) OPR is thermally degraded by heating, in the absence or near absence of air, in the range of 400–500 ºC. The charred OPR is now upgraded into a more efcient type of fuel (COPR) with a heating value that increases by 50% (Braadbaart et al. , 2017). The weight (mass) of the fuel is also considerably reduced, which facilitates transportation. Thus, a hotter and smoke-free fre is created, compared to a fuel that is just air dried and not thermally degraded as explained above. The temperatures of around 400 ºC measured on the charred olive stone samples could be a confrmation that COPR was indeed used as fuel (Table 1a). In the case of the charcoal samples the temperature is 380 ºC, somewhat lower compared to the olive stones in spite of the fact that both types of samples were retrieved from approximately the same spot, i.e. 178CC and 178OS in the EC period (Table 1b). Yet, even at this temperature, a similarly efcient fuel like COPR could have been produced from wood, although the quality might have been less in the sense of its physical strength. Since the refectance just shows the highest temperature at which the charred material was heated, it can be argued that olive stones and wood were thermally upgraded or charred under the described conditions outside the city before being used as fuel by its occupants. Thus, the measured temperatures of 380 to 400 ºC are not the temperatures necessary for the purpose of the fres. They are too high for fres that were probably only used for cooking where a temperature of around 300 ºC or even less is normally more than sufcient, which would mean that less fuel would be used. It is, therefore, tentatively suggested that the more efcient thermally-upgraded COPR and wood had been used as fuel. It could have been upgraded at Akrotiri, or on Thera itself, where enough olives and/or
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 139 woody material were available. Subsequently, the light-weight, thermally-upgraded materials would have been transported to the city from its vicinity and, perhaps, along with the possible importation of wood from the macro- environment of the island and/or from other areas further afeld, used for the construction of houses (Palyvou, 2005, p.112). On the line of these arguments, one notes that already in the EC period this system would have been applied for the production of fuels, a rather advanced technology. However, it cannot be excluded that the higher temperatures were used for other activities where higher heat was needed, such as metalworking, faience and so forth.For the LC period, in principle, the same explanation is valid as discussed above, but the process becomes more complicated to explain, since the material was suddenly buried by hot pumice, i.e. the volcanic heat source. In order, to study this process a number of characteristic samples of diferent materials were selected and analysed (Table 2). The results show that the olive stone samples 715 and 923 were heated at temperatures of 400 ºC, which is comparable to the samples from the EC and MC periods. Apparently, these samples were not afected by the heat of the hot pumice, which must have had a lower temperature. The temperatures of the other samples are much lower and are further discussed in a following section describing the volcanic heat source. 5.2 Ash In addition to the charred material, two samples of ash from the LCI period were investigated (Table 5). The results show that hardly any calcium was present, indicating that wood was not burned as fuel (Braadbaart et al. , 2017). Moreover, sample 4-10 found in the vicinity did not show the typical cell walls of wood and is attributed to, probably, animal dung (Sarpaki and Asouti, 2008). As far as the opal phytoliths are concerned, the dendritic morphotypes strongly dominate the samples and are characteristic of cereals (Ball et al. , 2009). Assuming that animal dung could have been used as one of the fuels, it is suggested that harvest residues of cereals were used as feed for goats and in turn goat dung could have been used as fuel (Owen, 1994), especially as cereals were grown on the island. The use of animal dung as fuel would be in accord with the possible fndings in Sarpaki and Asouti (2008). No typical grass phytoliths were found, so dung from other animals that feed on grass were not used as fuel in that particular hearth. However, the study of fuel remains an open question at Akrotiri, which needs to be further studied more closely in the future (Shahack-Gross, 2011). 5.3 Volcanic heat source After the thin layer of precursory material, the frst phase of the Minoan eruption deposited a layer of hot pumice on the city. It buried the VDL layer and spilled through windows and doors. The roofs were loaded with the pumice and some collapsed. The second phase consisted of hot pyroclastic surges and fows. This caused probably less damage, since it may have fowed along the buildings through the streets. The third phase does not seem to be present in Akrotiri, but there are indications that the fourth phase consisting of ignimbrites is present in Akrotiri (Druitt, 2014).The organic material got buried by the hot tephra and accordingly was heated and also charred since no air was present between the reacting surfaces of the tephra and the organic material. The temperatures based on the refectance measurements of the charred wood samples recovered from the post holes show that two groups of temperatures can be distinguished (Table 3). One group was heated and charred at 310 to 340 ºC and a second group at 370 to 410 ºC. Apparently, these groups represent the frst two phases of tephra, each with diferent emplacement temperatures. Samples presented in Table 2 also show temperatures that are comparable to the samples of the frst group excavated from the post holes. Olive stone sample 7-10 and the two samples of seeds (2-11 and 6-10) were recovered from pots found on the ground foor of the West House building and it is not very likely that they were charred before being deposited in the pots. Since they were charred it can be concluded that they were heated by the hot pumice at a temperature of around 320 ºC. The beam from shaft 65N (sample 1-11) must have been part of the building material and was heated at its outside by the hot pumice at a temperature of 300 ºC. The two charcoal samples (5-10 and 8-12) were charred at 340 ºC. The samples of charred olive wood recovered from the wall of the Caldera and covered by pumice showed a temperature of 310 ºC. This temperature is comparable to the samples from Akrotiri and may suggest that the emplacement temperature of the frst phase consisting of the pumice was in the range of 300–340 ºC. It is suggested that the second group of charred material from the post holes represents the second phase of tephra with temperatures of 370 to 410 ºC. In relation to their height, the exact location of the samples from the post holes is confusing, since the original level is not clear. Charcoal sample 18-12 was charred at 500 ºC, which is considerable higher than the samples heated by the tephra of the frst two phases. However, the two charcoal samples from the site of Megalochori show comparable temperatures of 420 and 580 ºC. Temperatures like these are also measured in the ignimbrites of phase 4 (McCoy, Heiken, 2000, p.55 and Druitt, 2014). This would mean that phase four of the volcanic deposits must have had some efects in Akrotiri.Earlier studies that have measured emplacement temperatures used magnetic methods (Bardot, 2000 and references cited therein). The measurements were applied on lithics found on various places on the island of Thera. For Akrotiri, magnetic methods have been applied on lithics as well as pottery (Tema et al. , 2013; 2015). The results show measurements that were in the order of 260 to 280 ºC for material covered by the pumice of the frst phase, but higher temperatures to the order of 310 ºC were also shown at a few places. The temperatures, therefore, measured by magnetic methods on the inorganic material are lower than the temperatures found on charred organic material charred by tephra studied in this paper. For organic material to be charred the temperature has to be at least 280 to 300 ºC and
image/svg+xmlIANSA 2019 ● X/2 ● 129–141 Freek Braadbaart † , Anaya Sarpaki, Harry Veld, Bertil van Os: Charred Organic Material, Heated by Anthropogenic Fires and Hot Volcanic Products from the Minoan Eruption, Excavated from the Bronze Age Site of Akrotiri on the Cycladic Island of Thera (Greece) 140 these materials become, as a result, always black. Since the refectance measure only the highest temperatures, the samples that were even exposed to a possible cooling of the tephra after deposition, will not change the refectance and the temperatures. It is therefore tentatively suggested that the emplacement temperatures of the tephra of phases 1, 2 and 4 are as described above. The pH values of the pumice are such that no corrections for the refectance are necessary (Braadbaart et al. , 2009). In the literature no descriptions could be found of fres that have raged the city during or after the eruption. On the other hand, hardly any wooden construction material used for the buildings was recovered. Only wood imprints were all that was found, indicating where wood must have been present (Palyvou, 2005, Figure 5). An explanation for this phenomenon could be that the buildings and thus also the wooden material were buried by substantial layers of hot pumice without air. Therefore, as the wooden material was heated without air, as a result it became charred. Probably this process occurred over a limited time and only had an efect on the outside of the wooden beams (sample 1-11) and other architectural wooden parts, which means that the inside of the beams were not charred and have not survived the natural disintegrating processes that usually occur after deposition. Only the char formed on the outside was left along with some remnants that could still be recovered from the imprints. 6. Conclusions The settlement of Akrotiri developed from the Late Neolithic through the Early and Middle Cycladic period into a fourishing city up to the beginning of the Late Cycladic period, covering a period of over 1500 years. Then, at a late seventeenth century BC date, as a result of the Minoan eruption, the city was buried and at the same time preserved by four phases of hot tephra. During this whole Cycladic period, fre structures were found and excavated that were most probably used for activities primarily concerned with the preparation of food. However, the practice of other pyro-technologies should not be excluded. The charred organic materials recovered from these fre structures showed that the temperatures at which they must have been exposed to range from 380 to 490 ºC (Table 1a). It is argued that such temperatures are too high for an activity such as cooking, as these temperatures consume too much fuel and the food may well burn. When olive-pressing residues and wood are thermally upgraded by charring at 400 to 500 ºC, excellent fuels are produced (Braadbaart et al. , 2016). This process could have been performed outside the city in places where enough olive trees and woody material were present. It probably explains both the presence of the charred olive stones and charcoal showing high temperatures. It has been noted from this study that already since the EC period this advanced technology was applied in the settlement. Another residue of burned fuel is the ash. Unfortunately, the very summary work and the few samples indicate, through some elemental analyses, the presence of opal phytoliths (Shahack-Gross, 2011) and some observations under refective microscopy that animal dung could also have been used as a fuel resource. When the city was buried by the frst main phase of tephra, this being the hot pumice, the buildings and all the material they contained were exposed to this heat source. The wooden parts of the architecture were heated and covered and since no air was present anymore at the reacting surfaces, they became charred. The lack of air may explain why in the literature no indication of fres in the city after the eruption was found. By using this charred organic material, the temperature at which the material had been exposed to can be measured using the refectance method. In this way the emplacement temperatures of the various phases of tephra can be determined. The results show that: for the frst phase, the pumice, the temperature must have been between 310 and 340 ºC; for the second phase, between 370 and 410 ºC; and samples showing temperatures of around 500 ºC are attributed to the ignimbrite of phase four of the tephra. These temperatures are somewhat higher than the earlier obtained temperatures using magnetic methods measured on lithics and pottery, both being inorganic materials. The applied methods are especially valid for the circumstances encountered in this archaeological and volcanological site. It was observed that not all black material was charcoal, but could be charred dung or other charred organic material. It is suggested that, apart from botanical identifcation, another important part of an archaeological investigation should be, as suggested in this study, the analysis of charred organic material and ash, as well as the remains of the various used fuels. This could add much valuable information related to not only how ancient societies selected the wide spectrum of their fuels for their pyro-technical needs, but also open up avenues of research into the technological know-how and those aspects of the environment and economy that can only be indirectly inferred. Acknowledgements We thank Ch. Doumas for allowing us to work and take samples at the site Akrotiri on Thera (Greece). For the preparation of phytolith microphotographs we thank Ton van Brussel of the Faculty of Science, Institute of Biology, Leiden University (The Netherlands). We would also like to thank Sampson Katsipis who walked with us and indicated the site on the edge of the caldera where Prof. Friedrich had found the olive tree trunk, as well as Lefteris Zorzos who provided the Megalochori samples. Last but not least, we thank Mieke Verheem, Freek Braadbaart’s widow, for she has been instrumental in fnding leads which helped with complying with the reviewers’ suggestions and the completion of this work.
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