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Experimental explorations into the aceramic dry distillation of Betula pubescens (downy birch) bark tar

  • Peter Groom
  • Tine Schenck
  • Grethe Moéll Pedersen
Original Paper

Abstract

A range of experiments were conducted in an attempt to create tar from the bark of Betula pubescens (downy birch) using an aceramic dry distillation process. Fire structures based on small pits and small kiln-like mounds were explored with a focus on fire intensities and differing burn times under field-based conditions. Heat penetration presented itself as an all-important factor, and the depth of the construction of the structures and features was considered to directly correlate to the impact of the heat. Single variable experiments confirmed that the necessary reducing atmosphere was achievable despite friable soil, but that heat would not penetrate a 50- to 80-mm-deep layer of grass turf. The evolution of structures from pits towards a raised type resulted in kiln-like structures which proved more successful. Though the experiments did not successfully produce tar as a finished product, they did lead to a better understanding of the dry distillation process of the established technology of birch bark tar extraction in aceramic societies.

Keywords

Dry distillation Betula pubescens Birch bark tar Middle Palaeolithic Mesolithic Experimental archaeology 

Introduction

Tar extracted from the bark of Betula sp. (birch species) has a long and widespread history of use throughout Europe, from Middle Palaeolithic Germany to modern-day Fenno-Scandinavia. It has been widely used as an adhesive for hafting and repairs. Finds of this nature are numerous throughout prehistory and range from, amongst others, the very oldest finds with flint imprints from Campitello quarry, possibly of Mousterian origin (Mazza et al. 2006), and from Königsaue, Germany, AMS-dated to the Upper Palaeolithic (Koller 2001). In the Neolithic, ceramic sherds were glued together with birch bark tar at sites in amongst others Belgium and Greece (Bosquet et al. 2001; Urem-Kotsou et al. 2002), and later at Iron Age sites in Austria, France and Great Britain (e.g. Regert et al. 2003; Charters et al. 1993; Sauter et al. 2002). In addition, the Ojibwa in North America have used birch bark tar (mainly from Betula papyrifera) for waterproofing birch bark canoes (Lyford 1943), and it has potentially also been used as an odoriferous product for ceremonial use in the Jersey Neolithic (Pomstra and Meijer 2010).

One of the more unusual find categories is the lumps of birch bark tar with teeth imprints, interpreted by some as a medicinal chewing gum (Hiltemann 2012; Aveling and Heron 1999), that have been found across Scandinavia and Central Europe. Many of these are from the Neolithic, and the use of birch bark tar in general seems to increase from the Neolithic onwards (Aveling and Heron 1999). Nevertheless, it must be emphasised that quite a few of the teeth-imprinted lumps are from pre-ceramic contexts, such as the 11 specimens dated to the Early Mesolithic Maglemose period (9100–8700 uncal. BP) at the site of Huseby Klev in southwestern Sweden (Wadley 2010; Hiltemann 2012). The earliest datings demonstrate that tar is one of the oldest constructed synthetic materials (Grünberg 2002), and the cognitive abilities of Neanderthals with regard to adhesive production is currently entering the Archaeological debate. It must be considered that Neanderthals might in fact have been able to create synthetic adhesives (see Wadley 2010).

As the chemical process needed for the forming of Betula sp. bark tar relies on strict control of temperature and requires a dry distillation that excludes oxygen (Koller et al. 2001, p. 394), the question of production method arises in these early periods. It has been shown that a substance much like birch bark tar can be produced in small quantities by means other than dry distillation (Pomstra and Meijer 2010).

A common way of later production amongst modern humans has been by dry-distillation in ceramic pots. A sealed pot provides limited intake of oxygen, it can be placed directly in a bonfire, and its contents are effectively heated due to the thin, ceramic walls. From the Neolithic onwards, pottery is found from sites such as Fischergasse in Lower Bavaria with residues that may indicate the dry distillation of Betula sp. bark tar (Ottaway 1992), and it seems clear that the use of pottery as a type of kiln/furnace then became the preferred method of operation for the distillation, a method that survived well into modern times in certain countries (Piotrowski 1999). The authors have successfully produced birch bark tar with the double-pot method described by Piotrowski (1999) in as little a time as 15 min.

Despite the evidence for Betula sp. bark tar, just how it was produced without ceramic containers remains a puzzle (Grünberg 2002). Czarnowski and Neubauer (1992) have previously experimented with attempts involving dry distillation in pits with heated stones, however unsuccessfully. Although pit structures with heated stones do not exist in large numbers from pre-ceramic periods, there are some examples, for instance from Late Mesolithic eastern Norway (Risbøl et al. 2002). On the other hand, generic pit structures without additional features are innumerable and can be considered almost intangible at times as they can be both hard to observe, date and determine as a definite cultural feature. The cognitive level needed for the construction of pit structures is likely to also have been achievable for Neanderthals if their production was in fact controlled and intentional (see Koller 2001). As such, and since there is a lack of diagnostic tar production structures from pre-ceramic societies in the archaeological record, it is of interest to explore whether small pits could be used for the dry distillation of birch bark tar.

The distillation process

Betula bark tar production relies on strict control of temperature together with the exclusion of oxygen (Koller et al. 2001, p. 392). If oxygen is present during the tar-making process, then tar molecules will combust at relatively low temperatures. However, if oxygen is excluded, tar will be formed from the chemical components lupeol and betulin; the extent of formation is dependent upon temperature and the stability of the individual molecules (Jens Glastrup, 2009, personal communication). Therefore, the process of tar production is likely to be a combination of the wood/bark being heated in a reducing atmosphere together with complex steam distillation of the non-polymer component present in the wood/bark.

The process is a balance of controlled temperatures in relation to the heat source (in this case, the differing burn times of wood fires), potentially with the running off of what is a highly combustible organic mixture into a tar catchment whilst at all times being airtight. Grünberg (2002, p. 16) suggests that for instance during the Middle Palaeolithic, Betula sp. bark pitch (tar) was produced using less energy and at a much lower temperature, approx. 350 °C, than in the Neolithic and later.

The practicalities of a dry distillation of Betula sp. bark into tar will include an airtight container-like structure that allows smouldering at around 350–400 °C for the duration of the distillation (Koller et al. 2001, p. 394). It is relatively easy to produce Betula sp. bark tar using the double-pot method (Piotrowski 1999, p. 148), and previous firings conducted by Schenck and Pedersen demonstrated that a double-pot structure can produce tar in as little as 15 min using wood and dried bark. The remains of the Betula sp. bark are turned into charcoal in this process.

However, the question that this set of experiments sought to answer was how people without fire-resistant, oxygen-excluding vessels may have managed to keep such strict control of temperature and aridity, restricting ventilation as is necessary in the smouldering process of dry distillation with which tar is procured from Betula sp. bark.

Experiment method

The experiments were in experimental archaeological terminology completely ‘field-based’ and as ‘actualistic’ or representative as an unknown process can possibly be. To allow the experiment to move forwards without measuring temperatures, and by only making the coupled structures comparable to each other through the near-identical conditions, we created a truly free exploratory process. This allowed for an inductive process that we consider crucial in the formation of hypotheses (and see Schenck 2011). In many regards, the set of experiments referred to in this article can be seen as hypothesis-forming experiences or pre-experiments rather than true, scientific experiments.

Production structure design

The traditional double-pot method provides both air exclusion and heat conduction in the shape of a ceramic wall (Piotrowski 1999, p. 148). The model for the experiments was to mimic these conditions without the use of ceramic containers. The initial pit structure form was derived from a generic idea of a small-scale tar production structure from the Norwegian Middle Ages, called ‘tjørehjell’ (see Farbregd 1989). This became a starting point for a development of structures, each relying on experiences with the previous. For a generic representation of structure types, see Fig. 1.
Fig. 1

General development of pit structures

The experiments were conducted at the Lejre Research Centre in Denmark during the summer of 2009. The structures were created by digging small pits into a grassy turf with an underlying dry, compacted sand layer. Clay and sand were occasionally used as linings and these were acquired from the site. The wood fuel came from trees at the site; the location for the experiments was on the brow of a small slope. Though the dimensions of the various structures were often spontaneous, each one building on the knowledge or experience gained from the former, most of them turned out to be of similar proportions to our previous double-pot processes. Thus, 11 small structures were created and three separate bark sheet experiments were conducted. In addition, one double-ceramic pot distillation was used as a standard for the firewood and burn time under similar conditions. Although this firing was successful, it will not be reiterated in detail.

Experiment design

Each firing was conducted in pairs of two parallel structures with similar structural properties but different dimensions. It became clear that pits were difficult to heat sufficiently without any additional cooking stones or other heat-conducting media. We decided not to measure temperatures due to the fact that the firings would be compared only inwards in pairs rather than across structural types. This was because our intent was to explore rather than to control since the distillation structures are not known.

Experiments were structured to keep firing conditions as similar as possible for each pair of structures, with approximately the same amount of firewood, firing, and cool-down times and the same exposure to wind. Structures were constructed on the approximate same slope and dry ground, with thickness of cover kept as homologous as possible. One firing (S7) was a freestanding attempt in which we started moving towards raised structures.

Since only small quantities of Betula sp. bark tar have been found at any given time, we did not consider the use of sizeable structures, instead concentrating on structures that could hold a maximum of 2.5 L of Betula pubescens bark each. Between 1.5 and 2 L of tightly packed, dried birch bark from B. pubescens was used for each experiment.

We decided that the presence of tar odour, which has been a gauge for a successful distillation in previous double-pot firings, would be the guideline to the achievement of a crucial temperature. This turned out to not be a good guideline for individual structures since it proved impossible to judge which structure exuded the odour. Rather, it told us that the conditions could have been favourable for tar production, although excavations proved that in neither experiment had actual production taken place.

A summary of each structure design is presented in Tables 1, 2, 3, 4, 5, 6 and 7.
Table 1

Summary of experiments S1 and S2

Structure

S1

S2

Description

Medium shallow pit structure with catchment channel and pool placed on slope. Sand and turf cover. Fire built on top of cover

Shallow pit structure with catchment channel and pool placed on slope. Sand and turf cover. Fire built on top of cover. Constructed as a variant of S1

Depth

200 mm

80 mm

Dimensions plan

Ø = 400 mm

Ø = 360 mm

Catchment channel dimensions

L = 350 mm

L = 260 mm

D = 200 mm

D = 80 mm

Bark volume (packed)

2 L

2 L

Cover

Dry sand and grass turf

Dry sand and grass turf

Cover thickness

Approx. 60 mm

Approx. 60 mm

Tar odour in air

3 h, 15 min

3 h, 10 min

Burn time before die-down

3 h, 30 min

3 h, 25 min

Experiment termination

Dowsed with water after 4 and 5 h

Dowsed with water after 4 and 5 h

Comments

Pungent, acrid odours were released upon dowsing, which suggested that aromatic volatiles may have been present in the sub fire surface. No tar was observed in the collection channel and bulb or upon excavation of the pit the following morning. A loose bundle of bark was excavated and no charring was visible.

Dowsing resulted in pungent, acrid smell as possible aromatic volatiles vented from the sub-fire surface. This latter observation appeared to be correct as on the following morning, excavation of S2 showed tar-stained/coloured soil and burnt tar residues on the surface of the B. pubescens bark. In addition, the bark itself was charred and caked with a gloss on parts of the exterior surface.

Table 2

Summary of experiments S3 and S4

Structure

S3

S4

Description

Central pit, containing bark and covered with sand and turf, within surrounding ditch. The wood fire was lit within the ditch, which meant that the fire was lower and therefore closer to more of the bark within the covered central pit.

Shallow, funnel-shaped structure with deeper central pit and catchment channel. Sand and turf cover. Fire built on top of cover

Depth (D)

180 mm

80–110 mm sloping, 150 mm centrally.

Dimensions plan

Ø = 1,000 mm (moat)

750 × 650 mm

Central pit dimensions

D = 180 mm

D = 150 mm

Ø = 320 mm

Ø = 130 mm

Catchment channel dimensions

L = 625 mm

D = 150–50 mm (at pool)

Bark volume (packed)

1.5 L

1.5 L

Cover

Dry sand and grass turf

Dry sand and grass turf

Cover thickness

Approx. 60 mm

Approx. 60 mm

Tar odour in air

3 h, 5 min

3 h, 5 min

Burn time before die-down

6 h

6 h

Experiment termination

Die-down

Die-down

Comments

The hypothesis up to this point had been that fire temperatures had not been high enough and this structure was fashioned in an attempt to redress this. A slow burn rate on one side of the ‘moat’ led to limited heating through the structure. The results of this could be observed on excavation, with the contents displaying greater charring on the side where the fire was started. On this side, there was a black sheen coating of approx. 40 %, with charring on the bark towards the top of the pit; the rest of the underlying bark was incompletely burnt.

Windy day resulted in noticeably hot fire and rapid wood consumption. Resulted in incomplete charring of the bark, except for at the very top of the bundle. The collection channels appeared to be ineffective and were hereafter dropped as part of the structures.

Table 3

Summary of experiments S5 and S6

Structure

S5

S6

Description

Pit structure with central circular pit where bark package was placed

Shallow rectangular form with a central round pit where bark package was placed

Depth (D)

30 mm

80 mm

Dimensions plan

Ø = 200 mm

L = 750 mm

W = 650 mm

Dimensions circular pit

D = 100 mm

D = 120 mm

Ø = 200 mm

Ø = 300 mm

Bark volume

1 L

1 L

Cover

Sand and turf

Inverted turf

Cover thickness

60–70 mm

Approx. 60–70 mm

Tar odour in air

None

None

Burn time

4 h

Right side of structure: 2 h

Left side of structure: 4 h

Experiment termination

Dowsing with water

Dowsing with water

Bark state after burning

Approx. 30 % charring. Surface charring and gloss/burnt tar present at the top of the bundle. Limited charring towards the bottom

Approx. 40 % overall charring together with several small deposits of tar and gloss

Comments

Present on the bark were condensation droplets of an unknown chemical, which quickly evaporated upon excavation.

Fire was started on the right of the central pit, the logs arranged at a right angle to be fanned by the wind. This reduced fire spread and the right-hand side experienced heat for 4 h, left-hand side for approx. 2 h. On excavation, it was revealed that the slow-burning low fuel fire had not produced enough heat to burn the grass on the inverted turf; however, the bark sheet at the top was charred. Condensation was also present amongst the tightly packed bark.

Table 4

Summary of experiment S7

Structure

S7

Description

A square of turf was placed on four small stones; bark was folded into a ball and placed on the turf and then covered with turf, sand and clay.

Dimensions plan

H = 300 mm

W = 320 mm

L = 320 mm

Bark volume (packed)

1 L

Cover

Sand and turf

Cover thickness

70–100 mm

Tar odour in air

2 h

Burn time before die-down

5 h

Experiment termination

Die-down

Bark state after burning

Obliterated

Comments

Singular experiment. Damp blustery conditions meant that it took about 1.5 h for the wood fire to light properly; the fire then burned with extreme heat for the duration of the experiment. Excavation the following morning revealed a completely destroyed structure with no trace of the bark. This was most likely due to crumbling of the structure during firing, leading to intake of air and combustion. All four stones were cracked and discoloured from the heat. Fired simultaneously with S8–S10

Table 5

Summary of experiments S8, S9 and S10

Structure

S8

S9

S10

Description

Turf squares were removed; into these squares were placed sheets of bark flat to the ground. The turf was then replaced and a wood fire was lit on top and kept burning for a set time. Fire was aligned with the wind to encourage a high heat and ‘fast burn’.

Turf squares were removed; into these squares were placed sheets of bark flat to the ground. The turf was then replaced and a wood fire was lit on top and kept burning for a set time. Fire was aligned with the wind to encourage a high heat and ‘fast burn’.

Turf squares were removed; into these squares were placed sheets of bark flat to the ground. The turf was then replaced and a wood fire was lit on top and kept burning for a set time. Fire was aligned with the wind to encourage a high heat and ‘fast burn’.

Depth (D)

50 mm

50 mm

50 mm

Dimensions plan

W = 800 mm

W = 800 mm

W = 800 mm

L = 800

L = 800

L = 800

Bark volume (sheet)

700 × 650 × 2 mm

650 × 650 mm × 2 mm

650 × 650 mm × 2 mm

Cover

Turf

Turf

Turf

Cover thickness

50 mm

50 mm

50 mm

Tar odour in air

2 h (?)

2 h (?)

2 h (?)

Burn time before die-down

6 h

4 h

2 h

Experiment termination

Dowsing with water

Dowsing with water

Dowsing with water

Bark state after burning

Overall discolouration with approx. 3 % charring. A glossy charred residue was present in small amounts on parts of the bark.

Approx. 60 % discolouration with no charring

Approx. 8–10 % discolouration had occurred with no charring.

Comments

The charring occurred in the corner where the fire had been started. This corner got approximately 20 min more heat exposure than the far corner.

Discolouration was clearly directional and coincided with where the fire had been started.

Discolouration was clearly directional and coincided with where the fire had been started. Burnt intensely for 2 h and had to be dowsed twice to be extinguished.

Table 6

Summary of experiments S11 and S12

Structure

S11

S12

Description

Funnel-shaped pit lined with wet clay in an attempt to seal off air spaces within the friable soil. A small bark collection pit with a twig grid at bottom. On top of this was placed bark, wet F. excelsior leaves and grass before being covered with turf and sand.

Conical-shaped pit structure lined with wet clay, with a small collection pit covered with a twig grid at the bottom. On top of this was placed bark, wet F. excelsior leaves and grass before being covered with turf and sand. Intended as a shallower version of S11

Depth

D = 530 mm (top)

D = 210 mm

Diameter

Ø = 550 mm (top)

Ø = 500 mm

Bark volume

2.5 L

2 L

Cover

Fresh ash leaves, dried grass, turf and sand

Fresh ash leaves, dried grass, turf and sand

Cover thickness

150 mm

150 mm

Tar odour in air

Burn time before die-down

5 h

4 h, 45 min

Experiment termination

Dowsing with water

Dowsing with water

Bark state after burning

Unaffected

Unaffected

Comments

Upon starting the excavation the following morning, the structure was still hot and the excavation stopped until it had cooled. The structure proved to be extremely odorous, although the smell was not one of tar. The wet leaves were mostly intact, albeit discoloured, the grass was in a charred state, and the bark was uncharred and had not even discoloured, suggesting that the pit was too deep to be efficiently fired. The last hour of firing was without feeding.

The last hour of firing was without feeding. The lower half of the pit still held moist clay.

Table 7

Summary of experiments S13 and S14

Structure

S13

S14

Description

Raised conical kiln-like structure fashioned out of wet sand encasing a cylinder of bark, wrapped in green F. excelsior leaves

Raised conical kiln-like structure fashioned out of wet sand encasing a cylinder of bark

Height

H = 250 mm

H = 250 mm

Diameter

Ø = 300 mm (base)

Ø = 300 mm (base)

Bark volume

Approx. 2 L

Approx. 2 L

Cover

Fresh ash leaves, wet sand

Wet sand

Cover thickness

70 mm

50 mm

Tar odour in air

3 h, 10 min

2 h, 45 min

Burn time before die-down

5 h

5 h, 30 min

Experiment termination

Dowsing with water

Dowsing with water

Bark state after burning

Bark was discoloured, but not at all charred.

Top of bark bundle was charred and one of the corners of the bark cylinder exhibited a gummy tar film.

Comments

S13 was surrounded by wood fire; however, rather than continually feeding fresh fuel wood, the burnt wood was broken up and moved around the ‘raised kiln’ so that the hot charcoal covered the structure. Even so, the temperatures were noticeably lower than when using the fuel wood, and midway through the experiment, this technique was abandoned and firewood was again piled around the structure for a further 2 h. Bark discolouration may have been produced by the leaf wrap.

S14 was built as a direct comparison to S13, the same type of structure but without leaves. A 150 × 150-mm cylinder of bark was placed at the base and the cone built up. After an initial firing using the charcoal method described in S13, fuel wood was piled around the structure for the remaining 2 h. S14 proved to be the most efficient attempt with definite gummy tar film.

Results

Figure 2 displays the general plan of the site after all experiments. One of the structures (S9) was recycled to do a double-pot firing. As the structures had differing dimensions and formed different phases in a continuous process, they are presented in comparable sets below. Details for each structure are presented in Tables 1, 2, 3, 4, 5, 6 and 7.
Fig. 2

Plan sketch of the experimental area

Experiments S1 and S2

Experiments S1 and S2 (Table 1) were the very beginnings of what we assumed a tar distillation pit structure and was largely based on the idea of a small-scale medieval tjørehjell (Farbregd 1989). The collection channels caused some concern with regard to air intake, and we attempted to seal them off using stones (Fig. 3).
Fig. 3

Schematic sketch of S2

Experiments S3 and S4

With S3, we addressed the problem of insufficient heat penetration for the first time, with a surrounding ditch where firewood would be placed to better penetrate the pit from all sides (Table 2). We also continued the idea from S1 and S2 which now developed into S4 (Fig. 4), a combination between a tjørehjell and a sunken feature as displayed in the Polish Middle Ages (Piotrowski 1999).
Fig. 4

Schematic sketch of S4

Experiments S5 and S6

With S5 and S6 (Table 3), we discontinued the idea of a collection channel to focus on the pit structure itself. S5 (Fig. 5) and S6 were more or less shallow attempts at the Polish medieval sunken feature (Piotrowski 1999).
Fig. 5

Preparation of experiment S5

Experiment S7

Having had little success with pit structures, the idea from S3 was developed into a standing structure with S7 (Table 4 and Fig. 6). However, the idea to create space underneath to accommodate heat penetration from four sides soon had to be abandoned as the entire structure was near obliterated in the hot fire.
Fig. 6

Schematic sketch of S7

Experiments S8, S9 and S10

At this point, we wanted to see whether there was a direct correlation between heat exposure time and tar processing and whether the heat would be sufficiently penetrating through the turf layer we were using as cover. We also wanted to check whether we were in fact creating a reducing atmosphere by just using the ground as structure base. Experiments S8–S10 (Table 5 and Fig. 7) were kept as simple as possible, with bark sheets of the approximate same size laid flat underneath a layer of turf of approximately the same thickness.
Fig. 7

Experiments S8, S9 and S10. These bark sheets displayed a level of discolouration and charring directly corresponding to heat exposure time

The sheet experiments (S8–S10) proved that the duration of exposure to high-temperature fire was directly related to the amount of charring of the bark. From the presence of the bark sheets after intense fires, it was assumed that we had managed to limit oxygen supplies as the sheets were largely intact and had not combusted. Therefore, the main difference between the bark sheets seemed to be directly related to heat exposure over time. Given that in two of the pit structures (S5 and S6) we had previously produced a tarry substance in <6 h, it may be that the turf covering the bark sheets was too thick for sufficient heat penetration in experiments S8–S10; alternatively, it may have been that the fire was too unidirectional with the heat concentrated in one area. However, even though S8 (6-h fire) was sustained and intense, effective dry distillation did not occur, and we concluded that this was due to poor heat penetration through the turf cover.

Experiments S11 and S12

Experiments S11 and S12 were our last attempts to fire in a pit structure. With experiments S11 (Fig. 8) and S12 (Table 6), we went back to the pit structure taking into account the experience with pine tar distillation as it is still done traditionally in Finland and around Scandinavia, involving the introduction of moisture (see Egenberg and Glastrup 1999). As dry distillation of birch bark tar is generally done with dried bark and was assumed to be a dry process altogether, this option had not previously been investigated. Hot moisture was added to the process to inhibit air intake through the friable soil and to help produce a reducing atmosphere. We decided on adding both a lining of wet clay and a layer of fresh leaves on top of the birch bark. The leaves were overlain with a layer of dry grass, which by smouldering ideally would conduct heat downwards in the structure (Jens Glastrup, 2009, personal communication). The introduction of moisture was not beneficial to the process in the present structural form, and upon excavation, the bark displayed the least discolouration of all the experiments.
Fig. 8

Schematic sketch of S11

Experiments S13 and S14

With experiments S13 and S14 (Table 7 and Figs. 9 and 10), we returned to the idea of a standing structure because it allows heat penetration from multiple sides. We also decided to use sand alone as cover, without the turf. This allowed us to carefully mould and control the thickness of the cover. One of the experiments, S13, also included fresh leaves to give the hypothesis of moisture introduction a second attempt in a different type of structure. We used wet sand for cover, so that it would be airtight at least at the start of the experiments.
Fig. 9

Schematic sketch of S13

Fig. 10

Experiment S14 prior to firing

It was clear that the raised structures performed better than pits for tar production under the given circumstances. The efficiency of heating a small raised structure such as S7, S13 or S14 reduces the amount of fuel required compared to underground pits, where greater temperatures or longer burns are required to enable heat to penetrate through the turf covering. The existence of this type of small raised ‘kiln’ is conceivable in aceramic societies, where we must assume that the presence of cooking pits and other fire structures was commonplace. As such, it could be envisaged that a small raised mound of sand containing curls of Betula sp. bark could have been fashioned adjacent to a fire. These structures would, however, be hard to determine archaeologically.

Discussion

The variables that can influence the production of Betula sp. bark tar are most likely anaerobic conditions/reducing atmosphere, form/shape of structure, temperature, heat exposure time and heat penetration. The importance of a reducing atmosphere, however, does appear paramount to the production of tar. With reference to our experiments, the fact that several pits displayed what appeared to be initial tar production with subsequent incineration could suggest that the very friable soil with its low moisture content enabled the introduction of air into the pits during the burn via capillary action.

On advice from Jens Glastrup, the concerns with potential air intake led to modifications to other experiments, where prior to fuelling and burning moisture was added to the structures to inhibit capillary intake of air (S11, S12 and S13). However, the introduction of moisture by adding wet clay and leaves did not improve the conditions, as our last experiments S13 and S14 demonstrated. Here, the driest structure yielded the most successful result—namely, a gummy residue on one corner of the bark. As the bark was only burnt to destruction in one structure (S7), we can conclude that we succeeded to exclude air from the standing structures as the suberin in birch bark will incinerate at 410 °C (KGaA 2006). The two most successful pit structures (S2 and S4) were both fired on sunny days when the soil was extremely dry, whereas the rest of the structures were fired on days with more humid atmospheric conditions. If we also managed to provide oxygen-poor circumstances in these two structures, or if it was rather low temperatures resulting from poor heat penetration that prevented the bark from combustion, is unsure. However, if the latter was in fact the case, it is not expected that the bark would be charred to the extent that it was.

The fact that tarring did not occur is likely to have to do with the thickness and lack of heat conduction of the turf cover. As was displayed through the bark sheet experiments (S8–S10), the process was in fact beginning and the main difference between the bark sheets seemed to be directly related to heat exposure over time. The heat needed at least 6 h firing under the circumstances of the experiments to penetrate sufficiently to initiate the process. However, to achieve sufficient heat penetration through 50–70 mm of turf cover, it seems that the burn time and heat exposure must be increased. Since pits have unilateral heating, it is possible that a heat-conducting agent has to be introduced if they are to produce tar, such as was attempted in the experiments of Czarnowski and Neubauer (1992). In the future, pits with heated stones should be given closer scrutiny in light of aceramic tar distillation, both experimentally and in the archaeological record.

The later experiments that were developed into kiln-like structures (S7, S13 and S14) immediately seemed to yield better results. This is most likely to do with increased heat exposure time and, hence, temperature efficiency as, the fire now surrounded the structure instead of radiating heat from a single direction.

Since the conditions were not measured with instruments, it is hard to determine the moisture content or aridity of the soil; further experiments will focus on accurate measurements and documentation of temperatures and humidity.

Conclusion

By simplifying the otherwise technical method of tar production, the experiments referred above have gone some way towards suggesting possible production methods by pre-ceramic societies. Overall, the field experiments demonstrated that the use of simple fire structures and strategies could have resulted in the production of B. pubescens bark tar. The range and diversity of our initial experimental attempts generated data by which we modified subsequent experiments. Building from these, our evidence suggests that with some modifications, a small, raised ‘kiln’-like structure such as S7, S13 or S14 which excludes oxygen may be the most effective method for simple aceramic Betula sp. bark tar production. Contributory factors such as wind speed, wind direction, and fuel type and availability need to be considered.

Whilst acknowledging that we did not include added features such as hot stones or other heat-inducing elements to the pit structures, we suggest that these elements must be considered for later experiments with tar extraction pits. The balance between heat penetration through a protective medium (such as turf or sand) whilst excluding air to prevent combustion will be fundamental to our future experiments. Though the initial experiments here iterated did not successfully produce tar as a finished product, the results from the raised kiln-like structures suggest that this form may be most suitable in addressing this balance. The authors aim to start experiments with a second phase of structures in 2013 to further the exploration of aceramic tar distillation.

Notes

Acknowledgments

We wish to thank Jens Glastrup, Nationalmuseet in Copenhagen, for guidance in the chemical process of tar distillation and Sagnlandet Lejre for funding and accommodating our experiments. We would like to thank the reviewers for their useful comments and suggestions.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2013

Authors and Affiliations

  • Peter Groom
    • 1
  • Tine Schenck
    • 2
  • Grethe Moéll Pedersen
    • 3
  1. 1.StaffordUK
  2. 2.Department of ArchaeologyUniversity of ExeterExeterUK
  3. 3.StavangerNorway

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