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Direct 14C-dating of Roman and late antique purple dye sites by murex shells

Datation directe par le radiocarbone de coquillages murex provenant de teintureries de l’Antiquité romaine et tardive
Mark Van Strydonck, Mathieu Boudin et Damià Ramis
p. 15-23

Résumés

Jusqu’à la fin de l’Antiquité, les coquilles de murex étaient utilisées pour la production de la véritable pourpre. Des sites de production de murex ont été découverts sur tout le pourtour méditerranéen. Le présent article étudie quatre sites des Iles Baléares. Les datations radiocarbones d’os d’animaux et de charbon de bois, censés être concomitants de la production de teinture à partir du murex, sont comparées avec les datations directes des coquilles de murex. Sauf dans un cas, les échantillons terrestres étaient contradictoires. D’un autre côté, les coquilles de murex fournissaient des résultats cohérents. Le charbon de bois et les os d’animaux étaient donc sans nul doute parasites.

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Texte intégral

1. Introduction

1First used by the Phoenicians, murex shells were used for the production of true purple until late antique times (Gleba and Vanden Berghe, in print). Murex production sites are found all around the Mediterranean, and the Balearic Islands form no exception (Costa, 2009). The purple dyestuff is not present in the living animal, but after the animal dies it can develop through enzymatic hydrolysis of uncoloured components. This process has to be initiated immediately after collection of the shells. During this preparation an unpleasant smell is produced (rotting of the animal), hence it was forbidden to perform this activity in urban areas. Consequently purple dye production was done on site. As a result these sites contain very limited archaeological remains, except for large quantities of murex shell debris (Alfaro et al., 2004). In this paper shells, charcoal and bones coming from 4 Balearic sites were analysed by radiocarbon dating in order to date their use. All shells analysed in this study are Murex trunculus (1).

Figure 1: Murex trunculus from purple dye production sites.

Figure 1: Murex trunculus provenant d’une site de production de la teinture pourpre.

Figure 1: Murex trunculus provenant d’une site de production de la teinture pourpre.

2. Materials and methods

Sample selection

Figure 2: The Balearic Islands: 1) Cabrera; 2) Pedret de Bóquer; 3) Pou des Lleó - Cala Olivera

Figure 2 : Les Îles Baléares : 1) Cabrera ; 2) Pedret de Bóquer ; 3) Pou des Lleó - Cala Olivera

Figure 2 : Les Îles Baléares : 1) Cabrera ; 2) Pedret de Bóquer ; 3) Pou des Lleó - Cala Olivera

Mallorca

Cabrera

2A letter written by Pope Gregorius Magnum in 603 AD gives unmistakable evidence of the presence of a monastic congregation in the Cabrera Archipelago. Several locations near the port of Cabrera showed evidence of the presence of monks on the archipelago including a necropolis overlaying a purple dye tannery with a high concentration of murex debris (Riera 2010, Riera & Riera, 2005, Ramis & Pons, 2005). Bone samples from domesticated animals (goat) and murex shells from two locations were analyzed, both supposed to date from the period of the Roman purple dye production (figure 1).

Pedret de Bóquer

3Located just outside the port of Pollença, this is the area where the Civitas Bocchoritana was supposedly founded. During the excavation a reservoir lined with mortar was found together with a lot of shells, most of them murex (Estarellas & Merino, 2005). Samples from domesticated animals (bovine and goat) and murex samples, both supposed to date from the period of the purple dye production, were analysed.

Ibiza

Pou des Lleó

4The site of Canal d’en Martí is located in the Cala del Pou des Lleó, at the north-east coast of Ibiza. It has been recognized as a well-organized area for purple dye production, with a long history. Some architectural remains were recorded: mainly some pits and very eroded walls. The seashell debris related to purple production is very abundant and the characteristic breakage patterns of the specimens are quite common (Alfaro et al., 2002, 2004). In this case charcoal and murex shells were dated, both supposed to date from the period of the purple dye production.

Cala Olivera

5Cala Olivera is situated near the city of Ibiza, on the east coast of the island. A small collection of seashells —mainly murex— has been excavated there. The archaeological site is interpreted as a small purple dye workshop without any visible architectonic remains (Costa & Alfaro, 2007, Alfaro & Costa, 2008). Also in this case charcoal and murex shells were dated, both supposed to date from the period of the purple dye production.

Sample pre-treatment

6Collagen from the bone samples was extracted following the Longin method (Longin 1971) with an additional NaOH-wash. Collagen quality was tested by C:N (carbon/nitrogen) ratio and collagen yield. A small portion of each collagen sample was transferred in duplicate into tin capsules, which were analyzed using a ThermoFinnigan delta +XL (continuous flow type) isotopic mass spectrometer, interfaced with a Flash EA1112 elemental analyzer via a Conflo III interface. Both elements were measured together and a Helium-dilution was applied for carbon as the amount of C is much greater than the amount of N in the samples. Furthermore the visual appearance of the collagen and the amount of retrieved collagen were considered (Van Strydonck et al. 2005).

7Charcoal was successively washed in hot HCl (1%), NaOH (1%) and HCl (1%) (acid-alkali-acid) method and then again washed with Milli-Q water and dried (Van Strydonck et al. 2010).

8Shells were cleaned by removing the surface layer of the shell. Sample quality was checked by δ13C measurements (marine samples have δ 13C > 0 ‰) using a Finnigan-Mat- δ isotopic mass spectrometer and the crystal structure was measured by XRD using a Bruker-D8. The XRD measurements are necessary because in the case of marine carbonates, contamination appears usually in the form of secondary low-magnesium calcite – the stable polymorph of calcium carbonate (CaCO3) – and byproduct of the post-mortem recrystallization or replacement of the autochthonous phase, originally in the form of high-magnesium calcite or aragonite. Depending on the nature of the depositional environment, the recrystallized phase may be contemporary in age with the original shell-CaCO3 and may have even derived from it by dissolution-recrystallization reactions, or can be an exogenous contaminant of younger or older age (Magnani et al. 2007).

Graphitization and AMS measurements

9CO2 from the organic samples (charcoal and collagen) was obtained by combustion (in the presence of CuO) and in the case of shells by extraction with phosphoric acid (H3PO4).

10Graphitisation of CO2 was carried out using H2 over a Fe catalyst.

11Targets were prepared at the Royal Institute for Cultural Heritage in Brussels (Belgium) (M. Van Strydonck, K. van der Borg 1990-91) and measured at the Leibniz Labor für Altersbestimmung und Isotopenforschung in Kiel (Germany) (Nadeau et al., 1998).

3. Results and discussion

Results

12All results are summarized in table 1, the uncalibrated radiocarbon results are depicted in figure 3.

Figure 3: uncalibrated radiocarbon analyses (BP).

Figure 3: uncalibrated radiocarbon analyses (BP).

Discussion

Sample quality control

13All charcoal samples resisted very well the AAA pre-treatment.

14The quality of the bones was also very good. Although the amount of collagen was sometimes low, other parameters such as the C:N ratio, and the general appearance of the collagen were excellent. δ13C and δ15N correspond to terrestrial herbivores, no shift due to contamination was observed (Van Strydonck et al., 2005).

15All shells show positive δ13C values, indicating a 100 % marine provenance of the carbon. In some of the samples however calcite was detected. The presence of calcite is a strong indication of the presence of secondary carbonate. This secondary carbonate can have a different 14C signal than the original aragonite from the murex shells. Fortunately our results exclude this possibility. All murex shells from UE501 at Bóquer statistically have the same age although one of the samples contains about 16.2 % calcite. The murex samples from Cabrera statistically have the same age as well, in spite of the presence of different amounts of calcite. This proves that the deposition of secondary carbonate happened shortly after the shells were harvested. This situation is in agreement with the production process of the purple dye.

Sample integrity

16While the sample quality concerns the reliability of the 14C date, the relation between the samples age and the archaeological phenomenon to be dated is defined by the sample integrity. There should be a close relationship between the radiocarbon age of the material and the human event of interest (Van Strydonck et al., 1999). Only in the case of the Bóquer samples the differences in age between the marine murex shells and the terrestrial bones from domesticated animals are almost constant as one should expect from marine/terrestrial pairs with the same real age (figure 3, table 1). Using the Bóquer samples to calculate the R value (local deviation from the global ocean reservoir age)for the Mediterranean surface waters around the Balearic archipelago, a value of 26±24 14C year is obtained (Van Strydonck et al. 2010). It should be noted here that the used R value is in agreement with previous research (Siani et al., 2000)

17In the case of the Cabrera samples important age differences are observed between the terrestrial samples [χ² test fails: df=4, T= 204.014 (5%= 9.5)]. The murex samples on the other hand form a much more coherent series.

18In the case of the two sites from Ibiza, the charcoal samples are unrealistically young.

19Figure 3 depicts the calibrated radiocarbon results (Bronk Ramsey 2001) using the marine data from Reimer et al. (2009) and a R = 26±24 14C year for the different sites and units, calculated from the murex dates. From Bóquer UE 501 as well as from Cabrera PF05 the average of 3 results is calculated, from Cabrera 155 the average of 2 results is calculated. A χ² test shows that samples originating from the same layer can have the same real age. The results are also summarized in table 2.

Table 1: Samples and results from dye sites.

Tableau 1 : Echantillons et résultats des sites de teintureries.

reference

sample

lab. ref.


(KIA-)

BP

± 1σ

% collagen

appearance collagen

δ 13C ‰

δ 15N ‰

C:N

% aragonite

% calcite

Bóquer (Mallorca)

BO6 UE 401

animal bone

38439

1605

25

3.36

white & fluffy

-20.2

+6.1

3.2

BO6 UE 501 A

bovine

38442

1705

25

0.79

white & fluffy

-20.0

+4.7

3.2

BO6 UE 501 B

domesticated animal

38447

1675

25

3.00

white & fluffy

-19.7

+4.9

3.2

BO6 UE 501 C

horne pit goat

38446

1695

25

3.33

light yellow & fluffy

-19.0

+6.5

3.3

BO6 UE 501 A

murex

38443

2065

25

+2.4

100.0

BO6 UE 501 B

murex

38444

2050

30

+2.0

83.8

16.2

BO6 UE 501 C

murex

38445

2070

25

+2.5

100.0

BO6 UE 401

murex

38440

1980

25

+2.2

100.0

Cabrera (Mallorca)


Test site 1

PF 03 UE 116-1

goat

38449

1290

25

5.25

white & fluffy

-19.0

+8.4

3.2

PF 03 UE 116-2

goat

38448

1280

25

1.80

white & fluffy

-19.0

+8.5

3.2

UE 155-1

murex

38456

1765

25

+0.9

91.7

8.3

UE 155 -2

murex

38441

1825

25

+1.3

87.6

12.4

Cabrera (Mallorca)


Test site 2

PF 05 UE 263

goat

40857

1425

25

6.10

white & fluffy

-19.6

6.2

3.1

PF 05 UE 265

goat

40858

940

25

8.40

white & fluffy

-19.3

7.0

3.1

PF 05 UE 305

goat

40859

1285

25

3.50

white & fluffy

-19.7

6.2

3.1

 

 

PF 05 UE 263

murex

40789

1855

20

 

 

+0.4

 

 

90.9

9.1

PF 05 UE 265

murex

40788

1800

25

 

 

+1.1

 

 

89.8

10.2

PF 05 UE 305

murex

40790

1840

20

 

 

+1.5

 

 

85.5

14.5

Pou des Lleó (Ibiza)

Unit 2204

charcoal

43313

130

30

Unit 2204

murex

43311

2010

25

+1.8

77.9

22.1

Cala Oliver (Ibiza)

UE 8 – D1

charcoal

43312

65

25

UE 8 – C2

murex

43033

2050

30

+0.7

100

Figure 4: Calibrated radiocarbon dates from murex samples.

Figure 4 : Résultats radiocarbone calibrés des échantillons de murex.

Figure 4 : Résultats radiocarbone calibrés des échantillons de murex.

Table 2: Calibrated radiocarbon dates from murex shells.

Tableau 2 : Dates radiocarbones calibrés des coquillages de murex

Cala Oliver: 2050±30BP


300AD (68.2%) 430AD


240AD (95.4%) 460AD

Bóquer 401: 1980±25BP


390AD (68.2%) 510AD


340AD (95.4%) 560AD

Pou des Lleó: 2010±25BP


350AD (68.2%) 460AD


290AD (95.4%) 530AD


Cabrera PF05: 1836±12BP


570AD (68.2%) 645AD


530AD (95.4%) 680AD


χ 2-Test: df=2 T=3.0(5% 6.0)

Bóquer 501: 2063±15BP


290AD (68.2%) 400AD


250AD (95.4%) 430AD


χ2-Test: df=2 T=0.3(5% 6.0)


Cabrera 155: 1795±18BP


610AD (68.2%) 670AD


560AD (95.4%) 700AD


χ 2-Test: df=1 T=2.9(5% 3.8)

20The results in table 2 and figure 4 are coherent and correspond to the archaeological expectations. This is not the case for the charcoal samples and the bones from domesticated animals, both yielding dates that are contradictory and too recent.

4. Conclusion

21Most archaeologists prefer terrestrial samples to marine samples for dating because the marine reservoir age of large parts of the world is unknown and because of the possibility of contamination. In many cases however sample integrity should be a much more decisive factor than sample quality. There is a much more direct relationship between the murex shells and the human event of interest – purple dye production – (Van Strydonck et al., 1999) than there is between this event and the bones or the charcoal found on site. Consequently the ideal radiocarbon sampling strategy should exhibit a good equilibrium between sample quality and sample integrity.

This study was financed by the European project DressID: Clothing and identities. New perspectives on textiles in the Roman empire.

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Bibliographie

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Table des illustrations

Titre Figure 1: Murex trunculus provenant d’une site de production de la teinture pourpre.
URL http://archeosciences.revues.org/docannexe/image/3687/img-1.jpg
Fichier image/jpeg, 1,7M
Titre Figure 2 : Les Îles Baléares : 1) Cabrera ; 2) Pedret de Bóquer ; 3) Pou des Lleó - Cala Olivera
URL http://archeosciences.revues.org/docannexe/image/3687/img-2.jpg
Fichier image/jpeg, 1,3M
Titre Figure 3: uncalibrated radiocarbon analyses (BP).
URL http://archeosciences.revues.org/docannexe/image/3687/img-3.jpg
Fichier image/jpeg, 253k
Titre Figure 4 : Résultats radiocarbone calibrés des échantillons de murex.
URL http://archeosciences.revues.org/docannexe/image/3687/img-4.jpg
Fichier image/jpeg, 561k
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Pour citer cet article

Référence papier

Mark Van Strydonck, Mathieu Boudin et Damià Ramis, « Direct 14C-dating of Roman and late antique purple dye sites by murex shells », ArcheoSciences, 36 | 2012, 15-23.

Référence électronique

Mark Van Strydonck, Mathieu Boudin et Damià Ramis, « Direct 14C-dating of Roman and late antique purple dye sites by murex shells », ArcheoSciences [En ligne], 36 | 2012, mis en ligne le 31 décembre 2014, consulté le 26 mai 2017. URL : http://archeosciences.revues.org/3687 ; DOI : 10.4000/archeosciences.3687

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Auteurs

Mark Van Strydonck

Royal Institute for Cultural Heritage, Jubelpark 1, 1000 Brussels, Belgium. (Mark.vanstrydonck@kikirpa.be)

Mathieu Boudin

Royal Institute for Cultural Heritage, Jubelpark 1, 1000 Brussels, Belgium

Damià Ramis

Natural History Society of the Balearic Islands, Margarida Xirgú 16, 07011 Palma de Mallorca, Balearic Islands, Spain

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Droits d’auteur

Article L.111-1 du Code de la propriété intellectuelle.

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