<figure><img src="image_93.png" /><figcaption>Bény 23: Ascia ad alette, greggio di fusione frammento, evidente eccesso di materiale sul tagliente e sui fianchi del corpo, presenza dipatina di corrosione</figcaption></figure>

<caption>Bény 35: Ascia ad alette lavorata, evidenti segni di usura sultagliente, presenza di patina di corrosione</caption>
<figure><img src="image_94.png" /><figcaption>Bény 161: Ascia ad alette, greggio di fusione, sbavature di materialesui fianchi, presenza di patina di corrosione</figcaption></figure>

<caption>Bény 174: Ascia ad alette, greggio di fusione, presenza di anelloanche se difettoso, frammentata sul tagliente probabile defunzionalizzazione, presenza di patina di corrosione</caption>
## 8.2 Microscopio ottico (LOM) Campo Chiaro

All'interno di questa miscellanea di fotografie al Microscopio ottico di tutti i campioni, sono riscontrabili le strutture descritte all'interno del corpo della tesi: matrice omogenea, matrice eterogenea, presenza di dendriti. Esse sono il prodotto dell'assenza di lavorazione o il prodotto di trattamenti termomeccanici specifici.

È inoltre ravvisabile corrosione intergranulare, che ha causate cricche all'interno della lega. In certi casi la corrosione ha letteralmente mangiato la lega lasciando lacune

1) Bény 1: immagine LOM, matrice omogenea di Cu con residui di struttura bifasica eutettoide-, corrosione intergranulare che si sviluppa lungo i bordi grano, microcracking

2) Bény 10: immagine LOM, matrice bifasica con presenza di grani dendritici. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking. Si evidenzia una frattura che parte dallo strato di corrosione e continua per il campione.

3) Bény 14: immagine LOM, matrice tendenzialmente omogenea con residui di struttura bifasica, si evidenzia una frattura e corrosione

4) Bény 23 C, immagine LOM, matrice altamente bifasica con presenza di strutture dendritiche. Si evidenzia la presenza di un residuo di Cu non legato. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking.

5) Bény 23 T, immagine LOM, matrice eterogenea bifasica con strutture dendritiche. Si evidenziano fasi aciculari, dovute al rapido raffreddamento.

6) Bény 35, immagine LOM, matrice abbastanza omogenea con residui di struttura bifasica. Rilevata la presenza di corrosione e leggero microcracking. Si evidenzia la presenza del bordo del campione di geminati meccanici, prodotto di lavorazione meccanica sull'oggetto.

7) Bény 37: matrice omogenea. Sono presenti microfratture e forte corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking.

8) Bény 46: immagine LOM, matrice omogenea. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking; evidente una frattura al di sotto del bordo come diretta conseguenza della corrosione intergranulare. Sono presenti geminati meccanici. La cricca che si evidenza nella parte più interna dell'immagine è riscontrabile nella fase δ dovuta alla lavorazione a freddo.

9) Bény 46, immagine LOM, matrice omogenea. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking; evidente una frattura al di sotto del bordo come diretta conseguenza della corrosione intergranulare. Sono presenti geminati meccanici incrociati, è presente un po' di fase α+δ, conseguenti a ricottura e deformazione.

10) Bény 131, immagine LOM, matrice eterogenea bifasica con strutture dendritiche. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking. Assenza di lavorazione, non sono presenti cricche ed è presente ancora il dendritismo di partenza; la seconda fase presenta dendriti notevolmente pronunciate

11) Bény 131, immagine LOM, matrice eterogenea bifasica con strutture dendritiche. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeño del microcracking. Assenza di lavorazione, la cricca presente è dovuta alla corrosione

12) Bény 46, immagine LOM, matrice omogenea. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking; evidente una frattura al di sotto del bordo come diretta conseguenza della corrosione intergranulare. Sono presenti geminati meccanici incrociati, è presente un po' di fase α+δ, conseguenti a ricottura e deformazione

13) Bény 153, immagine LOM, matrice bifasica con strutture dendritiche. Sono presenti microfratture e corrosione intergranulare che si sviluppa lungo i bordi grano aumentando il fenomeno del microcracking, forse grossa frattura dovuta all'avanzare della corrosione.
1

<figure><img src="image_95.png" /></figure>

2

<figure><img src="image_96.png" /></figure>
3

<figure><img src="image_97.png" /></figure>

4

<figure><img src="image_98.png" /></figure>
5

<figure><img src="image_99.png" /></figure>

6

<figure><img src="image_100.png" /></figure>
7

<figure><img src="image_101.png" /></figure>

8

<figure><img src="image_102.png" /></figure>
<figure><img src="image_103.png" /></figure>

<figure><img src="image_104.png" /></figure>
11

<figure><img src="image_105.png" /></figure>

12

<figure><img src="image_106.png" /></figure>
<figure><img src="image_107.png" /></figure>
8.3 Microscopio ottico (LOM) Campo Scuro

All'interno di questa miscellanea di fotografie al Microscopio Ottico di tutti i campioni sono ravvisabili tutti i prodotti di corrosione descritti all'interno del corpo del testo della tesi: ossidi (cuprite) e carbonati di rame (azzurrite e malachite). In talune foto si vede chiaramente la stratificazione dei prodotti di corrosione oltre che, appunto, l'avanzare della corrosione all'interno della matrice.

1) Bèny 1, immagine LOM, grosso cristallo di azzurrite nello strato di corrosione,

2) Bény 10, immagine LOM, presenza di cuprite, azzurrite e malachite nello strato di corrosione. Risulta anche ricca in Sn (bianco) e ossido di Sn (giallo)

3) Bény 21, immagine LOM, presenza di cuprite e azzurrite in cristalli dispersi all'interno della malachite nello strato di corrosione

4) Bény 21, immagine LOM, presenza di cuprite e azzurrite in cristalli dispersi all'interno della malachite nello strato di corrosione

5) Bény 23 C, immagine LOM, presenza di un grosso cristallo di cuprite, oltre a quella già presente nello strato di corrosione insieme all'azzurrite

6) Bény 23 T, immagine LOM, presenza di forte corrosione lungo i bordi del campione con evidenza di cuprite

7) Bény 23, immagine LOM, si evidenzia un grosso e ben definito cristallo di cuprite e scarsa presenza di malachite

8) Bény 35, immagine LOM, presenza di malachite in cristalli dispersi nello strato di corrosione, pochissimi cristalli di cuprite

9) Bény 35, immagine LOM, dettaglio dei cristalli di malachite

10) Bény 37, immagine LOM, presenza di cuprite, malachite e azzurrite nello strato di corrosione. Si evidenza molto bene l'avanzamento verso l'interno della corrosione

11) Bény 153, immagine LOM, Presenza di cuprite, malachite e azzurrite nello strato di corrosione

12) Bény 153, immagine LOM, dettaglio dei cristalli di malachite

13) Bény 153, immagine LOM, dettaglio dei cristalli di malachite e cuprite

14) Bény 174, immagine LOM, presenza di cuprite, malachite e azzurrite nello strato di corrosione
<figure><img src="image_108.png" /></figure>
2

<figure><img src="image_109.png" /></figure>

3

<figure><img src="image_110.png" /></figure>
4

<figure><img src="image_111.png" /></figure>

5

<figure><img src="image_112.png" /></figure>
6

<figure><img src="image_113.png" /></figure>

7

<figure><img src="image_114.png" /></figure>
8

<figure><img src="image_115.png" /></figure>

9

<figure><img src="image_116.png" /></figure>
10

<figure><img src="image_117.png" /></figure>

11

<figure><img src="image_118.png" /></figure>
12

<figure><img src="image_119.png" /></figure>

13

<figure><img src="image_120.png" /></figure>
<figure><img src="image_121.png" /></figure>
8.4 Microscopio Elettronico a scansione (SEM)

All'interno di questa miscellanea di fotografie al Microscopio Elettronico a scansione di tutti i campioni, sono riscontrabili le strutture descritte all'interno del corpo della tesi: matrice omogenea, matrice eterogenea, struttura dendritica, solfuri tondeggianti, polilobati ed allungati, geminati meccanici che sono indice di lavorazione meccanica. Tutte sono comunque o il prodotto dell'assenza di lavorazione oil prodotto di trattamenti termomeccanici specifici.

1) Bény 21, immagine SEM, solfuri per la maggior parte polilobati immersi in una matrice eterogenea con presenza di struttura bifasica

2) Bény 21, immagine SEM, dettaglio a maggior ingrandimento della struttura bifasica e dei solfuri polilobati

3) Bény 21, immagine SEM, dettaglio dello strato di corrosione, evidente la corrosione intergranulare, presenza di grossa cricca lineare e parallela che evidenzia maggiormente l'avanzamento della corrosione

4) Bény 37, immagine SEM, solfuri sia polilobati che tondeggianti immersi in una matrice omogenea. Quasi assenza totale di seconda fase

5) Bény 37, immagine SEM, dettaglio dello strato di corrosione, evidente la corrosione intergranulare, presenza delle due grosse cricche lineari e parallele che evidenziano maggiormente l'avanzamento della corrosione.

6) Bény 46, immagine SEM, solfuri tondeggianti immersi in una matrice omogenea, c'è qualche residuo di struttura bifasica

7) Bény 46, immagine SEM, dettaglio dello strato di corrosione, evidente la corrosione intergranulare che avanza verso l'interno testimoniata dalla presenza inoltre di una cricca.

8) Bény 131, immagine SEM, solfuri polilobati immersi in una matrice eterogenea bifasica con strutture dendritiche

9) Bény 131, immagine SEM, dettaglio a maggiore ingrandimento dei solfuri polilobati e della presenza della bifasicità

10) Bény 131, immagine SEM, dettaglio dello strato di corrosione che avanza verso l'interno

11) Bény 132, immagine SEM, solfuri polilobati immersi in una matrice eterogenea bifasica con strutture dendritiche

12) Bény 132, immagine SEM, dettaglio dello strato di corrosione molto pronunciata

13) Bény 153, immagine SEM, solfuri allungati senza una specifica orientazione immersi in una matrice omogenea bifasica con strutture dendritiche. Dendriti abbastanza dritte e non deformate, dalla morfologia più arrotondata

14) Bény 153, immagine SEM, dettaglio a maggiore ingrandimento delle dendriti e dei solfuri tondeggianti

15) Bény 153, immagine SEM, dettaglio dello strato di corrosione che avanza verso l'interno, con formazione di microcracking dovuto all'avanzamento della corrosione.

16) Bény 153, immagine SEM; dettaglio a maggiore ingrandimento dello strato di corrosione, con evidente cricca

17) Bény 23 C, immagine SEM, Solfuri allungati (non per lavorazione, ma per trasformazioni termiche che hanno schiacciato i solfuri) senza un'orientazione specifica immersi in una matrice altamente bifasica con presenza di strutture dendritiche.

18) Bény 23 C, immagine SEM, dettaglio a maggiore ingrandimento delle strutture dendritiche

19) Bény 23 T, immagine SEM, dettaglio a maggiore ingrandimento della matrice bifasica, con presenza di solfuri allungati non per lavorazione, come Bény 23 C

20) Bény 29, immagine SEM, solfuri tondeggianti dispersi in una matrice eterogenea con evidenti strutture dendritiche
21) Bèny 29, immagine SEM, dettaglio a maggiore ingrandimento della struttura bifasica e deisolfuri tondeggianti

22) Bény 168, immagine SEM, solfuri polilobati e tondeggianti immersi in una matrice omogenea con residui di struttura bifasica

23) Bény 180, immagine SEM, solfuri polilobati immersi in una matrice omogenea con residui di strutture bifasiche con grani poligonali. Pochissima presenza di seconda fase, quindi ha subito trattamento di omogeneizzazione e ricristallizzazione.
1

<figure><img src="image_122.png" /></figure>

20 µm
H

EHT = 20.00 kV

WD = 8.0 mm

Signal A = NTS BSD

Mag = 500 X

Sample ID = Hache Beny 21

2
<figure><img src="image_123.png" /></figure>

2 µm
H

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 5.00 K X

Sample ID = Hache Beny 21

<figure><img src="image_124.png" /></figure>

20 µm
H

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 500 X

Sample ID = Hache Beny 21
4

<figure><img src="image_125.png" /><figcaption>20 µm
EHT = 20.00 kV
WD = 8.0 mm</figcaption></figure>

Signal A = NTS BSD
Mag = 1.00 K X
Sample ID = ascia Beny 37

5

<figure><img src="image_126.png" /><figcaption>100 µm
EHT = 20.00 kV
WD = 8.0 mm</figcaption></figure>

Signal A = NTS BSD
Mag = 300 X
Sample ID = ascia Beny 37
<figure><img src="image_127.png" /></figure>

<figure><img src="image_128.png" /></figure>
<figure><img src="image_129.png" /></figure>

<figure><img src="image_130.png" /></figure>

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 500 X

Sample ID = Hache Beny 131

<figure><img src="image_131.png" /></figure>

<figure><img src="image_132.png" /></figure>

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 5.00 K X

Sample ID = Ascia Beny 131
10

<figure><img src="image_133.png" /></figure>

10 µm
H

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 1.00 K X

Sample ID = Ascia Beny 131

11

<figure><img src="image_134.png" /></figure>

20 µm
H

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 1.00 K X

Sample ID = hache 132 Beny polished
<figure><img src="image_135.png" /></figure>

<figure><img src="image_136.png" /></figure>
14

<figure><img src="image_137.png" /></figure>

10 µm
H

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 1.00 K X

Sample ID = 153 Beny

15

<figure><img src="image_138.png" /></figure>

20 µm
H

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 500 X

Sample ID = 153 Beny
16

<figure><img src="image_139.png" /></figure>

20 µm

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 2.00 K X

Sample ID = 153 Beny

17

<figure><img src="image_140.png" /></figure>

20 µm

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 500 X

Sample ID =
18

<figure><img src="image_141.png" /></figure>

19

<figure><img src="image_142.png" /></figure>
20

<figure><img src="image_143.png" /></figure>

20 µm

$$EHT = 20.00 \text{ kV}$$

$$\text{Signal A} = \text{NTS BSD}$$

$$WD = 8.0 \text{ mm}$$

$$\text{Mag} = 1.00 \text{ K X}$$

$$\text{Sample ID} =$$

21

<figure><img src="image_144.png" /></figure>

10 µm

$$EHT = 20.00 \text{ kV}$$

$$\text{Signal A} = \text{NTS BSD}$$

$$WD = 8.0 \text{ mm}$$

$$\text{Mag} = 3.00 \text{ K X}$$

$$\text{Sample ID} =$$
<figure><img src="image_145.png" /></figure>

<figure><img src="image_146.png" /></figure>

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 500 X

Sample ID =

<figure><img src="image_147.png" /></figure>

<figure><img src="image_148.png" /></figure>

EHT = 20.00 kV

Signal A = NTS BSD

WD = 8.0 mm

Mag = 1.00 K X

Sample ID =
## 8.5 Attacco acido

All'interno di questa miscellanea di fotografie al Microscopio ottico di tutti i campioni, sono riscontrabili le strutture descritte all'interno del corpo della tesi in seguito ad attacco acido conFeCl₃: dendriti, grani poligonali, grani fortemente ingrossati, dendriti ingrossate. Esse sono o il prodotto dell'assenza di lavorazione o il prodotto di trattamenti termomeccanici specifici.

<figure><img src="image_149.png" /><figcaption>Bèny 132, immagine LOM, l'attacco acido con FeCl₃ ha messo in risalto le strutture dendritiche</figcaption></figure>
<figure><img src="image_150.png" /></figure>

Bény 10, immagine LOM, l'attacco acido con FeCl$_3$ ha messo in risalto unostadio intermedio di ricottura in cui è presente parzialmente l'eterogeneità della matrice ed una parziale ricristallizzazione
<figure><img src="image_151.png" /><figcaption>Bèny 23 C, immagine LOM, l'attacco acido con FeCl₃ ha messo in risalto le strutture dendritiche</figcaption></figure>
<figure><img src="image_152.png" /><figcaption>Bény 23T, immagine LOM, l'attacco acido con FeCl$_3$ ha messo in risalto lestrutture dendritiche</figcaption></figure>
<figure><img src="image_153.png" /><figcaption>Bény 35, immagine LOM, l'attacco acido con FeCl$_3$ ha messo in risalto igrani poligonali</figcaption></figure>
<figure><img src="image_154.png" /></figure>

Bény 37, immagine LOM, l'attacco acido con FeCl$_3$ ha messo in risalto igrani poligonali e geminati meccanici. Presente un processo di ingrossamento del grano con cristalli grandi e bordi tondeggianti che indicano un trattamento termico a T alte o tempi lunghi di cottura.
<figure><img src="image_155.png" /></figure>

Bény 49, immagine LOM, l'attacco acido con FeCl$_3$ ha messo in risalto igrani poligonali e geminati meccanici. Presente un processo di ingrossamento del grano con cristalli grandi e bordi tondeggianti che indicano un trattamento termico a T alte o tempi lunghi di cottura.
<figure><img src="image_156.png" /></figure>

Bény 168, immagine LOM, l'attacco acido con FeCl₃ ha messo in risalto igrani poligonali, molto ingrossati probabilmente sempre causa di trattamenti termici a T elevate o sottoposti a cotture prolungate
<figure><img src="image_157.png" /></figure>

Bény 180, immagine LOM, l'attacco acido con FeCl$_3$ ha messo in risalto igrani poligonali, molto ingrossati probabilmente sempre causa di trattamenti termici a T elevate o sottoposti a cotture prolungate
9. BIBLIOGRAFIA

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[3] P. Piccardo, G. Ghiara, J. Vernet, Mise en œuvre des alliages cuivreux : faire parler le métal grâce à la science des matériaux. In : M. Pernot, Quatre mille ans d'histoire du cuivre, fragments d'une suite de rebonds, Presses universitaires de Bordeaux, collection « THEA », p. 41-60, 2017

[4] Dominique Cliquet, Maréva Gabillot et Claude Escolano, « Bény-sur-Mer – La Grande Hache » [notice archéologique], ADLFI. Archéologie de la France - Informations [En ligne], Normandie, mis en ligne le 04 juin 2021, consulté le 16 octobre 2022. URL: http://journals.openedition.org/adlfi/74568

[5] A J Wilson, The Living Rock: The story of metals since earliest times and their impact on developing civilization, Cambridge, Woodhead Publishing Ltd., 1996 [1994], p. 8.

[6] P.J. Bray, A.M. Pollard, A new interpretative approach to the chemistry of copper-alloy objects: source, recycling and technology, Antiquity 86, p. 853-867 2012

[7] M. Cavallini, Fortutitum & sordidum opus. Appunti di storia della Metallurgia, Crace, Perugia, 2005

[8] P. Piccardo, R. Amendola, A. Adobati, C. Faletti, Studio della fluidità di leghe a base rame, Metall Ital 5:31-38, 2009

[9] S. Kumar, C. A. Handwerker, M. A. Dayananda, Intrinsic and Interdiffusion in Cu-Sn System, Journal of Phase Equilibria and Diffusion, 2011

[10] Paul, C. Ghosh, W.J. Boettinger, Diffusion Parameters and Growth Mechanism of Phases in the Cu-Sn System. In: Metallurgical and Materials Transactions, 2011

[11] P. Piccardo, R. Amendola, A. Ervas, Metallographic investigation and experimental replication of an Etruscan bronze mirror, Hist Metall 44: 10-14, 2010

[12] C. Fan, A. Blair, L. Mayer, J. Abys, New Technical and Economical Developments in Nickel/Palladium Leadframe Surface Finishing, AESF SUR/FIN, Chicago, 2000

[13] G. J. Davies, Solidification and casting, Halsted Press 1973

[14] H. Fredriksson, U. Akerlind, Materials Processing during Casting, John Wiley & Sons, Ltd, England 2006

[15] G. F. Vander Voort, Metallography and Microstructures, ASM Metals Handbook, 9, 2004

[16] M. Leoni, Elementi di Metallurgia applicata al restauro delle opere d'arte, corrosione conservazione dei manufatti metallici, Opus Libri, Firenze 1984

[17] K. Geels, D.B. Fowler, W.U. Kopp, M. Ruckert, Metallographic and Materialographic Specimen Preparation, Light Microscopy, Image Analysis and Hardness Testing, ASTM International, New York, 2006

[18] J. I. Goldstein, D. E. Newbury, J. R. Michael, N. W. M. Ritchie, J. H. J. Scott, D. C.
Joy, Scanning electron microscopy and X-ray microanalysis, Springer, New York, 2018

[19] D. Hanson, W. T. Pell-Walpole, Chill-cast tin bronzes, E. Arnold & Co, London, 1951

[20] J. M. Vernet, Out-of-equilibrium behaviour of copper-based alloys in industrial, artistic and historical gravity casting processes, PhD thesis, 2018

[21] P. Piccardo, M. Pernot, Studio analitico e strutturale di alcuni vasi celtici in bronzo, Metal Ital 11: 43-52, 1997

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Matteini, ICVBC, Nardini Editore, Firenze 2004

[23] D. A. Scott, Copper and Bronze in Art, corrosion, colorants, conservation, Getty Conservation Institute, Los Angeles, 2002

[24] P. Piccardo, B. Mille, L. Robbiola, Tin and copper oxides in corroded archaeological bronzes. In: P. Dillman, G. Béranger, P. Piccardo, H. Matthiesen (EDS), Corrosion of Metallic heritage artefacts: investigation, conservation and prediction for long term behaviour, WoodHead publishing, Cambridge, p. 239-262, 2007

[25] I. De Ryck, A. Adriaens, F. Adams, An overview of Mesopotamian bronze metallurgy during the 3rd millennium BC, Journal of Cultural Heritage, 6, pp 261-268, 2006

[26] S. Hurst, Metal casting: appropriate technologies in the small foundry, ITDG publishing, London, 1996
# Archaeometallurgical investigation of a Middle Bronze Age hoard from Bény-sur-Mer

Camilla Criaco<fnref n="1" />, Paolo Piccardo <fnref n="1" />,* Mareva Gabillot <fnref n="2" />, Céline Lagarde³, Flavia Boragina<fnref n="1" /> and Roberto Spotorno<fnref n="1" />

**Abstract:** This paper presents a detailed analysis of a Middle Bronze Age (1600-1350 B.C.) hoard containing 182 axes between intact objects and fragments. Our research is focused on evaluating 19 axes by chemical and metallographic examination. Furthermore, the techniques of the ancient smiths of Northern France were defined from that historical period, along with the composition of the materials and the quality of the axes themselves from a functional and material point of view and whether there had been any trade. All the objects were sampled and mounted in cold epoxy resin and polished for microstructural analyses with Light Optical Microscope (LOM). The chemical analyses were performed using Scanning Electron Microscopy – Energy Dispersive X-Ray Spectroscopy (SEM-EDS), and each sample was etched for metallographic examination with ferric chloride. The analyzed axes show the presence of Cu and Sn with a variable percentage of Sn ranging from 12 wt% to 15 wt%, falling within the specifications of Middle Bronze Age alloys.

**Keywords:** Middle Bronze Age; Bény-sur-Mer; Metallography; Chemical analysis; Copper alloys; Bronze axes

## 1. Introduction

The archaeological finds presented in this paper come from northern France, specifically from Bény-Sur-Mer, a small town in Normandy. The area of discovery, close to the D-Day beaches, was entirely fortuitous; during a survey of the area by C. Escolano, fragments, and 15 axes were found on the surface. This immediately led to the opening of an excavation campaign to counter even the activity of amateur detectorists focused on the D-Day beaches. The excavated area unearthed a hoard of axes stacked one on top of the other in an elliptical pit 60 cm by 50 cm in shape, with a bowl-shaped profile about 50 cm deep in an orange-colored clay soil. The uniqueness of this find is due to the multiple types of axes found within it, which refer to "Atlantique-Manche" productions. In addition, typological studies have already been carried out by Maréva Gabillot [1] (UMR ARTeHIS, Dijon) and Céline Lagarde (Cardona, UMR IRAMA-CRP2A, Bordeaux). It is interesting to observe that 44 have peculiar marks on the cutting edge, since they were probably used to

<footnote n="1">¹ Affiliation 1; e-mail:e-mail.com</footnote>

<footnote n="2">² Affiliation 2; e-mail:e-mail.com</footnote>

<footnote>* Correspondence: e-mail@e-mail.com; Tel.: (optional; include country code; if there are multiple corresponding authors, add author initials)</footnote>
defunctionalize or break other axes, 82 are ready to use or already partially used, and 100 are non-functional pieces, but to be considered as casting blanks or ingots of metal alloy [2].

All these preliminary data could be of interest in uncovering trade and craftsmanship contacts and exchanges between Northern France and Middle Bronze Age Britain.[3] The finds are currently kept at INRAP in Bourguébus (Calvados).

Metallic materials, due to their versatility of use and being easily recyclable, have always accompanied human history. Different metal alloys correspond to different uses, so this is why it is so crucial to know the metallurgy of bronze and copper. In fact, due to this information, it is possible to reconstruct various aspects of ancient society in each historical period and geographical area. Metallurgy, thanks to the data collected through metallurgical characterization, which is then compared with the typological studies of archaeologists and experimental activities, allows us to see how mankind over the millennia has managed, even by making mistakes, to improve himself in the techniques of producing metal objects [4].

The main purpose of this work is to understand the manufacturing techniques of these axes through metallographic and chemical analyses, and to formulate hypotheses about the techniques used to obtain the finished object. The focused investigation of the nonmetallic inclusions present in the alloy makes possible to formulate concrete hypotheses about the thermomechanical processes adopted in the manufacture of the artifacts. Similarly, the different percentages of Sn present in the alloy made possible to better understand the different qualities of the axes according to the different concentrations of Sn. In fact, tin is an element deliberately included in the alloy along with copper, because it modifies the fluidity and, therefore, the castability of the alloy. In addition to this, it improves its mechanical qualities such as hardness, strength, and toughness if a certain concentration within the alloy is respected [5]. Indeed, by increasing the Sn percentage, the alloys are characterized by greater hardness, but at the same time, the eutectoid is created. Thus, without proper heat treatment, an as-cast piece will have poor mechanical properties due to its high brittleness. It is also important to note that within the corpus of axes, there are both wrought and as-cast axes. The latter can therefore be considered as casting blanks, transportable by smiths and then cast again (Figure 1).
<figure><img src="image_158.png" /><figcaption>**Figure 1.** a) Bény 131 unmachined axe, casting blank; b) Bény 35 machined and defunctionalized axe. Looking at the cutting edge of the blade of Bény 35, it is possible to see the defunctionalization mark.</figcaption></figure>

Moreover, we seek to identify macro and microstructural witnesses of the thermomechanical history of artifacts, which can provide valuable information on the shaping steps and the last phases of the operating chain (annealing, mechanical deformation). The following are thus studied: the typology of grains (primary dendritic, dendritic after partial annealing, annealed without deformation, annealed with deformation) and their dimensions, the presence of thermal or mechanical twins, the presence of phases or inclusions and their proportions, and finally the presence of defects related to the forming process (porosities, shrinkage, etc.), use, defunctionalization.

Regarding the defunctionalization of the axes, archaeologists have given a ritual interpretation of the gesture. Given the arrangement of the tools within the pit, with such an orderly stacking, and a large number of buried axes, the hypothesis has been made that this was more than a hoard of materials but was probably a votive pit by the community that then inhabited the area.

## 2. Materials and Methods

### 2.1 *Description of the axes*

The typological study carried out by Maréva Gabillot and Céline Lagarde shows a great diversity of types, among which those from Lower
Normandy dominate and the recognition of new ones called Bény types (ranging from 1 to 6) previously undistinguished among the axes. According to the nomenclature of the French Prehistoric Society, there are 130 axes of the Norman type, 29 of the Breton type, 3 items mixing characteristics of these two types, 2 pieces of the Central-Western type, and 2 items of the Medoc type. Six axes are identical to a "Picard" type, and 10 are like types well represented in Britain.

## 2.2. *Definition of Conservation State*

The samples were analyzed macroscopically and microscopically to understand the state of conservation, the type of corrosion present on the surface, the metal microstructures, the composition of the materials, and thermomechanical processes to which they were subjected to make comparisons with other coeval productions from those parts of France.

## 2.3. *From sampling to microscopy*

All objects were observed macroscopically, photographed, and then subsequently, 19 axes were selected for sampling. Micro samples were taken at specific spots on the axes, those that could be of most interest to the study. To reduce the ?? of the object and to maximize the quality and the amount of the results a ?? was agreed to sample with a precision jewelry saw one microsample for each axe. The site of the sample is small enough to not damage the readability and the typological interpretation of the object while is large enough to offer plenty of information though a well consolidated protocol of investigation.

The samples were mounted in cold epoxy resin to be manipulated, and metallographic observations could proceed using LOM and SEM-EDS. To be able to proceed with microscopic observation, the samples must have a surface that is free of alterations due to sampling. This procedure has three steps: grinding, polishing, and lapping. These procedures were carried out by Struers' TergaForce-5 cleaner, and in the first stage, silicon carbide (SiC) abrasive papers with progressively finer grit sizes from 180 to 1000 Grit were employed, using water as a lubricant. Then, fabric discs were used with a diamond suspension of granulometry ranging from 6 µm to ¼ µm, step by step, with alcohol-based lubricant. The result should be a mirror-like surface free of artefacts.

The polished surfaces of the samples observed at LOM in bright field revealed the presence of inclusions and their shape, cracks and fractures, and corrosion ?? Dark field contrast, on the other hand, allowed us to observe the corrosion products, which are present and highlighted by the typical color of the mineral phases that characterize them.

For SEM-EDS observation, the samples have been made conductive by Au coating as conductive material. This allowed to define chemical nature of the inclusions, and of the alloys. The presence of second phases and the heterogeneity of the solid solution was also highlighted and quantified. The analyses of the corrosion products completed their characterization. To obtain this the metallographic etching (by FeCl₃-HCl-EtOH) was applied as final step with purpose to investigate the metallographic features and thus to formulate hypothesis on the manufacturing process and/or the usage of the axes.
## 3. Results

Results concerning metal alloying, inclusions, and corrosion will be treated separately.

The search was carried out with the following microscopical techniques:

* Optical microscopy in both bright and dark fields (using Leica MEF4 M equipped with six different magnifications: 25x, 50x, 100x, 200x, 500x, 1000x).

* Energy Dispersive X-Ray Spectroscopy (using a PENTAFET® EDXS detector sensitive to light elements) connected to a Scanning Electron Microscope (SEM) Evo40 Zeiss and INCA 300 software for BSE (backscattered electrons).

### 3.1. *Composition of the alloys*

In art and archaeology, the most commonly encountered type of alloy is bronze, which is a binary Cu-Sn alloy generally containing tenors of Sn up to about 15 wt%. The presence of Pb tends to be 1-2 wt%, and other elements are in traces, thus to be considered as purely random and unintentional elements; they were usually residues of the source ores. In these materials, the presence of Pb was detected as a trace element (<1wt%) and, therefore, unwilling to improve the castability of the alloy.

The analysed composition of the objects reported two compositional groups with different tin contents. The first group of samples confirmed the experimental literature data indicative of that historical period, as reported in Table 1.

<table><caption>Table 1. Composition of samples in wt% considering 15.8 wt% as the maximum value (percentage of maximum solubility of Sn in the Cu-Sn diagram).</caption><thead><tr><td>Sample</td><td>Cu*</td><td>Sn*</td></tr></thead><tbody><tr><td>Bény 1</td><td>85</td><td>15</td></tr><tr><td>Bény 10</td><td>85</td><td>15</td></tr><tr><td>Bény 14</td><td>86</td><td>14</td></tr><tr><td>Bény 29</td><td>87</td><td>13</td></tr><tr><td>Bény 37</td><td>88</td><td>12</td></tr><tr><td>Bény 46</td><td>86</td><td>14</td></tr><tr><td>Bény 49</td><td>88</td><td>12</td></tr><tr><td>Bény 54</td><td>87</td><td>13</td></tr><tr><td>Bény 131</td><td>87</td><td>13</td></tr><tr><td>Bény 132</td><td>87</td><td>13</td></tr><tr><td>Bény 153</td><td>86</td><td>14</td></tr><tr><td>Bény 168</td><td>85</td><td>15</td></tr><tr><td>Bény 180</td><td>86</td><td>14</td></tr></tbody></table>

The second group, on the other hand, recorded extremely high tin contents, as shown in Table 2.

<footnote>* Weight-normalized (wt %).</footnote>
Lead was always detected in trace amounts in each sample. Sulfur was mainly present in inclusions distributed in the matrix.

<table><caption>Table 2. Composition of samples in wt% with values above 15.8 wt%</caption><thead><tr><td>Sample</td><td>Cu *</td><td>Sn*</td></tr></thead><tbody><tr><td>Bény 21</td><td>84</td><td>16</td></tr><tr><td>Bény 23 C</td><td>79</td><td>21</td></tr><tr><td>Bény 23 T</td><td>71</td><td>29</td></tr><tr><td>Bény 35</td><td>83</td><td>17</td></tr><tr><td>Bény 161</td><td>83</td><td>17</td></tr><tr><td>Bény 174</td><td>81</td><td>19</td></tr></tbody></table>

## 3.2 Metallography

The samples, were studied by SEM, before etched for the LOM characterization. All these results made it possible to further subdivide the samples into different belonging groups, having noted several structures. In addition, the observation of the corrosion layers made possible to identify the different products that have been formed over the centuries, consequent to burial and exposure to soil moisture and pH as described in paragraph 3.3.

Samples #10, #29, #131, #132, #153 (Table 1) showed the structure typical of an as-cast blank, i.e., dendritic (Figure 2) and with a strong heterogeneity of the solid solution due to microsegregation during solidification. These structures are best seen after the etching with FeCl₃ (Figure 3).

<figure><img src="image_159.png" /><figcaption>**Figure 2.** Bény 10, SEM image: the dendritic structure typical of solidification blanks.</figcaption></figure>

<footnote>* Weight-normalized (wt %).</footnote>
<figure><img src="image_160.png" /><figcaption>**Figure 3.** Bright field (BF)-LOM image Bény 132, etched with FeCl₃. The original dendritic structure and heterogeneous solid solution are visible.</figcaption></figure>

Samples #1, #14, #37, #46, #49, #54, #168, and #180 (Table 1) showed homogenized solid solution and dissolution of the δ phase, thus obtaining a homogeneous metal alloy (Figure 4). This is possible by homogenization treatment at high temperatures i.e., 500-600° C. The acid etching for these samples, on the other hand, showed enlarged polygonal grains due to the heat treatment (Figure 5).

<figure><img src="image_161.png" /><figcaption>**Figure 4** Bény 1 SEM image: homogeneous matrix, absence of dendrites and microsegregation.</figcaption></figure>
<figure><img src="image_162.png" /><figcaption>**Figure 5.** Brightfield (BF)-LOM image Bény 180 etched with $FeCl_3$. Extremely enlarged grains with polygonal shape.</figcaption></figure>

Similarly, samples with Sn content above 15.8 wt% also exhibited the same features as the previous two subgroups. The main difference is the increased amount of eutectoid $\alpha+\delta$ as second phase surrounding the $\alpha$ primary cristals. Such a microstructure is expected to be hard and brittle this not suitable for cold working while it could be hot worked at a suitable temperature in the range 600°-800° C when $\beta$ phase substitute the $\alpha+\delta$ eutectoid.

Samples #23, #161, #174 were simply cast while samples #21 and #35 were hot worked as testified by their microstructural features as the presence of an homogeneous solid structure ($\alpha$ matrix) characterized by polygonal grains and thermal twins in reaction to hot working. The reduced amount of residual second phase is realed to the solubilization effect of the combined thermal treatment and plastic transformation.
Then #23C, #23T, #161 and #174 presented the characteristics of solidification blanks (Figures 6 and 7), while #21 and #35 presented homogeneous matrix and polygonal grains (Figures 8 and 9).

<figure><img src="image_163.png" /><figcaption>**Figure 6** Bény 161 SEM image: the dendritic structure typical of solidification blanks</figcaption></figure>

<figure><img src="image_164.png" /><figcaption>**Figure 7** Brightfield (BF)-LOM image Bény 23 C etched with FeCl₃. Dendrites.</figcaption></figure>
<figure><img src="image_165.png" /><figcaption>**Figure 8** Bény 35 SEM image: homogeneous matrix, absence of dendrites and microsegregation</figcaption></figure>

<figure><img src="image_166.png" /><figcaption>**Figure 9.** Brightfield (BF) LOM image Bény 35 etched with $FeCl_3$. Grains with polygonal shape.</figcaption></figure>

Regarding inclusions, all samples showed a high number of them within the matrix. Therefore, what confirms the presence of thermomechanical treatments is the type of inclusions that can be found. Those found showed a dark black-gray color and were already visible on initial observation at LOM; they are sulfides of different morphology: rounded, polylobated, elongated.
Confirming what has been described before for the metal alloy within the casting blanks, inclusions of rounded (Figure 10), polylobated shape (Figure 11) and elongated shape (Figure 12) were found.

<figure><img src="image_167.png" /><figcaption>**Figure 10.** Bény 46 SEM image: rounded sulfides.</figcaption></figure>

<figure><img src="image_168.png" /><figcaption>**Figure 11.** Bény 153 SEM image: polylobated sulfides.</figcaption></figure>
<figure><img src="image_169.png" /><figcaption>**Figure 12.** Bény 168 SEM image: elongates sulfides.</figcaption></figure>

The inclusions were analyzed by SEM-EDS and had similar chemical compositions in at% with Cu averaging 75 at% and S 25 at%, deviating little from the standard percentages of Cu 66 at% and S 33 at% calculating that they have stoichiometry Cu₂S.

It is very complex to remove the presence of sulfides residual from ores, which explains the systematic presence of these inclusions in the totality of samples analyzed.

Lastly, regarding corrosion, it could be seen that the green patina, typical of burial copper alloys, was macroscopically evenly distributed. Following sampling, corrosion products were observed at LOM by dark field (Figure 13). This mainly allowed us to identify three different types:

* Cuprite (Cu₂O): ranges in color from red to orange to brown depending on the presence or absence of SnO₂; it is the first corrosion product to form and is also the most frequent. It has also sometimes been observed with adamantine luster.

* Malachite (Cu₂(OH)₂CO₃): it has green color and several shades. It tends to occur on artifacts that have been buried for a long time and it is layered on top of Cuprite. It is formed in the presence of CO₂ and carbonate ions (CO₃²⁻); this is usually indicative of soils with an acid pH.

* Azurite (Cu₃(OH)₂(CO₃)₂): ranges in color from blue to deep blue, usually found as small crystals embedded in a matrix of malachite.
<figure><img src="image_170.png" /><figcaption>Figure 13. Dark Field (DF)-LOM image Bény 132 corrosion product layering.</figcaption></figure>

4. Discussion

The microstructure of the as-cast depends only on the elemental
composition, the cooling rate and the features of those
thermomechanically treated ?? as well as their amount of applied
deformation if this occurred at room T or at high T (e.g. heat deformation)
and on the annealing condition (T, t) the set of treatments the metal has
undergone; in fact, it can be changed by the combination of mechanical
processing and heat treatments that are collected under the heading of
thermomechanical history; therefore, it determines the final properties of
the fabricated object.

Normally, the craftsman makes a strategic choice that is related to
technological, cultural habits, or economic logic. This corresponds to
choosing either a homogenization heat treatment that precedes all
subsequent processing, or a solubility quenching followed by a
combination of homogeneity and recrystallization leading to partial
homogenization of the solid solution. The resulting microstructures
testify quite clearly and completely to which of these strategies was
selected. In our specific case, the morphology of the findings and their
intended use suggests that a homogenization heat treatment may have
taken place, although not explicitly required, and that the machining of
the cutting part of the blade may be the result of localized deformation
possibly alternating with annealing. The cutting edge is for sure left ?? to
acquire hig performance. This can be obtained also by the abrading
sharpening process.

From the data collected through analysis, we would expect, looking at the
Cu-Sn equilibrium diagram, samples with a two-phase alloy with an α
(Cu) solid solution matrix and precipitation at the grain joint of pure δ.
Looking at the samples metallographically, however, this was not found.
This is due to the extreme slowness of certain reactions and diffusion
phenomena, and the equilibrium diagram therefore becomes purely
theoretical. [6] Therefore, it is better to refer to the out-of-equilibrium
diagram (Figure 14). Referring to this, our samples will be characterized
by a biphasic matrix i.e., consisting of α(Cu) and α+δ residues. This assumption is confirmed by the formation of β during solidification which gave rise first to αγ and then α+δ to the eutectoid (Figure 15).

<figure><img src="image_171.png" /><figcaption>**Figure 14.** Out-of-equilibrium diagram of Cu-Sn.</figcaption></figure>

<figure><img src="image_172.png" /><figcaption>**Figure 15.** Bény 1 SEM image: eutectoid structure.</figcaption></figure>

Given the above considerations, the as-cast samples exhibit dendritic and heterogeneous microstructure. This is due to the microsegregation, since the coexistence range of liquidus and solidus α (Cu) is very wide, and, thus, the concentration differences between the liquid and solid phases are very high during solidification [7]. During solidification, therefore, these alloys will be more susceptible to microsegregation: a casting in its
raw state will therefore have strongly heterogeneous solid solution dendrites. This can be easily distinguished by the different coloration of the dendrites after acid etching. If, on the other hand, the solidification blank undergoes homogenization at high temperatures (500-600° C), then homogenization of the solid solution and dissolution of the out-of-equilibrium δ phase will occur by diffusion to the solid state, resulting in a homogeneous metal alloy.

These observations about the metal alloy are equally valid for samples exceeding the maximum solubility degree of 15.8 wt%. In fact, both solidification as cast and wrought axes exhibit the same characteristics as above. This is possible because, in this concentration range, an alloy with the mentioned characteristics would also be obtained according to the equilibrium diagram.

Regarding the thermomechanical treatments, it is found that some specimens at the hafting have undergone cold mechanical treatments following casting. In fact, mechanical twins are present: they are visible as parallel and intersecting lines within the grains that testify to the work hardening and displacement of the slip planes (Figure 16).

Some samples show inhomogeneity in terms of grain size related to the solidification process: the areas in contact with the mold, being the first to solidify, have higher cooling rates, and therefore, under these conditions, grain nucleation is encouraged as opposed to growth. The number of grains formed is higher the faster the solidification rate, and consequently, the size of the grains will also be smaller. So, in crude casting at high cooling rates, many grains having small sizes are noticed because their growth is hindered by the accretion of nearby ones [8]. Conversely, for low cooling rates, the casting blank will result more homogeneous and with larger grains.

<figure><img src="image_173.png" /><figcaption>**Figure 16.** Bény 46 SEM image: mechanical twins highlighted by the corrosion process.</figcaption></figure>
About inclusions with a rounded shape, they are the outcome of the presence or absence of mechanical processing, as an indication of an absence of machining. In the worked axes samples, on the other hand, sulfides with an elongated shape were found, denoting precisely mechanical processing. They do not change their volume, but only their shape giving the possibility to reconstruct the amount of deformation undergone.[9]

The defunctionalization observed in some axes being located on the cutting edge would indicate the use of the blade to destroy others that are no longer functional. The blades of the axes used showed clearly an enlargement of the cutting-edge area with respect to the profile of those that have not undergone a subsequent shaping after casting. All axes showed, however, clear signs of fusion in bivalve mold with evident signs of removal of the lateral burrs. The junction area between the two halves of the casting mold remains visible.

The axes were covered with green or blue corrosion products, mainly imputable to copper carbonates (azurite Cu₃(OH)₂(CO₃)₂ and malachite Cu₂(OH)₂CO₃. Some samples showed areas with a metallic luster with a golden color [10] when they have a composition Sn< 16 wt% and silvery in areas or parts that exceed that content. The presence of Sn increases corrosion resistance thanks to the tendency to form a thin layer of passive oxide [11] (not very reactive to the environment in which it is formed). It should also be noted that the castability increases thanks to the presence of Sn because the melting point drops drastically (while the casting temperature is often maintained), the surface tension changes as well as the capillarity. Then a layer enriched in Sn will form on the surface of the axes during solidification and this is regularly observed for Sn>16 wt% but it can also happen when after the solidification of the outermost part of the object there is, near the surface, a residual still liquid, because it is very rich in Sn. This will be expelled to the surface, interposing itself between the object and the mold model.

Future developments of the project include mold castings to reproduce the original subjects thanks to the knowledge about the morphology and the chemical composition of the alloy. The research will also be aimed at the production of axes to study the manufacturing techniques better, and the mechanical qualities of the alloys depending on the percentages of tin contained in them without affecting, with further withdrawals, the original axes. This is a part of a precise plan for conserving the cultural heritage, including a fruition of the asset in the museum environment.

## 5. Conclusions

The purpose of this research project was to understand the manufacturing techniques, state of preservation and material aspects of some archaeological metal artifacts by metallographic and chemical analysis. Therefore, through the results obtained, it can be stated that univocal production can be found as common features are evident among the 19 axes considered in this research. Therefore, it was possible to divide the axes into No. 10 worked and No. 9 solidification blanks.

Considering the unprocessed axes, these do not show any signs, detectable microscopically, of mechanical processing. From the microstructural point of view, dendritic grains were observed, which
precisely testify to the absence of heat treatment. As further proof, the rounded and polylobated inclusions confirm what was said before. On the other hand, the worked axes show clear signs of machining, especially on the blade, observable only macroscopically since sampling took place solely on the axe hafting. In fact, hammering marks on the cutting edge are visible. Instead, the microstructural analyses of the samples indicate evidence of heat treatment through the presence of greatly enlarged dendrites or recrystallized (polygonal) grains. The sulfides, in this case, also confirm the above by finding evidence in the elongated shape.

From the total of the 19 axes, it was found that for 13 of them, tin percentages settled between 12-15 wt% falling within the compositional parameters typical of the Middle Bronze Age. The fact that these are within such a narrow range confirms, in part, that the craftsmen had found, after several experimental attempts, optimal percentages for this manufacturing activity. This hypothesis would be confirmed by the presence of 6 samples having a significantly higher percentage of tin. This assumption could be evidence both by the experimental activity and the presence of ingots that were intended to produce a prealloy for future purposes.

The presence of oxides and carbonates is consistent with the environment in which the samples were found, typical of humid environments with an acid pH. The presence in some samples of stratified corrosion patinas is precisely compatible with an agricultural area and frequented over the centuries by anthropic activity. This could be justified by soil stirring and, thus an alternation of different microclimates that led to corrosion stratification.

And lastly, three broken axes were found within the archaeological corpus: two of them functional, with different morphology, used and later defunctionalized, i.e., broken deliberately, with a tin concentration of between 14-15 wt%, thus with a consistent alloy composition between them. The third, with an average tin concentration of 25 wt%, differs from the first two in that the breakage is not deliberate but due to the brittleness of the alloy, probably because of attempted processing.

## Ringraziamenti a Mareva cosa bisogna scrivere?

## References

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2. D. Cliquet, M. Gabillot et C. Escolano, «Bény-sur-Mer – La Grande Hache» [notice archéologique], *ADLFI. Archéologie de la France - Informations [En ligne]*, Normandie, **mis en ligne le 04 juin 2021**, *consulté le 16 octobre 2022*. URL : http://journals.openedition.org/adlfi/74568

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4. P. Piccardo, G. Ghiara, J. Vernet, Mise en œuvre des alliages cuivreux: faire parler le métal grâce à la science des matériaux. In: M. Pernot, *Quatre mille ans d'histoire du cuivre, fragments d'une suite de rebonds*, Presses universitaires de Bordeaux, collection « THEA », France, 2017, pp. 41-60

5. P. Piccardo, R. Amendola, A. Adobati, C. Faletti, Studio della fluidità di leghe a base rame, *La metallurgia italiana*, **2009**, 101 pp. 31-38.

6. S. Kumar, C. A. Handwerker, M. A. Dayananda, Intrinsic and Interdiffusion in Cu-Sn System, *Journal of Phase Equilibria and Diffusion*, **2011**
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8. J. M. Vernet, Out-of-equilibrium behaviour of copper-based alloys in industrial, artistic and historical gravity casting processes, PhD thesis, Università degli Studi di Genova, Genova2018

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10. N. Ammanati, E. Martellucci, S. Natali, C. Colibri, P. Piccardo, Valutazione del colore dei bronzi tramite misurazione CieL*a*b, In: E. Formigli, *Colore e luce nella statuaria antica in bronzo*, «L’Erma» di Bretschneider, Via Cassiodoro 19, Roma; Italia, 2009, pp 41-48

11. P. Piccardo, B. Mille, L. Robbiola, Tin and copper oxides in corroded archaeological bronzes, In: P. Dillman, G.Béranger, P. Piccardo, H. Matthiesen (EDS), *Corrosion of Metallic heritage artefacts: investigation, conservation and prediction for long term behaviour*, WoodHead publishing, Cambridge, United Kingdom, 2007, pp. 239-262

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## Nouvelles investigations sur le dépôt de Bény-sur-Mer en novembre 2023

Les 20 et 21 novembre 2023, une campagne de prospection thématique sur le premier lieu de découverte du dépôt. Cette campagne a été menée sous la direction de Cyril Marcigny, dans le cadre d'un PCR consacré à l'étude archéologique du site, financée par la DRAC Normandie.

Il s'agissait d'un décapage à la pelle mécanique d'une zone de 40 m2 autour du lieu de découverte. Cette campagne a permis de mettre au jour d'autres artefacts identiques aux premiers 182 découverts initialement.

Il s'agira par la suite de réviser la composition typologique du dépôt.

<figure><img src="image_174.png" /><figcaption>Vue générale d'une partie de l'équipe sur le site en cours de décapage.</figcaption></figure>
Un protocole classique a été mis en œuvre : décapage au godet lisse, passes fines de moins de 5cm d'épaisseur, puis passage de détecteurs de métaux.

<figure><img src="image_175.png" /></figure>
Passage au détecteur après une passe à la pelle mécanique.

<figure><img src="image_176.png" /></figure>

Le détecteur vient de sonner « cuivreux ».

<figure><img src="image_177.png" /></figure>

Hache in situ.
<figure><img src="image_178.png" /></figure>

Découverte d'un fragment mis au jour après passage du détecteur de métaux.
<figure><img src="image_179.png" /><figcaption>Nouvelles haches découvertes lors de la campagne 2023, ici sur site, en sachets.</figcaption></figure>
<figure><img src="image_180.png" /></figure>

Hache à talon, anneau sans nervure, identique à un lot figurant dans la première découverte.
<figure><img src="image_181.png" /></figure>

Hache à talon, anneau et nervure, identique à un lot figurant dans la première découverte.
<figure><img src="image_182.png" /><figcaption>Hache à talon de type « normand » décorée d'un trident sous la butée.</figcaption></figure>
<figure><img src="image_183.png" /></figure>

Hache à talon de type du Centre-Ouest. L'intérêt de cet exemplaire est sa présence, avec un autre découvert lors de la première campagne, au sein d'un lot de 190 haches de type essentiellement « normand ».
<figure><img src="image_184.png" /></figure>

Fragment proximal de hache à talon de type normand ; cet objet est précieux dans le cadre d'analyses métallographiques car le prélèvement peut « aisément » se faire à l'endroit de la fracture.
### 3c Mission de recherches au département de métallurgie et matériaux de l'université de chimie et physique à Gênes

Durant le mois de novembre 2023, Mareva Gabillot et Léonard Dumont se sont rendus à l'université de Gênes (Italie) au sein du département de chimie et chimie industrielle. Accueillis par Paolo Piccardo (professeur) et deux de ses doctorantes (Camilla Criaco et Flavia Boragina), l'objectif de ce séjour de recherches était de se familiariser avec le traitement des échantillons prélevés sur les haches en vue de l'étude métallographique.

Le premier jour (mardi 7 novembre 2023) a été dédié à la visite du laboratoire et à la présentation des différents équipements. Le reste du séjour a été dédié à la présentation des traitements des échantillons, à leur observation et à leur analyse. La première étape consiste à réaliser un enrobage à froid des fragments métalliques prélevés dans un cylindre de résine époxy. Une fois la résine prise (environ 12h), le polissage peut commencer. Cette longue opération s'effectue sur une polisseuse, machine doter d'un plateau rotatif sur lequel se place des disques abrasifs plus ou moins fins selon le résultat recherché. Un bras terminé par porte-échantillons de la taille adapté permet de placer les cylindres de résine contenant les prélèvements, qui sont plaqués contre le disque abrasif à l'aide d'un piston (fig. 1).

<figure><img src="image_185.png" /><figcaption>Fig. 1 : Polissage d'un échantillon en cours par P. Piccardo et échantillons après polissage.
Cliché : L. Dumont</figcaption></figure>

Un premier passage est effectué à l'aide d'un disque abrasif (grain 800), qui a pour but d'attaquer l'échantillon pour rendre visible une surface métallique. Celle-ci doit ensuite être polie de manière à obtenir une surface totalement lisse et réfléchissante. Cela s'effectue par quatre passages successifs en commençant par un disque de grain 1000 et un disque très fin au grain d'un micron. Chaque passage consiste en trois étapes :
1. 1:30 min sur disque humide avec ajout d'une suspension diamantée d'un grain correspondant à celui du disque.

2. 30 s avec ajout de détergent afin d'évacuer la suspension de diamants.

3. 30 s avec lavage à l'eau courante de manière à évacuer le détergent.

Pour les échantillons de haches préparés lors de notre séjour, le plateau tournait à une vitesse de 250 tours par minute et les échantillons étaient maintenus en exerçant une force de 20 N. À l'issue du processus, les échantillons ont été séchés par air comprimé afin d'enlever toute trace d'eau sur la surface métallique.

Une première observation a alors été réalisé par microscopie optique. Le laboratoire de métallurgie dispose d'un microscope métallographique MEF4 M (fig. 2) qui permet des prises de vues à la fois en champ clair pour l'examen des parties métalliques, et en champ sombre, offrant un haut degré de contraste afin de visualiser les parties corrodées du métal. À ce stade, une partie des échantillons ont subi une étape supplémentaire de polissage à l'aide d'un disque très fin (0,25 micron), qui a parfois dû être répétée plusieurs fois pour totalement supprimer les rayures et traces d'humidité nuisant à l'observation de la microstructure métallique.

<figure><img src="image_186.png" /><figcaption>Fig. 2 : microscope employé pour les analyses métallographiques au département de chimie et chimie industrielle de l'université de Gênes. Cliché : L. Dumont.</figcaption></figure>

Une fois les échantillons correctement polis, l'étude métallographique peut débuter. La présence de différentes phases formées au cours de la fonte (fig. 3) et leurs microstructures livrent des informations sur la manière dont le bronze a été coulé puis mis en forme post-fonderie (Kienlin 2008 ; Vernet 2018).
<figure><img src="image_187.png" /><figcaption>Fig. 3 : Diagramme représentant les différentes phases de l'alliage Cu-Sn formées selon la teneur en étain et la température.</figcaption></figure>

Lors de la première observation de l'échantillon simplement poli par microscopie optique, la forme de la phase α + δ (alpha-delta), riche en étain, ainsi que celle des inclusions de sulfure de cuivre (CuS), livrent des informations sur l'état du métal, qui peut être brut ou retravaillé par déformation à froid et/ou avec recuits. L'élongation des sulfures, normalement arrondies, indique une déformation plastique, tout comme la fracturation de la phase α + δ ou encore la présence d'un réseau de fissures formant des lignes parallèles et perpendiculaires. L'organisation de la phase α + δ sous la forme d'un fin réseau au sein de la phase α est typique d'un métal brut de coulée. Lorsque cette phase est moins importante et se présente comme des zones anguleuses, il s'agit en revanche d'un métal recuit (fig. 4).
<figure><img src="image_188.png" /></figure>

Fig. 4 : Observations des échantillons des haches du Musée National de la Préhistoire par microscopie optique avant attaque chimique. A. Phase α + δ formant des « poches » et fissures parallèles typiques d'un métal non recuit déformé à froid. B Métal homogénéisé, phase α + δ anguleuses et sulfures allongés témoignant d'un métal recuit et déformé à froid. Clichés : C. Criaco ; L. Dumont.

Une fois ces premières observations réalisées, les échantillons doivent être préparés pour pouvoir être observés au microscope électronique à balayage (MEB). Cette technique repose sur l'interaction entre un faisceau d'électrons et la matière qu'ils rencontrent, générant des électrons secondaires selon la nature et la topographie de l'échantillon. Un capteur permet de traduire ces électrons secondaires en signal électrique converti lui-même en un pixel en niveau de gris pour chaque point d'impact, permettant d'obtenir des images en très haute résolution, avec des grossissements bien supérieurs à un microscope optique. Dans la mesure où l'intégralité de la surface étudiée au MEB doit être conductrice, une fine couche d'or (6,5 µm) est appliquée sur l'échantillon avant son observation. Les très forts grossissements atteints permettent d'étudier en détail la structure de la phase α + δ et des sulfures, révélant parfois des structures complexes, comme des incrustations associant des sulfures de cuivre et de zinc, ou encore des oxydes d'étain (cassitérite), en fonction des éléments résiduels issus des minerais d'où proviennent les métaux employés (fig. 5).
<figure><img src="image_189.png" /><figcaption>Fig. 5 : Observation d'un échantillon de la hache 46 de Bény au microscope électronique à balayage. Les sulfures de forme ronde témoignent d'un bronze n'ayant pas été déformé. Cliché : P. Piccardo.</figcaption></figure>

Par ailleurs, le MEB offre également la possibilité de réaliser des analyses élémentaires grâce au rayonnement X secondaire produit par l'interaction de la matière avec les électrons. En utilisant la spectroscopie de rayons X à dispersion d'énergie (EDS), il est possible d'obtenir la composition des différentes phases et éléments observés à une échelle très fine.

Finalement, une dernière observation des échantillons a été réalisée après attaque chimique permettant d'améliorer le contraste entre les phases métalliques et leur visibilité. Un polissage fin (grain de 1 µm) a été réalisé afin de supprimer la couche d'or appliquée précédemment.

L'attaque de la surface de l'échantillon a ensuite été réalisée par une exposition durant quelques secondes à une solution de chlorure de fer. Ce procédé permet d'obtenir une lecture plus aisée en révélant des structures pas ou peu visible avant attaque, comme les dendrites se formant lors du refroidissement du bronze après la fonte, ou bien la structure polygonale typique se formant après recuit (fig. 6).
<figure><img src="image_190.png" /></figure>

Fig. 6 : Observations de l'échantillon 25 des haches du Musée National de la Préhistoire par microscopie optique après attaque chimique. La structure comprend à la fois des dendrites (éléments arrondis irréguliers) et des polygones, témoignant d'un état intermédiaire entre le métal brut de coulée et le bronze recuit. Cliché : L. Dumont.

Les observations réalisées par microscopie optique avant et après attaque ainsi qu'avec le MEB permettent ainsi d'obtenir tout une gamme d'informations sur la microstructure du bronze et sa composition, nous renseignant sur une partie de la chaîne opératoire, en particulier les étapes de post-fonderie, le tout à partir d'un unique échantillon. Des recherches récentes montrent qu'il est également possible de déterminer la nature du moule en fonction de la taille et de la répartition des grains de la phase α ainsi que du volume de la phase α + δ (Vernet 2018). Ces caractères varient en effet selon la conductivité du moule, qui influe sur la vitesse de refroidissement de l'objet.

Le laboratoire de métallurgie étant équipé pour la réalisation de coulées expérimentales (fig. 7), nous réfléchissons actuellement à la mise en place d'un référentiel expérimental permettant de distinguer l'emploi de différents types de moule en fonction de la microstructure observée à l'aide de différentes techniques.

Il est à noter qu'une coulée expérimentale type « fonte au sable » a été réalisée lors de ce séjour ; elle a révélé qu'il était tout à fait possible d'obtenir des objets du même type que les lames que nous étudions. Les objets issus de cette fonte au sable portent des « bavures de coulée » latérales, comme on peut les voir sur des objets identifiés comme ayant pu être coulés dans des moules bivalves en pierre.

La fonte au sable peut ne laisser aucune trace éventuellement retrouvée par l'archéologie. Cela permet d'avancer une réponse quant au manque criant de moules, d'ateliers, de structures de
manufacture d'objets : si la fonte au sable a été massivement utilisée, cela explique le manque
de données archéologiques dont nous disposons.

<figure><img src="image_191.png" /><figcaption>Fig. 7 : Coulée expérimentale d'un objet en bronze dans un moule en sable au sein du laboratoire de métallurgie. Cliché : M. Gabillot.</figcaption></figure>

<figure><img src="image_192.png" /><figcaption>Moule au sable utilisé pour la coulée expérimentale. État à la fin de la coulée.</figcaption></figure>
<figure><img src="image_193.png" /><figcaption>Objet « brut » issu de la fonte expérimentale.</figcaption></figure>

<figure><img src="image_194.png" /><figcaption>Objet issu de la fonte expérimentale après polissage de surface.</figcaption></figure>
Détail des références bibliographiques citées dans le paragraphe :

Kienlin T. L. (2008). *Frühes Metall in nordalpinen Raum. Eine Untersuchung zu technologischen und kognitiven Aspekte früher Metallurgie anhand der Gefüge frühbronzezeitlicher Beile*, Bonn: Verlag Dr. Rudolf Habelt (Universitätsforschungen zur prähistorischen Archäologie, 162), 799 p.

Vernet J. M. (2018). *Out-of-equilibrium behaviour of copper-based alloys in industrial, artistic and historical gravity casting processes*, thèse de doctorat, Università di Genova, 156 p.

**4. Perspectives**

Les résultats issus des recherches menées en 2023 encouragent l'équipe à poursuivre les investigations dans le cadre d'un nouveau PCR, décrit dans la demande 2024, intitulé « Le Morvan producteur de métal à la fin de la Préhistoire ».