Bronze axe head - Tin Bronze - Middle Bronze Age

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

The schematic representation below gives an overview of the corrosion layers encountered on the head axe from a first visual macroscopic observation.

Fig. 10: Stratigraphic representation of the object in cross-section using the MiCorr application. This presentation can be compared to Fig. 18. CP1 corresponds to CP1c.
Fig. 11: Stratigraphic representation. of the object in cross-section using the MiCorr application. This presentation can be compared to Fig. 18. CP1 corresponds to CP1a.

SEM-EDS, FTIR.

The FTIR device used is a Biorad Excalibur FTS 3000 spectrometer coupled to an IR microscope UMA500. The measurements were performed between 4000 and 650 cm-1.

SEM/EDS analysis of corroded particles indicates that the alloy is composed of copper and tin. The core metal is most likely a tin bronze.

 

Light yellow particles constituting sample 1 are predominantly composed of Sn and O with minor presence of C, Cu, Fe, S, Cl and As (Fig. 10), while green and red particles of sample 2 are respectively composed of O, Cu, Cl, and Sn, with some minor presence of Fe and S (Fig. 11) and Cu,O (Fig. 12, cuprite?). Sample 3 is made of green particles composed mostly of Sn, Cl, Cu and O with S, Al and Fe as minor elements (Fig. 13). Sample 4 is constituted of green and white particles. The former have the same composition as particles of sample 3 while the white particles are significantly different in composition: large amount of C with a small amount of O, Cl, S, K and Cu (Fig. 14). The FTIR spectrum of the white particles shows several characteristic peaks of an organic compound: 2919 cm-1 and 2850 cm-1 (C-H- bond), 3445 cm-1 and 3323 cm-1 (O-H- bond) or 1030cm-1, 1006cm-1 and 951cm-1 (C-O-C- bond). 

 

Table 1: Identification of chemical elements in the corrosion products developing on the head axe. Method of analysis: SEM/EDX, HEI-Arc.

 

Sample / particles

Major elements

Minor elements

Green powdery corrosion product

Sample 1

Sn + O

C + Cu + Fe + S + Cl + As

Sample 2 – Green particles

O + Cu + Cl + Sn

Fe + S

Sample 2 – Red-brown particles

Cu + O

 

Sample 3

Sn + Cl+ Cu + O

S + Al + Fe

Darker green corrosion product

Sample 4 – Green particles

Cl + Sn + Cu + O

S + C

Sample 4 – White particles

C

O + Cl + S + K + Cu

The schematic representation of corrosion layers below integrates additional information based on the analyses carried out.

The axe head is probably a tin bronze containing a small amount of As. It is difficult to interpret the corrosion layer only by macroscopic observation. The analyses show that all green corrosion products contain chlorides that might originate from strip-cleaning by hydrochloric acid (not documented though). Active corrosion is not proved but we suspect the presence of atacamite, paratacamite or clinoatacamite (Cu2Cl(OH)3). FTIR revealed the presence of an organic compound on the metal surface (protective coating?).

 

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2.   Degrigny, Christian and Senn, Marianne. Methodology to study and analyse the microstructures and corrosion forms of ancient and historic metals: application to metallographic samples from Swiss collections MIFAC-Métal. Projet Sagex, Final Report, Haute école de conservation-restauration ARC, Neuchâtel, 2012.

3.     Selwyn, Lyndsie. Métaux et corrosion Un manuel pour le professionnel de la conservation. Institut Canadien de Conservation, Ottawa, 2004.