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Metallography, Microstructure, and Analysis

Open Access 06-05-2024 | Original Research Article

Feasibility Examination of the Field Multi-focal Metallography Method (FMM) for Characterisation of Metallic Marine Artefacts

Authors: N. Iddan, D. Ashkenazi, D. Cvikel

Published in: Metallography, Microstructure, and Analysis

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Abstract

Field multi-focal metallography (FMM) is a development of field metallographic replication (FMR). It is an innovative minimally destructive technique that facilitates high-resolution metallographic observations of metallic object surfaces, regardless of their orientation to the optical axis of the microscope. Several artefacts retrieved during underwater excavations (a bronze powder chamber, a stud-link anchor chain, a winch, a heart-shaped shackle, a deadeye strap with a futtock plate, and an iron stud-link chain controller) were examined. The FMM results were compared with conventional metallography, where the sampling process inflicts substantial damage to the item. This FMM trial produced results of comparable quality to conventional metallography for both the bronze and the ferrous objects. It revealed the microstructure of the archaeological objects with minimal damage. The FMM method was shown to be a suitable tool for the study of ancient metal objects retrieved from shipwrecks.

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Introduction

The study of shipwreck sites can contribute new and important information and enhance our understanding of the past. Methods for the study of archaeological objects retrieved from shipwrecks are continually being developed, allowing the development of materials characterisation methods for investigating microstructure, as well as the relationship between microstructure, manufacturing processes, and material properties [1]. Archaeometallurgy adapts various methods to study different aspects of early manufacturing processes of metals and alloys, and the production stages of metal objects [2, p. 738, 3, pp. 132–133, 4, p. 174]. Archaeological objects serve as material records of the past, providing significant information [2, p. 739, 5].

Metallography can assist in determining the microstructure, composition, and manufacturing techniques of metallic archaeological artefacts [2, 5‐11]. In some cases, it can also determine their origin and dating [12]. The conventional metallographic process of preparation of standard specimens, according to standard ASTM E3-11 (2017) [13], involves cutting a significant portion of the object, and thus damaging it. For unique ancient metal artefacts the use of non-destructive testing (NDT) techniques is obligatory.

Methodology

Field metallurgical replication, also known as field metallographic replication (FMR), is a NDT technique used for in situ microstructure analysis of the surface of metal objects. FMR involves the preparation of irregular or curved surfaces by grinding, polishing, and etching by swabbing the external surface of the object, followed by pressing a celluloid film to the surface. The celluloid film is used to copy the microstructure, including grain boundaries, phases, imperfections, and cracks, creating a ‘negative’ replication of the microstructure. The recorded microstructure is observed by light microscopy (LM) and/or scanning electron microscope (SEM) [14, 15, p. 307]. FMR metallography does not always yield high-quality micrographs obtained by conventional destructive metallography.

FMM is an improvement on NDT FMR. It involves the preparation of an irregular or curved surface in the same way as the FMR method, and then imaging the surface by multi-focal LM observation instead of a standard LM, omitting the plastic film replication procedure used in the FMR method. It enables the direct observation of the microstructure and identifying microscopic imperfections connected to the manufacturing process on-site after appropriate surface preparation, creating a high-quality image without having to cut and destroy unique archaeological objects [16, 17].

The current study was made to establish whether the novel NDT FMM method is feasible, producing satisfactory results comparable with the conventional destructive metallography technique for both ferrous and non-ferrous archaeological artefacts retrieved from marine environments. FMM and conventional destructive metallography by LM and SEM were used to study the microstructure of metal objects, and the results of the two metallographic methods were compared.

Materials

Bronze Powder Chamber

The bronze powder chamber was discovered near the Akko Tower shipwreck (Akko, Israel), and retrieved during the 2016 underwater excavation [18, 19, pp. 68–70, 20, 21]. Breech-loading swivel guns that used powder chambers were commonly used in European and Ottoman navies from the 15th century [21, p. 71, 22, p. 2]. The cylindrical powder chamber (Fig. 1) weighed 7.56 kg, and was 19 cm long, with a minimum top diameter of 6.4 cm and maximum base diameter of 8.0 cm. The chamber included a handle, a main bore for insertion of the powder, and a touchhole at the bottom ornamented with four intersecting lines [16].

The powder chamber was previously studied by several metallurgical methods comprising X-ray fluorescence (XRF), conventional destructive metallography, novel NDT field multi-focal metallography (FMM), LM observation, optical emission spectroscopy (OES), microhardness indentation measurements, and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDS) analysis, as well as lead isotope analysis (LIA) with inductively coupled plasma mass spectrometry (ICP-MS) [16].

Iron Artefacts from the Akko Tower shipwreck

Chain links, studs, and a rod retrieved from the Akko Tower shipwreck were previously analysed by radiographic testing (RT), XRF, conventional destructive metallography, LM, SEM-EDS, OES, and microhardness measurements [12]. Other ferrous objects: a winch, a heart-shaped shackle, a deadeye strap with a futtock plate, and a stud-link chain controller, all retrieved from the shipwreck, were studied by different methods, including conventional metallography, LM, FMM, SEM-EDS, OES, and microhardness measurements [17]. Below is a short description of the finds.

Stud-Link Anchor Chain

The stud-link chain is an important part in the anchoring and mooring system of a ship. A 2.3-m-long segment of a ferrous chain was retrieved from the north-western section of the Akko Tower shipwreck during the 2016 underwater excavation season. It was covered with thick encrustation and concretion, and a ferrous rod was also encased in the concretion (Fig. 2). The concretion was mechanically removed, followed by the exposure of 30 oval ferrous links, each with a stud at its centre. The major and minor axes of a representative link were 12.5 cm and 7.5 cm, respectively, with bar diameter between 17 and 22 mm (roughly equivalent to 11/16 and 7/8 inches). The diameter at the stud varied between 15 mm at the centre and 22 mm at the border with the link, and the typical stud was 3.6 cm long [12]. The chain links and studs were previously examined by conventional destructive metallography, but not by the FMM method; therefore, in the current research this gap was filled by performing FMM analysis of these items.

Winch

A winch (Fig. 3a) is a mechanical winding device used for adjusting the tension of, or extending or withdrawing, a rope, wire, or cable [17, 23, 24]. The total weight of the winch was 11.5 kg. The drum was 14.9 cm long with a maximum diameter was 14.6 cm, and an average diameter of 11.2 cm. The shaft was 29.6 cm long, and between 20 and 24 mm in diameter [17].

Heart-Shaped Shackle

The heart-shaped shackle (Fig. 3b) is a primary fastening link in the rigging system. It has a bolt, offering rapid connection and disconnection of rigging elements, including chain cables [17, 25, 26, p. 27]. The heart shape allows the joining of lines, chain, or rope, at various angles. The shackle weighed 3.2 kg. It was 28.1 cm long, with an average thickness of 18 mm. The heart-shaped shackle was 15.3 cm long and 5.5‒6.3 cm wide, with an average thickness of 19 mm. The bolt diameter was 27.5 mm [17].

Deadeye Strap with Futtock Plate

Deadeyes were part of the standing rigging system of traditional sailing ships. A typical deadeye was made of a circular hardwood block, grooved around the circumference and pierced with three holes for a lanyard. Deadeyes were used in pairs to tension stays (ropes or wires) and shrouds (sets of ropes producing lateral support to the ship’s masts). The main ferrous parts of a typical deadeye include a futtock plate, a strap wrapped around the wooden block, a loop forge-welded around a bolt, a securing element beneath the loop, and the shackle that connected the futtock plate to a chain with a bolt [7, 17, 26, p. 11, 27, 28]. This deadeye strap (Fig. 3c) weighed 1.7 kg. The futtock plate was 38.1 cm long and 6.5 cm wide, and 9.5 mm thick. The strap was 5.1 cm wide and 12 mm thick. The strap’s external diameter was 16.4 cm and its internal diameter was 13.9 cm. The bolt diameter was 29 mm [17].

Stud-Link Chain Controller

This stud-link chain controller (Fig. 3d) was possibly used to support the anchor stud-link chain [12]. This item weighed 12 kg. It was 40.5 cm long, 8.1 cm wide, and 13.5 cm high. The thickness of the stud-link chain controller’s arc varied between 4.0 and 7.0 cm. The hole had a diameter of 42 mm on the inside of the stud-link chain controller, and was 33 mm diameter on the outside (Fig. 3d). The internal width of the chain was 3.5 cm.

Experimental

To examine the feasibility of the FMM method, the bronze, wrought iron, and cast iron objects were further characterised as described below (Table 1):

Visual testing (VT) inspection was performed to detect visible macroscopic defects.

Minimally destructive FMM was done on a selected surface of each item. The FMM surface preparation [16, 17] included the following four steps: (1) A suitable external surface was located; (2) The selected area of each ferrous item was ground with a conventional handheld grinder/polisher with 80, 120, 240, 400, 600 grit silicon carbide papers, and the bronze powder chamber was ground with 150, 320 and 600 grit silicon carbide papers; (3) The surface of the ferrous objects was polished with a handheld grinder/polisher instrument, with polishing cloth of 9, 3 and 1 μm polishing compound, and the bronze powder chamber was polished with 15, 6, 3 and 1 μm polycrystalline diamond paste; and (4) The surface of the iron objects was etched with Nital (97% alcohol, 3% HNO₃) etchant, and the surface of the bronze powder chamber was etched by swabbing with an FeCl₃-HCl-ethanol etchant. The surface was washed with ethanol after each step. During the grinding process, a layer of less than 1 mm of metal was removed from the surface. At the last step of this procedure, the etched zone was observed with a HIROX RH-2000 digital 3D multi-focal LM. This is an advanced tool combining large depth of field (a measure of how much of an image is in focus), and large working distance (the distance between the front of the objective lens and the observed sample surface). This advanced microscope includes an ultra-fast auto-focus and multi-focal system, as well as an integrated stepping motor with Z-axis movements of 50 nm for each step. This tool also includes a motorised rotary head, which allows 360° rotation around the observed sample [1].

For the conventional metallographic inspection of the objects, samples were cut from the items and specimens were prepared according to ASTM E3-11 (2017) standard [13], longitudinal cross section (L-CS), transverse cross section (T-CS), and in a planar (rough-ground) section (P-section, which is the plane perpendicular to the L-CS and T-CS), and mounted in Bakelite. The surface of the powder chamber and the ferrous samples was ground with 80–4000 silicon carbide grit papers, polished with 1 μm aluminium oxide (Al₂O₃) polishing suspension, and then with 0.04 μm colloidal silica suspension. All samples were cleaned with ethanol between these steps to remove contaminants. Next, the polished ferrous specimens were etched with Nital (97% alcohol, 3% HNO₃), and the polished surface of the powder chamber was etched with hydrochloric acid in ferric chloride solution (FeCl₃–HCl–H₂O). The microstructure of the metallographic specimens was observed with an Olympus BX60M light microscope (LM), equipped with a DeltaPix Invenio 3SII camera.

A Zeiss Supra 40 SEM instrument was used at high vacuum mode. The composition of the objects was measured with energy-dispersive X-ray spectroscopy (EDS) equipped with a silicon drift Brucker XFlash 4010 detector with EDS resolution of 129 eV. The EDS detector was calibrated with standard samples with an approximate error of 0.1–1%.

Table 1

Description of metal objects and their characterisation methods

Artefact	Material	Characterisation method
Artefact	Material	Field multi-focal metallography (FMM)	Conventional metallography	VT	XRF	SEM-EDS	OES	HV
Powder chamber	Bronze	+	+	+	+	+	+	+
Anchor chain link	Wrought iron	+	+	+	+	+	+	+
Anchor chain stud	Grey cast iron	+	+	+	–	+	+	+
Winch drum	Grey cast iron	+	+	+	+	+	–	+
Winch shaft	Wrought iron	+	+	+	+	+	–	+
Heart-shaped shackle link	Wrought iron	+	+	+	+	+	–	+
Heart-shaped shackle	Wrought iron	+	+	+	+	+	–	+
Futtock plate	Wrought iron	+	+	+	+	+	+	+
Deadeye strap	Wrought iron	+	+	+	–	+	–	+
Stud-link chain controller	Grey cast iron	+	+	+	+	+	–	+

All objects were examined by VT, field multi-focal metallography (FMM) technique, conventional metallography, and SEM observation, including qualitative SEM-EDS analysis and elemental mapping. The XRF, SEM-EDS, OES elemental analyses of the objects, as well as their HV indentation results, were conducted in previous studies [12, 16, 17]. EDS and XRF methods were not employed for qualitative determination of the carbon content in the objects

Results

Bronze Powder Chamber

A well-preserved shiny yellow–brown metal was observed after the removal of the oxide cover by abrasive brushing (Fig. 1). VT inspection of the powder chamber following the abrasive brushing exposed a rough surface with interdendritic shrinkage porosity.

Typical dendritic microstructure was observed following the FMM procedure (Fig. 4a and b, multi-focal LM) and the standard destructive metallography (Fig. 4c and d, LM). Interdendritic shrinkage porosity was also observed by LM (Fig. 4c, dark area), typical of as-cast bronze items that were produced with no additional deformation treatment [9, 10, p. 274, 29, p. 321l]. The results of the two metallographic methods agree well (Fig. 4). In both methods, LM observation of the polished and etched dendritic surface revealed that it comprised two phases: an orange-coloured main phase and a silver-coloured minor interdendritic phase (Fig. 4).

SEM observation of the drilled specimen (conventional metallography) also revealed a dendritic microstructure with areas of shrinkage porosity (Fig. 5a, black area). This microstructure comprised a dark grey main phase and a bright minor interdendritic phase (Fig. 5a). EDS spectral analysis of a conventional metallographic sample of the bronze powder chamber revealed presence of Cu, Sn, Pb, Zn, As, Sb, Ag, and Ni (Fig. 5b). SEM-EDS analysis of the bronze powder chamber drilled specimen revealed an average composition of 86.6 ± 0.7 wt.% Cu, 10.1 ± 0.6 wt.% Sn, 1.4 ± 0.7 wt.% Pb, 0.7 ± 0.1 wt.% Sb, 0.5 ± 0.5 wt.% Zn, 0.3 ± 0.1 wt.% As, and 0.2 ± 0.1 wt.% of Ag and Ni. SEM–EDS elemental mapping of the powder chamber after conventional metallography of the drilled specimen revealed presence of Cu, Sn, Sb, Pb, As, Si, and Ag (Fig. 6).

Stud-Link Anchor Chain

A well-preserved dark metal was observed after the removal of the concretion (Fig. 2a and b). VT inspection of the links after sandblasting revealed a fibrous texture of parallel fine lines (Fig. 2c). A macro-cross section of the link (Fig. 2d) revealed shiny grey metal.

The FMM procedure (Fig. 7a and b, multi-focal LM) and the standard destructive metallography (Fig. 7c and d, LM), of the wrought iron link showed preferentially oriented slag inclusions (dark areas) surrounded by iron ferrite grains at the surface, typical of wrought iron products. The results obtained by both metallographic methods agreed well (Fig. 7). SEM-EDS elemental analysis of the wrought iron link (scanned area shown in Fig. 8a) revealed composition of 97.6 ± 2.0 wt.% Fe, 1.5 ± 1.4 wt.% O, 0.3 ± 0.2 wt.% Si, 0.3 ± 0.1 wt.% Mn, and 0.3 ± 0.1 wt.% P. SEM–EDS elemental mapping of the link specimen after conventional metallography revealed presence of Fe, O, Si, and P (Fig. 8), typical of wrought iron products. Due to different factors related to the carbon X-ray counts spotted by the EDS detector, SEM-EDS analysis results of carbon concentration in wrought iron and steel samples are not considered reliable quantitative outcomes [30]. Therefore, the carbon content was omitted from the SEM-EDS elemental analysis and SEM–EDS elemental mapping of the link specimen.

The FMM procedure (Fig. 9a and b, multi-focal LM), and the standard destructive metallography (Fig. 9c, LM) of the grey cast iron stud showed a dendritic pattern including black graphite flakes and graphite rosettes at the stud surface, typical of grey cast iron products. The results obtained by both metallographic methods agreed well (Fig. 9). The graphite flakes and graphite rosettes were surrounded by ferrite grains, perlite, and iron phosphide eutectic (Fig. 9b). SEM BSE mode observation also revealed graphite flakes surrounded by ferrite and perlite (lamellar microstructure) grains (conventional metallographic sample).

SEM-EDS analysis of the cast iron stud (Fig. 9d) revealed an average composition of 95.2 ± 0.9 wt.% Fe, 2.4 ± 0.4 wt.% Si, 1.2 ± 0.9 wt.% P, 1.1 ± 0.2 wt.% Mn, and 0.1 ± 0.1 wt.% S. The carbon content was omitted from the SEM–EDS elemental analysis. SEM–EDS qualitative elemental mapping of the grey cast iron stud conventional metallography (Fig. 10a‒f) revealed presence of Fe, P, C, S, and Mn, typical of graphite flakes (carbon), surrounded by iron ferrite matrix, and iron phosphide eutectic (Fe₃P), and manganese sulphide (MnS) precipitations.

Ferrous Winch

Although the surface of the ferrous winch drum and shaft were covered with corrosion products, their overall shape was well-preserved. Cast defects, including rounded cavities and interdendritic porosity, were observed by VT on the external surface of the cast iron winch drum.

A dendritic microstructure containing black graphite flakes and graphite rosettes was observed at the drum surface following the FMM procedure (Fig. 11a and b, multi-focal LM) and the standard destructive metallography (Fig. 11c and d, LM), typical of grey cast iron products. Higher magnification observations of the winch drum revealed that the graphite flakes and graphite rosettes were surrounded by ferrite grains and iron phosphide eutectic (Fig. 11b, d). SEM images of the drum (conventional metallography) revealed graphite flakes surrounded by ferrite grains (Fig. 11e), typical of as-cast iron. SEM-EDS analysis of the cast iron drum (scanned area shown in Fig. 11e) revealed an average composition of 95.5 ± 0.4 wt.% Fe, 2.7 ± 0.1 wt.% Si, 1.1 ± 0.1 wt.% Mn, 0.6 ± 0.3 wt.% P, and 0.1 ± 0.1 wt.% S. The carbon content was omitted from the SEM-EDS elemental analysis. SEM–EDS qualitative elemental mapping of the grey cast iron drum shows the distribution of the dark graphite flakes (Fig. 11f).

Field FMM observation revealed that the winch shaft was made of wrought iron (Fig. 12a), with preferentially oriented dark slag inclusions surrounded by a bright iron ferrite matrix. Both small and large ferrite grains were observed, with diameters of 10–100 µm and above 100 µm, respectively (Fig. 12a and b).

SEM images of the shaft revealed preferentially oriented slag inclusions (dark areas) surrounded by iron ferrite grains (Fig. 12c), typical of wrought iron products. LM observation of the conventional metallographic samples revealed small ferrite grains, 20–80 µm in diameter, and large ferrite grains, 100–400 µm (Fig. 12c). SEM-EDS analysis of the wrought iron shaft (Fig. 12c) revealed a composition of 99.5 ± 0.2 wt.% Fe, 0.3 ± 0.1 wt.% P, 0.1 ± 0.1 wt.% S, 0.1 ± 0.1 wt.% Si. The carbon content was eliminated from the SEM-EDS elemental analysis. SEM–EDS elemental mapping of the wrought iron shaft revealed that the preferentially oriented inclusions (Fig. 12d, dark pink areas) were mostly made of silicon oxide.

Ferrous Heart-Shaped Shackle

Although the surface of the ferrous heart-shaped shackle was covered with corrosion products, its general shape was well-preserved. VT inspection of the shackle link after sandblasting exposed a fibrous texture of parallel fine lines (Fig. 3b).

Preferentially oriented slag inclusions (dark areas) were observed surrounded by ferrite grains (with ferrite grain size of 10‒200 µm) at the shackles surface following the FMM procedure (Fig. 13a and b, multi-focal LM), and the standard destructive metallography of the shackle link surface (Fig. 13c and d, LM), typical of wrought iron products. The results obtained from both metallographic methods agree well for this item (Fig. 13).

SEM-EDS analysis of the heart-shaped shackle made of wrought iron with embedded preferentially oriented inclusions (after omitting the peaks of carbon) revealed composition of 98.0 ± 1.4 wt.% Fe, 1.1 ± 1.0 wt.% O, 0.4 ± 0.2 wt.% Si, 0.3 ± 0.2 wt.% P, 0.1 ± 0.1 wt.% S, and 0.1 ± 0.1 wt.% Mn. The carbon content was eliminated from the SEM-EDS elemental analysis.

Ferrous Deadeye Strap with Futtock Plate

The general shape of the ferrous deadeye strap with futtock plate was well-preserved (Fig. 3c). Straight parallel lines were observed by VT inspection of the external surface of the deadeye strap, showing that this item was wrought iron product worked by plastic deformation.

Deformed (elongated) iron ferrite grains and preferentially oriented slag inclusions were observed at the deadeye strap surface following the FMM procedure (Fig. 14a, multi-focal LM), and by the standard destructive metallography of the surface (Fig. 14b and c, LM), typical of wrought iron products. Large and small grains were observed in both the futtock plate and the strap, where the small grain size was 10–100 µm, and the large grain size was 100–500 µm at the shackle surface following the metallographic observations (Fig. 14a‒c). Small circular inclusions were also observed with diameter size up to 10 µm (Fig. 14c and d).

SEM-EDS analysis of the deadeye strap with futtock plate made of wrought iron with embedded preferentially oriented inclusions (after omitting the peaks of carbon) revealed composition of 97.6 ± 1.3 wt.% Fe, 1.6 ± 1.0 wt.% O, 0.3 ± 0.1 wt.% Mn, 0.2 ± 0.2 wt.% Si, 0.2 ± 0.1 wt.% P, and 0.1 ± 0.1 wt.% S.

Ferrous Stud-Link Chain Controller

Although the surface of the cast stud-link chain controller was covered with corrosion products, its overall shape was well-preserved. Cast defects, such as interdendritic porosity, were observed by VT on its surface, indicating that it was produced by casting.

A microstructure of graphite flakes surrounded by ferrite matrix, perlite, and white iron phosphide eutectic was observed at the chain controller surface following the FMM procedure (Fig. 15a and b, multi-focal LM), and the standard destructive metallography (Fig. 15c and d, LM), typical of grey cast iron. The results obtained from both metallographic methods agreed well (Fig. 15).

SEM-EDS analysis of the cast stud-link chain controller (after omitting the carbon content) revealed composition of 96.2 ± 0.5 wt.% Fe, 1.7 ± 0.3 wt.% Si, 0.8 ± 0.5 wt.% P, 0.8 ± 0.2 wt.% Mn, 0.4 ± 0.3 wt.% O, and 0.1 ± 0.1 wt.% S.

Discussion

Several maritime archaeological items, made of bronze, wrought iron and grey cast iron, were studied by the FMM method. The primary objective was to compare the results obtained by this innovative NDT technique with conventional destructive metallography. As demonstrated in the results of this study, and shown below, these metallographic methods yield similar results.

The powder chamber (Fig. 1) was made of cast bronze. Although the bronze was well-preserved, different corrosion products were observed on its external surface prior to cleaning by abrasively brushing, as expected from bronze archaeological objects, after a long burial period in a marine environment. This corrosion phenomenon is occasionally referred to as ‘bronze disease’ [17, 31]. LM and SEM examinations of the object, by both the FMM technique and by conventional metallography, revealed dendritic microstructure and interdendritic shrinkage porosity (Figs. 4 and 5a), typical of cast copper alloy objects [8, 10, 16, 32]. Both metallographic techniques revealed a dual-phase microstructure of an orange major phase and a silver minor interdendritic phase, as well as interdendritic porosity (Figs. 4 and 5a).

EDS spectral analysis and elemental mapping of the chamber’s conventional metallographic sample revealed presence of Cu, Sn, Pb, Zn, As, Sb, Ag, and Ni (Figs. 5b and 6). The reliability of the minor element composition detected by SEM-EDS analysis was confirmed by comparison with the XRF and OES analysis results [16, p. 5, table 2 and p. 6, table 3] and good agreement was achieved between the results. According to Iddan et al. [16], the elemental composition of the abrasively brushed surface of the powder chamber based on XRF results was 71.9–85.9 wt.% Cu, 11.2–20.1 wt.% Sn, and 1.1–4.7 wt.% Pb, as well as small amounts of Si, S, Sb, As, Ag, Ni, Zn, and Fe. The OES analysis of the abrasively brushed surface revealed that the item was made of homogeneous bronze with a composition of 84.6–86.2 wt.% Cu, 10.5–11.8 wt.% Sn, 1.9–2.2 wt.% Pb, and up to 0.6 wt.% of Sb, As, Ag, Ni, and Zn. The SEM–EDS analysis of the conventional metallography specimen produced similar results, with a composition of 85.2–87.0 wt.% Cu, 9.5–10.8 wt.% Sn, 1.0–2.4 wt.% Pb, and up to 0.8 wt.% of Sb, As, Ag, Ni, and Zn [16].

Three-phase texture was observed in the cast bronze powder chamber microstructure (Fig. 6): (1) an α-copper dendritic matrix as a main phase composed of a Cu-Sn solid solution (Fig. 6b), (2) an interdendritic Sn-rich α + δ phase scattered at the boundaries of the copper-rich phase (Fig. 6c), and (3) and a minor Pb-rich phase (Fig. 6e) [16, 33, p. 10, Fig. 5, 34, pp. 11–12]. The similarity between the elemental distribution of the tin (Fig. 6c) and antimony (Fig. 6d) detected in the SEM–EDS elemental mapping analysis of the powder chamber may result from an overlap uncertainty between the Sn and Sb peaks in the X-ray spectra (Fig. 5b). However, since both Sn and Sb were also detected in the OES analysis, their mutual appearance may originate in their ore mineral composition. The bright areas, observed by SEM–EDS elemental mapping, are mainly composed of lead (Fig. 6e). However, the parallel elemental distribution between the lead and the arsenic images (Fig. 6e and f, respectively) may result from an overlap uncertainty between the Pb and As peaks [35] (Fig. 5b). However, since both elements were also spotted in the OES analysis, their mutual appearance may be related to their ore composition.

According to Iddan et al. [16], based on the examination of the conventional metallography samples, the concentration of copper was higher in the core of the powder chamber than at its external abrasively brushed surface; whereas, the concentrations of tin and lead are higher at the surface. This phenomenon probably results from the solidification process during casting, as well as according to various diffusion and corrosion mechanisms during the long burial period at sea [16]. Lead was probably added to the molten copper alloy based on economic considerations, and/or to improve castability [8, 16, 36]. Sulphur, silicon, and iron were detected only at the surface of the powder chamber, and are therefore related to the long-term exposure to the underwater environment [16, 36].

The anchor chain links were made from wrought iron, although the studs themselves were of grey cast iron (Fig. 2c). The stud-link anchor chain was well-preserved (Fig. 2), although it was found covered with concretion. Beneath this concretion, different corrosion products were observed on the external surface before it was removed by sandblasting [12], as expected of a ferrous archaeological object after a long burial period in the sea, LM and SEM examinations of typical links removed from the chain by both the FMM technique and by conventional metallography revealed preferentially oriented slag inclusions (dark areas, longitudinal plan) surrounded by iron ferrite grains (Fig. 7), typical of wrought iron products [7, 11, 37‐39]. The absence of cementite morphology in the link’s microstructure indicates that it was made of wrought iron produced by the indirect smelting process of refining pig iron (the ‘puddling’ method) [7, 11, 37]. SEM–EDS elemental mapping of a typical link revealed presence of Fe, O, Si, and P (Fig. 8), typical of wrought iron products [7, 11]. According to Iddan et al. [16], the elemental composition of the links based on XRF analysis of the ground surface revealed that it was made of iron, with presence of only 0.3 ± 0.2 wt.% Si and 0.2 ± 0.2 wt.% P. Similar XRF analysis results were also observed in the rod, with a composition of 99.6 wt.% Fe and 0.4 wt.% Si. Since silicon does not reduce in either the direct or indirect smelting techniques, its presence in the link represents the distribution of the slag inclusion in the wrought iron matrix [7, 11, 12, 37].

According to the OES analysis results, which is considered as a reliable and accurate method for the determination of carbon concentration in iron alloys [40], the carbon content detected in the ground surface of a typical link was 0.0648 ± 0.0003 wt.% C, which is a typical value of ferritic iron [12]. The SEM–EDS analysis of the link’s ground surface revealed similar results, with a composition of 95.6–99.5 wt.% Fe, up to 2.9 wt.% O, 0.1–0.5 wt.% Si, and up to 0.4 wt.% of Mn, Pb, Ca, and Mg [12].

LM and SEM examinations of a typical stud (Fig. 2c) from the stud-link anchor chain, by both the FMM technique and by conventional metallography, revealed a dendritic pattern with black graphite flakes and graphite rosettes (Fig. 9), typical of type B according to ASTM A247-67 [41] grey cast iron products. SEM–EDS elemental mapping of a grey cast iron stud by conventional metallography revealed presence of Fe, P, C, S, and Mn (Fig. 10).

Based on Iddan et al. [12] XRF analysis of a typical stud demonstrated that this item was mainly composed of iron, with presence of 1.9 ± 0.4 wt.% Si, and 0.2 ± 0.1 wt.% Mn, as well as up to 0.4 wt.% of P, S, and Ti. The carbon content in the stud material, according to Iddan et al. [12] OES analysis, was 1.9333 ± 0.0455 wt.% C. The manganese was perhaps added deliberately to reduce gas porosity [5, 42] and to form manganese sulphide with the sulphur impurities in the metal. The presence of silicon, which serves as a graphite stabilizer material, is a typical element of grey cast iron.

LM and SEM examinations of the winch drum (Fig. 3a) by both the FMM technique and by conventional metallography revealed a dendritic microstructure with black graphite flakes and graphite rosettes surrounded by ferrite grains and iron phosphide eutectic (Fig. 11), typical of type B [40] grey cast iron products. A qualitative elemental mapping of the winch drum is presented in Fig. 11f, showing the distribution of the graphite flakes (dark blue flakes), surrounded by ferrite phase (bright green matrix).

SEM–EDS elemental mapping of the drum revealed presence of Fe, C, S, P, Ti and Mn (Fig. 11f). According to Iddan et al. [17], the elemental composition of the drum surface based on XRF analysis was 95.1–96.3 wt.% Fe, 2.6–3.8 wt.% Si, as well as less than 1.0 wt.% of P, Mn, and Ti. The SEM-EDS analysis of the drum’s conventional metallographic samples revealed a composition of 95.0–96.0 wt.% Fe, 2.6–2.8 wt.% Si, 0.9–1.1 wt.% Mn, 0.2–1.0 wt.% P, and up to 0.2 wt.% of Ti, S, and Cu; whereas, SEM-EDS analysis of black flakes revealed they were composed of 100 wt.% C [17]. Iddan et al. [17] found that the iron phosphide (Fe₃P) eutectic morphology was composed of 83.5–83.7 wt.% Fe 13.7–13.8 wt.% P, and 2.0–2.3 wt.% Mn, as well as presence of 2.0–2.3 wt.% Mn, and 0.1 wt.% of Ti and Cr. Manganese sulphide (MnS) precipitations were also observed on the drum surface by SEM-EDS analysis, with a composition of 59.2 wt.% Mn and 36.9 wt.% S [17].

LM and SEM examinations of the winch shaft (Fig. 3a) by both the FMM technique and the conventional metallography revealed preferentially oriented slag inclusions (dark areas) surrounded by iron ferrite grains (Fig. 12), typical of wrought iron products produced by refining pig iron by the indirect smelting process [7, 11, 37]. SEM–EDS qualitative elemental mapping of the wrought iron shaft revealed that the preferentially oriented slag inclusions were composed mainly of silicon oxide (SiO_2, pink-red areas, Fig. 11f) surrounded by ferrite phase (Fig. 12d, light green matrix).

According to Iddan et al. [17], the XRF analysis of the shaft revealed that the wrought iron was composed of iron 99.9 wt.% Fe and 0.1 wt.% Ti; whereas, the shaft’s composition according to the SEM-EDS analysis was 99.3–99.8 wt.% Fe, 0.2–0.4 wt.% P, up to 0.2 wt.% Si, and up to 0.1 wt.% S [17].

VT inspection of the heart-shaped shackle after sandblasting exposed a fibrous texture of parallel fine lines (Fig. 3b), typical of wrought iron rods [12]. LM examination by both the FMM technique and the conventional metallography revealed preferentially oriented slag inclusions, as well as small circular inclusions surrounded by iron ferrite grains (Fig. 13), typical of wrought iron products. According to Iddan et al., the XRF analysis of the heart-shaped shackle revealed that it was made of a wrought iron, with a composition of 98.6–99.8 wt.% Fe, and up to 0.8 w% of Si, P, S, Mn, and Ti [17]. The SEM-EDS analysis of the link’s wrought iron (conventional metallographic samples) revealed a composition of 96.3–99.2 wt.% Fe, 0.2–2.3 wt.% O, 0.5–0.6 wt.% Si, and up to 0.6 wt.% P, as well as up to 0.2 of S and Mn. The SEM-EDS analysis of the shackle revealed it was composed of 97.0 wt.% Fe, 1.8 wt.% O, 0.5 wt.% Si, and less than 0.5 wt.% of P, S, and Mn [17].

LM examination of the deadeye strap with futtock plate (Fig. 3c) by both the FMM technique and conventional metallography revealed deformed (elongated) iron ferrite grains, as well as preferentially oriented slag inclusions and small circular inclusions (Fig. 14), typical of hot-forged wrought iron products. According to Iddan et al. [17], the XRF analysis of the deadeye strap with a futtock plate showed that it was made of wrought iron, with a composition of 98.9–99.5 wt.% Fe and up to 0.7 wt % of Si, P, S, and Ti. The SEM-EDS analysis of this item (conventional metallographic samples) revealed a composition of 96.2–99.5 wt.% Fe, up to 2.8 wt.% O, and less than 0.5 wt.% Si, P, S, Mn, and Cu [17].

LM and SEM examinations of the stud-link chain controller (Fig. 3d) by both the FMM technique and conventional metallography revealed microstructure of graphite flakes surrounded by ferrite matrix, perlite, and white iron phosphide eutectic (Fig. 15), typical of type B grey cast iron products [41, 43]. The XRF analysis of the chain controller revealed a composition of 96.0–97.6 wt.% Fe, 1.2–2.3 wt.% Si, and 0.4–1.5 wt.% P, as well as less than 0.5 wt % of Mn and Ti [17]. The SEM-EDS analysis of this item (conventional metallography samples) after omitting carbon peaks revealed a composition of 95.7–96.9 wt.% Fe, 1.5–2.2 wt.% Si, and less than 1.0 wt.% of O, P, S, Mn, Ti, and V. The composition of a typical Fe₃P eutectic morphology according to SEM-EDS analysis was 88.9 wt.% Fe, 0.5 wt.% Si, 10.2 wt.% P, and 0.2 wt.% of Cr and V [17].

This study was aimed at assessing the feasibility of the FMM method as an advanced non-destructive technique for examining the microstructure of both ferrous and non-ferrous archaeological artefacts retrieved from marine environments. The advantages of the FMM method and conventional metallography are summarised in Table 2. The examination of the ferrous and non-ferrous archaeological objects by using the novel FMM technique produced metallographic images equivalent in their quality to the images achieved by conventional destructive metallography. Thus, results obtained in both metallographic methods agree well for all examined bronze, cast iron and wrought iron objects (Figs. 4, 5a, 7–9, 11, 12a‒c, 13–15).

Table 2

FMM and conventional destructive metallography methods and their main advantages and disadvantages

Method	Advantages	Disadvantages
FMM	Minimally destructive	Expensive
	Enables examination of crystalline grains, different solid-state phases and microstructural defects (with multi-focal LM observation)	Surface preparation is essential, skill needed to properly prepare surface
	Includes advanced microscopy with auto-focus and multi-focal system, with large depth of field with large working distance	Large objects cannot be inserted into a conventional scanning electron microscope
	Real-time in situ analysis	Skills required to prepare surface of small objects
	Possible to observe the surface of a convex object.	Near surface area is observed only after grinding, polishing, and etching
Conventional metallography	Inexpensive	Destructive
	The core of the material can be observed after grinding, polishing, and etching	Possible to observe only flat surfaces
	Enables examination of crystalline grains, different solid-state phases and microstructural defects (with LM and SEM observation)	Surface preparation is essential
		Skill needed to properly prepare specimens

Conclusions

The six examined objects retrieved from the underwater excavation of the Akko Tower shipwreck and its surrounding were divided into three groups based on their material and production processes: (1) one cast bronze object; (2) three type B grey cast iron objects and parts (ASTM A247-67); and (3) four wrought iron objects and parts. The FMM method, which is a novel NDT technique, offered high-resolution metallographic observation of the surface of these metallic objects without damaging them. The FMM results were compared with conventional metallography and the current results indicate that the results obtained by the FMM technique agree well with the results from conventional destructive metallography for both the bronze and the ferrous objects. The FMM is an advanced technique that can be used by others to expose the microstructure of archaeological metallic objects without damaging them, and consequently can contribute to a better understanding of early manufacturing techniques while retaining the objects display value.

Acknowledgements

The underwater excavations and research of the Akko Tower shipwreck were supported in part by the Israel Science Foundation (Grant No. 447/12), the Honor Frost Foundation, and the Rector and Research Authority of the University of Haifa, to whom the authors are grateful. The authors are grateful to H. Kravits, Microtech LTD (Israel), for his technical assistance with the HIROX microscope, to the anonymous reviewers for their valuable comments, and to J.B. Tresman for the English editing.

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Title: Feasibility Examination of the Field Multi-focal Metallography Method (FMM) for Characterisation of Metallic Marine Artefacts
Authors: N. Iddan
D. Ashkenazi
D. Cvikel
Publication date: 06-05-2024
Publisher: Springer US
Published in: Metallography, Microstructure, and Analysis
Print ISSN: 2192-9262
Electronic ISSN: 2192-9270
DOI: https://doi.org/10.1007/s13632-024-01074-1

Springer Professional

Feasibility Examination of the Field Multi-focal Metallography Method (FMM) for Characterisation of Metallic Marine Artefacts

Abstract

Publisher's Note

Introduction

Methodology

Materials

Bronze Powder Chamber

Iron Artefacts from the Akko Tower shipwreck

Stud-Link Anchor Chain

Winch

Heart-Shaped Shackle

Deadeye Strap with Futtock Plate

Stud-Link Chain Controller

Experimental

Results

Bronze Powder Chamber

Stud-Link Anchor Chain

Ferrous Winch

Ferrous Heart-Shaped Shackle

Ferrous Deadeye Strap with Futtock Plate

Ferrous Stud-Link Chain Controller

Discussion

Conclusions

Acknowledgements

Publisher's Note

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