analytical and characterisation study of heavily oxidized...

$: Analytical and Characterisation Study of Heavily Oxidized ...iasir.net/AIJRFANSpapers/AIJRFANS15-506.pdf · ray diffraction (XRD) were used to characterize coinage technology, surface$
ISSN (Print): 2328-3777, ISSN (Online): 2328-3785, ISSN (CD-ROM): 2328-3793

American International Journal of Research in Formal, Applied & Natural Sciences

AIJRFANS 15-506; © 2015, AIJRFANS All Rights Reserved Page 1

AIJRFANS is a refereed, indexed, peer-reviewed, multidisciplinary and open access journal published by International Association of Scientific Innovation and Research (IASIR), USA

(An Association Unifying the Sciences, Engineering, and Applied Research)

http://www.iasir.netAvailable online at

Analytical and Characterisation Study of Heavily Oxidized Billon Coin,

Tell Basta, Egypt Dr. Saleh Mohamed Saleh, Dr. Mohamed M. Megahed

Faculty of Archaeology, Fayoum University, Egypt

I. Introduction

Coins form a unique class of archaeological material (Seeley, 1980), and they are a prevalent cultural material

than other artifacts because coins have survival rate (Alsaa’d, 1999). Also, coins are made of different metal to

supply denomination to cover the range of transactions (Butcher and Ponting, 2014). They are often made at the

royal mint, and they have a technical attribute of its era. Silver is usually alloyed with copper, and even if there

is 50/50 Cu/Ag, the appearance of this alloy is still white (Grassini, Angelini, Palumbo and Ingo, 2008). Billon

alloy is consisting of gold or silver and a base metal, usually copper, used especially for coinage (Scheidel,

2010, Kelly, 2005) or it is an alloy of copper and silver, with more than half copper. Large quantities of billon

coins were produced in the Roman era (Clayton, 2013). The use of copper instead of gold or silver may have

raised the level of tolerance of coin overvaluation relative to metal value, the underlying principle was the same

as with precious-metal coins: the market value of coin was primarily a function of its intrinsic value and crude

attempts to introduce token coins were invariably unsuccessful. Billon alloy is a low-grade metal after gold

alloys (Hrenderson, 1953). Bimetallic system using silver and copper has been little studied to date.

Silver was one of the first metals to be used by humans, and it is found in many archaeological objects such as

coins, jewellery and ornaments (Selwyn, 2004). Silver has been known longer than recorded history (around

3000 B.C). The Romans are the first to mention the use of silver (www.footdefense.com).

Copper is the most important non-ferrous metal that was used in early times (Renfrew and Bahn, 2004), and

copper ore occurs within the geographical limits of Egypt in two different localities, namely in Sinai and the

eastern desert (Lucas, 1927).

II. Corrosion process of Billon Coin

Corrosion process of billon alloy is complicated. The in-situ corrosion caused by interaction between metals and

surrounding soil changed in the coin’s morphology. The burial corrosive environment is sandy soil rich with

high salts and chloride ions. The site is characterized by rising of lime and gypsum. Selwyn and other said that a

complete understanding of the corrosion of metal artifacts requires consideration of both the changes that occur

while the object is buried and how these changes affect corrosion after excavation (Selwyn, Sirois and

Argyropoulos, 1999). Soil or subterranean corrosion occurs within an electrolytic film, created by the moisture

constantly present within the inter-granular spaces and organic matter of soil or sediment. The variation of

chemical and physical properties of soil and soil water alter the properties of the electrolyte in terms of pH,

pollutants and oxidizing properties (Dracott, 2014). The amount of precipitation is the main effect of climate

and alloying elements, which results in electrons loss (Domoneand, and Jefferis, 2002, Ferreira, Ponciana,

Vaitsinan, and Pérez, 2007). Metallic corrosion in general produces metal ions oxidized in the hydrated form

and then precipitates a mass of corrosion compounds in the solid form (Sato, 2012). Electrolytic corrosion is a

result of direct current from outside sources entering and then leaving a particular metallic structure by way of

the electrolyte (Bushman, 2010). Also, electrochemical corrosion occurs when counterbalancing reduction

reaction at cathodic sites that consumes the generated electrons (Selwyn, Sirois and Argyropoulos, 1999). The

Abstract: Some coins have been excavated since 1998 by the Supreme Council of Antiquities Mission in Tell

Basta, El- Sharqyia governorate, Egypt. The coins were found in a salty soil suffering from large quantity of

corrosion products, which were mixed with soil deposits. This work aims to evaluate and identify the

influence of alloy compositions and corrosion processes in the burial environment on the corrosion rate.

Stereomicroscope, scanning electron microscope (SEM) with energy dispersive spectrometry (EDS) and X-

ray diffraction (XRD) were used to characterize coinage technology, surface morphology, corrosion

products, and chloride ion diffusion rate. The results signify that billon coins contain silver in metallic case

in the core with a small quantity of silver corrosion products on the surface. In contrast, copper was less in

the core, and the majority of it has totally turned the core into corrosion products.

Keywords: Archaeological Billon Coins, Copper, Silver, Coinage Technology, Burial Environment

Corrosion Products, Stereo Microscope, X-Ray Diffraction, Scanning Electron Microscope/ Energy

Dispersive Spectrometry.

http://www.iasir.net/

http://www.coins-of-the-uk.co.uk/pics/metal.html

http://www.coins-of-the-uk.co.uk/pics/metal.html

http://www.footdefense.com/

Saleh and Megahed, American International Journal of Research in Formal, Applied & Natural Sciences, 13(1), December, 2015-

February, 2016, pp. 01-09


copper-silver played a serious role in corrosion process, because of the difference between the two metals in the

potential, and surely the copper was the victim. The presence of copper causes the brittle of billon coins

(Thompson and Chatterjee, 1954). Corrosion and internal leaching and dissolving of copper can result in a

considerable loss of weight over time (Butcher and Ponting, 2014). Erosion corrosion is indicated by an

increased rate of deterioration and degradation on a metal, because of the relative movement between a

corrosive fluid and a metal surface (Macleod and Schindelholz, 2004). Chloride corrosion compounds form

under these conditions will tend to precipitate rapidly and be powdery rather than crystalline in texture (Sharkey

and Lewin, 1971). At the same time, chloride ions are not combined in a stable solid corrosion product. They

take part in the metal corrosion cycle, and once the artifact becomes completely mineralized, they diffuse into

the burial soil (Turgoose, 1982). Hydrated copper (II) salts are generally blue or green in colour, both as solids

and in aqueous solution. In concentrated hydrochloric acid, however, they become yellow or red-violet,

respectively, to the formation of complex anions. Anhydrous copper (II) salts are usually black or yellow-brown

from the sulphate which is white. Copper (II) sulphate, chloride and acetate are very soluble in water (Burns,

Townshend and Carter, 1981). Patina has indicated a desirable and attractive surface condition indicative of age

(Weil, 2007).

III. Description and Condition

Archaeological billon coins were discovered by The Supreme Council of Antiquities Mission in Tell Basta, El-

Sharqia governorate, Egypt, season 1998.

Figure 1: Sceenshot fom Google earth of Tell Basta

After: https://earth.google.com

Excavated antiquities and coins are dated back to the Roman period according to the archaeological

documentation from the decoration as shown in figure 2. After the excavation, coins were preserved in

uncontrolled environment, so post-corrosion resulted from chlorides led to transformation into corrosion. Billon

coins were in close by corrosion products and soil deposits. The most of coin decorative were removed

gradually during burial. In generally, corrosion products changed in the morphology of billon coins.

Archaeological coins removed from the soil are likely to have coloured cover were dependent on soil properties.

The predominantly coins had lost mint lustre, and fragments were not restored, because they were highly

mineralized.

Photo 1: (A) Front of the billon coin, (b) Back of the billon coin after treatment

A B

https://earth.google.com/




IV. Materials and Methods

A. Sample collection

Coin fragments were examined and analyzed by stereo microscope and SEM-EDS without embedding and

mounting. Random Powder samples were used in XRD analyses. Fragment surfaces were not cleaned prior

examination and analyses. Investigation and analysis were organized into one fragment from the excavated

coins.

B. Stereo microscope

A deep examination and photography for the coin fragments were performed by using stereo microscope in

order to declare the structure of the coin surface, and the stratified corrosion layers. It is the oldest and simplest

investigation method. The investigation was implemented using Zeiss microscope.

C. Scanning Electron Microscope and Energy Dispersive Spectrometry (SEM/EDS):- Scanning electron microscopy is a valuable tool for the examination and analyses of archaeological coins,

because of the possibility of high magnification, high depth resolution, and chemical analysis. Examination of

samples in high magnifications allows for identification of the metal artifacts microstructure and their oxidized

form. Scanning electron microscopy based on the detection and treatment of X-rays emitted after excitation of

the sample ray energy is characteristic from which emitted intensity characteristic of the concentration of the

sample (www.engr.sjsu.edu/MC2/SOP_EDAX.pdf). Magnification in the SEM is controlled by the ratio of the

dimensions of the Cathode Ray Tube (CRT) to the dimensions of the area being scanned (Dunlap and

Adaskaveg, 1997). SEM-EDS was used in investigation and measurement of the elements in the coin alloy,

corrosion compounds and soil deposits.

D. X-Ray diffraction Analysis

X-ray powder diffractometry involves characterization of materials by using data dependent on the atomic

arrangement in the crystal lattice. The technique uses single or multiphase specimens. Each 2Θ value is

converted to a d-spacing, using Bragg’s law to generate a list of d-spacing and intensity called a d/I list

(Winefordner, 2012). X-RD analysis was performed by a Phillips PW 1840 series powder diffractometer, using

CuK radiation. Measurements were carried out in the range 3˚ < 2Θ < 65◦ with a step of 0.01◦.

V. Results

A. Microscopic investigation

Photo 2: Stereo microscope photos of billon fragments indicate (a) Observe surface of the billon coin, (b)

Reverse surface of the billon coin, (c) Cross section of the billon coin, (d) The coin edge.

(a) (b) (c) (d)

The previous photos clarified that corrosion products were mixed with soil encrustations covering the coin

surfaces. Fragments have a thick layer of corrosion combined to soil deposits. As shown in photo (1a), the

nodule formations of red colour on the surface is due to formation of iron oxide, but the red colour in the core

resulting from formation of cuprite mineral. Layered corrosion is the main corrosion form of the fragments.

Some parts lost from the coin surface. The interior layers were formed with silver and cuprite. Fragment is in

worst state, because it’s completely oxidized.

Figure 3: schematic of corrosion layer stratigraphy

http://www.engr.sjsu.edu/MC2/SOP_EDAX.pdf




B. SEM-EDS results

SEM examination and EDS analyses display the morphology and microstructure of billon corrosion compounds

covering the coin surfaces and soil deposits.

FIGURE 4: SEM images of samples from billon coin fragments

A) SEM Mapping of Patina made up from silver and copper corrosion compounds,

B) SEM Mapping of billon coin surface fretting damage, C) SEM image of bassanite crystals resulting from soil

deposits, D) SEM of soil encrustations on the coin surface, E) SEM mapping of the soil deposits on the coin

surface, F) SEM image showing black spot of corrosion compounds on the coin surface, G) SEM-EDS result of

the soil deposits on the coin surface, H) SEM image showing surface porousity resulting from corrosion

compounds of Ag-Cu metals I) SEM image shows dendritic structure of silver surrounding corrosion products,

J) SEM image shows mass of silver covering by corrosion products

A B

C D

E F




G H

I J

Figure 4-A and 4-B show elemental mapping that indicates the high percentage of silver-copper. From EDS

result, silver concentration is nearly twice of copper in bulk and white spot. Sodium, calcium, Aluminum and

magnesium were found as surficial deposits of billon coins, but iron was not detected. Phosphorus was detected

on the surface caused by ancient organic remains of vegetation and animals.

As shown in figure 4-C and 4-D, there is a high percentage of soil deposits on the billon coin surface were

declared as sulpher, calcium, iron and silicon are detected. Sulphure and calcium are detected in high percentage

but sodium, magnesium, silicon, potassium and aluminum in percentage low. SEM image result shows bassanite

crystals (semi-hydrate gypsum) that adhered with the corrosion compounds resulting from the burial

environment.

Figure 4-E SEM mapping shows the corrosion compounds on the coin surface that was subjected to degradation

phenomena during the burial environment. From EDS result, we conclude that silver is considered the major

element in the white areas.

In the figure 4-F, we can see the soil deposits in close contact with coin surface. Also, EDS data of the same

sample reveals that sodium is the main element in black spot. The chemical active of sodium caused porousity

on the surface.

Figure 4-G shows halite crystals on the billon coin surface resulting from salt-rich sediments. SEM signifies a

deep crack on the surface. Silver and copper corrosion compounds are detected as traces. From EDS results in

table 4, Na and Cl elements are detected in high level, but chloride is greater than sodium.

Figure 4-H shows SEM image result of the coin surface that reveals high level of copper corrosion products.

Sodium and iron are detected in high level on the coin surface. SEM image declares localized corrosion that

penetrates inside the coins.

Figure 4-I clarifies the dendritic form that is the proof of minting method by casting method. Also, EDS shows

the high percentage of silver, but copper are not included. Other elements were declared in high level with silver

as sulpher and iron.

Figure 4-J clarifies limit metal residue that contains the main elements of billon alloy. The residual metal in this

area contains 77.19(wt)% by mass of silver and 3.84% mass of copper, because of its chemically active that

caused transformation into corrosion products.

In the following table 4, EDS result reveals the wide variation of the chemical composition of the billon coin

combined with soil minerals.




Table 4: EDS analyses of samples from billon coin fragments Sample %

Element

Α Β C D E F G H I J

Na 11.34 10.46 1.91 1.46 7.49 70.20 34.21 9.65 1.92 1.23

Mg 1.69 1.99 1.57 0.42 1.60 2.75 0.98 1.46 2.32 1.63

Al 1.06 0.77 1.52 0.87 0.89 2.95 0.89 0.71 0.74 1.61

Si 1.45 .0.95 1.93 3.06 1.03 ----- 0.74 ------ 0.62 1.29

S 0.77 ------ 34.58 33.37 1.45 ------ 1.02 ------ 9.42 0.86

P 0.87 ------ ---- ------ 0.65 ------ ------ ------- ----- ------

Cl 0.48 0.26 1.77 3.16 0.45 2.74 58.56 0.77 0.65 0.60

Ag 52.95 55.86 1.66 0.74 78.73 2.85 0.66 7.18 66.53 77.19

K ----- ------- 0.76 0.59 ------ 3.75 0.18 0.71 0.14 0.35

Ca 0.87 0.74 39.63 42.35 0.51 3.56 0.61 0.49 2.21 0.30

Fe ------- ------- 14.57 13.67 ------ ------ 1.46 5.86 15.45 9.42

Cu 28.47 28.97 ------- 0.31 7.20 11.20 0.69 73.12 ----- 5.52

C. X-RD result

X-ray powder diffraction data of bulk samples revealed the composition of the corrosion layered encrustations

and soil deposits.

Table 1 X-RD results of two samples (A and B) from the random corrosion products of the billon coins

combined with soil deposits Angle 2Θ d (A°) I/I0 Angle 2Θ d (A°) I/I0

4.35 20.32 14.6 5.46 16.17 22.5

6.74 13.11 22.2 6.11 14.46 18.0 7.87 11.22 15.6 8.09 10.92 25.6

10.09 8.76 20.5 10.69 8.27 20.8

12.41 7.13 16.1 13.02 6.80 14.9 14.80 5.98 17.1 14.75 5.46 23.8

16.17 5.48 72.8 16.23 5.08 91.3

17.03 5.21 21 17.71 5.02 26.3 17.89 4.96 17.1 22.32 3.98 10.5

31.48 2.84 65.4 24.92 3.58 10.8

32.21 2.78 97.5 32.30 2.78 96.2 33.52 2.67 23.4 39.68 2.27 100.0

36.46 2.46 56.5 42.39 2.13 21.9

39.62 2.27 100 50.17 1.82 59.6 42.22 2.14 33.6 53.58 1.71 23.1

47.92 1.90 38.2

49.98 1.82 32.1 53.34 1.72 22.8

57.06 1.61 10.6

A B

From the previous results illustrated in table 1 (A and B), d (A°) value and 2Θ peak positions extracted

manually from the reference pattern. When 2 were 32.21°A, chlorargyrite (AgCl) was known, but acanthite

was detected at 49.98°A and argentite was detected at 49.98°A. The peaks of atacamite were known at 16.17°A

and 39.62°A. Generally, average errors in 2Θ and/or d (A°) ranged from 0.1 and 0.2.

Figure 13: The percentage of the corrosion compounds and soil deposits

A B

Figure 13-A and 13-B shows billon patina that was constituted of cuprite, atacamite, chlorargyrite and malachite

in high level. Akagneite, halite and iron oxides were detected in low level in the external layer. Silver and

copper sulphides were identified as acanthite, argentite, spionkopite, and brochantite. Atacamite is the highest

corrosion compound in the random powder of billon alloy.




Table 2: The minerals of corrosion compounds of billon coins

Mineral Formula

Cuprite Cu2O

Atacamite CuCl2.3Cu(OH)2

Spionkopite CuS

Malachite CuCO3.Cu(OH)2

Atacamite CuCl2.3Cu(OH)2

Copper hydrogen phosphite hydrate CuHPO3.2H2O

Brochantite Cu4(SO4)(OH)6

chlorargyrite AgCl

Argentite Ag2S

Acanthite Ag2S

Wustite FeO

Hematite Fe2O3

Akaganite FeO(OH,Cl)

Siderite FeCO3

Lepidocrocite γ-FeOOH

Greenalite Fe23Si2O5(OH)4

Magnetite Fe3O4

Bassanite CaSO4.0.5H2O

VI. Discussion

There is a strong consistent correlation between the kind of the corrosion products and surrounding

environment. From the obtained results, sandy soil played an important role in their severe corrosion during

burial incubation resulting from chloride ions. This soil structure is a porous that changes from sub saturation to

saturation with water movement (Fjaestad, Nord and Tronter, 1997, Tylecate, 1979). Also, circulation of saline

water in the soil had a serious effect on the coins, which led to transformation the ductile metals completely into

corrosion products (Werner and Roland, 1998, Tennent and Antonio, 1981). The analysis and investigation

revealed halite and bassanite adhering to the coin surface. Halite was the source of chloride ions that make

copper and silver chlorides.

The result obtained confirms the presence of the two main metals of billon alloy. Because copper is less noble

than silver, the first metal corroded and dissolved more than the latter, so there is no residue of metallic copper

in the billon coin surface, caused by the different potential of silver-copper during the electrochemical corrosion.

The EDS and X-RD analysis result of the corrosion crust revealed a relatively high chloride content in the coins

with very low content of sulfide. Generally, copper corrosion patina in billon alloy involves cuprite,

paratacamite, chlorargyrite and atacamite. Cuprite appeared obviously from the cross section in the core of the

billon coin. Metal chlorides have been formed during the long-term archaeological burial environments thus

forming chloroargyrite, Cu[I] chloride and Cu [II] compounds and akaganiete were detected by XRD spectrum.

In addition to, it must be taken into account that, iron and there oxides were identified by the investigation and

analysis methods resulting from the original alloy. From XRD and EDS analyses results, Phosphorous and

Copper hydrogen phosphite hydrate are detected in low level resulting from organic material in the burial soil.

It can be deduced from the elongation of the silver grains and dendritic structure that the via casting method was

used in the manufacture of the coins. This result confirms the hypothesis proposed by metallurgists (see

figure14, 15).

Figure 14: shows SEM image of silver in the

core of billon coin

Figure 15 The most phase of metals solidify with

a dendritic structure

After: Ashby, and Jones, 1999




VII. Conclusions

The corrosion mineralogy and metallurgy of excavated billon coins from Tell Basta have been identified and

chracterised through a combination of stereo and scanning electron microscope with energy dispersive

spectrometry and X-ray diffraction for mineral characterisation.

Billon coins were excavated from the sandy soil in worse state, and they coalesced by corrosion products. The

fragility of the coin resulted from the impact of high level of salts in the burial environment as halite and

bassanite. Generally, corrosion products mainly consist of silver, copper and iron chlorides. The corrosion

products are found in 4 layers, in addition the core of the coin consists of silver and cuprite.

Neither the coin surface nor the core contains copper metal resulting from corrosion process during long periods

of burial. It is found that the changing in the thickness, diameter, size and weight of the coins could be allowed

measurement of corrosion rate. From EDS result, the average ratio of silver element is 57%, but the ratio of

copper element is 29% on the surface. So, there is a difference between the ratio of copper and silver and their

corrosion compounds in the exterior and interior layers. Chloride compounds as atacamite, paratacamite,

botallacite, chlorargyrite, and akaganeite must be removed to prevent the activity of re-corrosion.

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[36] www.footdefense.com

http://www.engr.sjsu.edu/MC2/SOP_EDAX.pdf

http://www.footdefense.com/

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