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Chemical cleaning of soiled deposits and encrustations on archaeological glass: A diagnostic and practical study
Journal of Cultural Heritage 14 (2013) 97–108
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Original article
Chemical cleaning of soiled deposits and encrustations on archaeological glass: A diagnostic and practical study Ramadan Abd-Allah a,∗,b,c a b c
Conservation Department, Faculty of Archaeology, Cairo University, Orman 12613, Giza, Egypt Institute of Archaeology, The University of Jordan, Amman 11942, Jordan Hamdi Mango Center for Scientific Research (HMCSR), The University of Jordan, Amman, 11942, Jordan
a r t i c l e
i n f o
Article history: Received 10 December 2011 Accepted 26 March 2012 Available online 30 April 2012 Keywords: Archaeological glass Soiled deposits Encrustations Examination Chemical cleaning
a b s t r a c t The aim of this study is practically to establish a chemical strategy for cleaning soiled deposits and encrustations on archaeological glasses. Investigations were performed on a series of Roman glass samples (Fragments and complete objects) coming from different excavations in northern Jordan. The chemical composition of the glass samples was determined by X-ray fluorescence spectrometer (XRF) analysis technique, whereas X-ray powder diffractometer (XRD) and Energy dispersive X-ray (EDX) methods were used to determine the mineralogical and elemental composition of the soiled deposits and encrustations on the glass surfaces. Furthermore, Scanning Electron Microscopy (SEM) examination and optical assessment were performed before and after cleaning glass. The glass samples were subjected to different cleaning protocols such as Calgon (Sodium hexametaphosphate), ethylenediaminetetraacetic acid (EDTA) at different pH values, citric and tartaric acids and piranha solution (a solution of sulphuric acid and hydrogen peroxide). Sepiolite poultices soaked by chemical agents were the most suitable methods used for applying chemical solutions on the glass surface. It can be concluded that EDTA is generally accepted as the most effective chelating agents recommended for cleaning encrustations on durable glass. It was more effective and safe at neutral pH with low concentrations around 5 to 7%. The calcareous crusts can safely be removed by using a piranha solution. Citric and tartaric acids appeared a moderate efficiency on cleaning weathered and stable glass. Calgon has a tendency to damage corroded and iridescent surfaces, and should be avoided when cleaning weathered glass. © 2012 Elsevier Masson SAS. All rights reserved.
1. Introduction A major objective of all conservation treatment is to increase the chemical stability of the object being treated. Cleaning often forms an important part of the stabilizing process. This is because dirt on an object can be a potent source of deterioration [1,2]. The cleaning of materials constituting artistic and/or archaeological handicrafts, such as glasses, pottery, frescoes, metals and woods, is a very important operation, because it takes off encrustations, deposits and dirt, rendering handcrafted articles and masterpieces more accessible from an aesthetic and a functional point of view. However, this kind of operation must be accomplished with great care in order to avoid any damage to the surfaces with an irrecoverable loss of material and information. Therefore, the cleaning procedures require the elimination of the extraneous substances present on the surface of the artefacts, these exogenous substances also being able to interact with the material by forming alteration products, and the respect of
∗ Tel.: +962 0 78 82 55 435; fax: +962 6 53 55 511. E-mail address: [email protected] 1296-2074/$ – see front matter © 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2012.03.010
the polychromy and the natural patina of the objects, by avoiding irreversible alteration of the surface [3]. A complex problem faced in the conservation of ancient artefacts is represented by the cleaning of thick encrustations from the surfaces of historical and artistic artefacts (ceramics, glasses, frescoes, stone monuments and statues, metals and wood). The removal of encrustations and hard-soiled deposits is an essential intervention, but yet controversial because the encrustations hinder significant decoration details and obscure the historical, social and artistic values of an ancient object but their removal could be harmful to the object itself. The commonly adopted procedure is mainly based on a mechanical cleaning that must be carried out in a very gentle and careful way, avoiding any surface damage that may produce material and/or information loss [4]. Sease (1994) [5] emphasised that in situ, it is usually desirable to remove surface dirt sooner rather than later in order to avoid the danger of it becoming drawn into the body or into cracks. In the case of wet or damp excavated objects, it may be important to remove surface dirt before it dries, both because it is often easier to do so before the dirt has hardened and because the dirt may shrink and cause damage as it dries. It was observed that the heavy carbonate crusts on wet glass objects are mechanically removed
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Fig. 1. Location map indicating the archaeological sites of Gadara, Dohaleh and Waggas in Northern Jordan.
before they allowed being dry out. Where these crusts are harder than the underlying surface, it is not possible to remove them completely by these methods. Resort to chemical removal of carbonate and sulphate crusts has to be made [1]. It was generally accepted that several parameters influenced the archaeological glasses pathology, like type of glass, burial conditions and the thermal (kiln) history. Jackson et al. (2012) [6] stated that the chemical durability of glasses is influenced by their composition, the conditions under which they were manufactured and their environment. It is the interplay between these factors that goes some way to explaining the results of the corrosion tests, in acid media, of two different glass compositions. Some work has been conducted on attack in alkaline environments, as this is the dominant surrounding matrix once the glass starts to corrode, and in the long term many commercial soda–lime–silica glasses are resistant to corrosion by weak mineral acids [7,8]. On the other hand, Rehren (2008) [9] assumed that the loss of alkali earth compounds during the preparation of ancient faience may explain why so little of the faience glazes are well preserved. Small objects, in particular, often made from crushed clean quartz rather than calcareous sand will not have enough alkali earth oxides in the glass to act as a stabilizer, resulting in rapid and complete corrosion of the glass phase once the object is buried. According to Abd-Allah et al. (2010) [2], before cleaning, it is essential to identify the type of glass, its composition as well as the nature of dirt and deposits. It is also important to understand that cleaning means the removal of soil or deposits and encrustation but not removal of original material or any opaque weathering crust or a patina, which has a protective action and archaeological feature. The choice of the cleaning procedure is crucial, since it is an irreversible process and it could induce irrecoverable damage. Essential for the conservator is the knowledge of the chemical nature and structure of the encrustations to be removed. They can be composed by insoluble salts, such as calcareous concretions, sulphated encrustations, alumino-silicate crusts also containing soluble salts (chlorides, phosphates and nitrates) as well as other inorganic species (iron, manganese, copper and black sulphides) or organic stains [4]. Mechanical cleaning, merely breaking the adhesion of dirt and moving it away, contrasts with chemical cleaning,
accomplished either by dissolving the dirt or by causing something to react with it. Although mechanical cleaning method have certain advantages for the object since nothing is added during them might cause further deterioration, such as solvents carrying the dirt further into porous materials, in most of cases the mechanical cleaning is not enough to remove all dirt and spots from the object, so we try to use wet and chemical cleaning methods for this purpose [10]. Chemical cleaning procedures generally consist of the direct use of chemical solutions or the application of poultices soaked by cleaning chemical and/or biological solutions and/or enzymatic systems. Traditionally, diluted acids or bases, ammonium carbonate, hydrazine hydroxide, hydroxylamine chloride and sodium hexametaphosphate are the most common reagents used for the cleaning of glass and ceramic materials [4]. In practice, water is the most important liquid cleaning agent, with the triple advantages of being very cheap, easily available and without hazard to the conservator, but it is rarely used alone as a solvent, and all kinds of additives are used to modify its properties. Wet cleaning may involve chemical reactions in the case of non-water soluble soils, sequestering or chelating agents, acids, alkalis, oxidation and reducing agents, and enzymes may be employed to improve the solubility and/or dispersion of soiling [1]. In the last few years, these procedures have been accomplished, in several cases, by using chelate systems. These consist of chelating species in a water solution that allow the elimination of the crusts coming from corrosion processes, being used both for cleaning metal materials and for the removal of crusts containing calcium salts (sulphate, oxalate, concretions or scialbo constituted by carbonate), and also being used for plaster, stone or glass supports [4]. These compounds are chemical species that succeed in eliminating the dirt and the encrustations present on the surface, by exploiting their polyelectrolyte nature and their ability to remove metal ions by chelating reactions. In fact, they are multi-dentate ligands that form very stable complexes that are, in particular, more stable than the alteration or incrustation products [11–13]. Moreover, in alkaline environments, they induce more wettable surfaces (having lower surface tension) and improve the electrostatic neutralization, facilitating the detachment of the dirt and the disintegration of the deposit that easily moves in the aqueous phase because of the reduction of aggregation, flocculation and deposition phenomena [14–15]. However, particular care has to be devoted to avoid the aggressive actions of chelating agents beneath the surfaces that can be attacked and partially dissolved because of the formation of complex species with uncontrolled and/or inappropriate pH conditions [16]. In this work, a detailed investigation of archaeological glass shards and objects found in three Jordanian sites, some of them also hardly encrusted were performed in order to identify the nature of the alteration deposits grown during the long-term archaeological burial. The morphological, chemical and structural characterization results were used for the setting up of a novel chemical cleaning experimental procedure for the removal of deposits and calcareous encrustations from glass artefacts to be used in the conservation practice.
2. Experimental 2.1. Glass artefacts 2.1.1. The selection of samples In order to evaluate the activity of the cleaning procedures, twenty glass fragments and three complete objects dated back to Roman period (1st–4th century AD) were selected. These glasses were uncovered during excavation works carried out by the Department of Antiquities of Jordan at the archaeological sites of Gadara
R. Abd-Allah / Journal of Cultural Heritage 14 (2013) 97–108
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Fig. 2. A view of glass fragments selected for the experimental cleaning.
(season 2009), Dohaleh (season 1994) and Waggas (season 2006) located in northern Jordan (Fig. 1). Since the excavations, these glasses are stored in uncontrolled condition storage at the National Museum of Irbid (Figs. 2 and 3). The choice of an ancient sample was made for two different reasons: in the early glasses there were no stabilizer elements, and Ca2+ is often contained in variable concentration with an uncontrolled Na/Ca ratio, so they are more sensitive to water-solution aggressiveness [17]. From a theoretical point of view, we believe that real samples are essential
to set a restoration protocol. In fact, in designing a new procedure, it is not easy to predict all the real problems that originate when old materials suffer chemical treatments. To evaluate the efficiency or aggressiveness of each cleaning agent, the glass fragments were classified into three groups, each experimentally cleaned by using only one chemical agent, whereas the complete or intact objects were finally cleaned by the appropriate cleaning agents and elected technique. Table 1 illustrates the description of these glasses.
Fig. 3. Complete glass objects selected for the final application of cleaning.
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Table 1 Description of the glasses selected for the analytical and practical study. Site
Sample No.
Colour
Deterioration
Gadara
1
Body fragment of thin-walled vessel
Light blue
Corroded and pitted
2
Body fragment of thin-walled vessel
Yellowish green
Completely corroded
3
Rim fragment of small flask
Colourless
Durable
4
Base fragment of round vessel
Yellowish green
Semi corroded
5
Neck and shoulder fragment of Small bottle
Greenish blue
Corroded
Dohaleh
Waggas
6
Base fragment of round plate
Opaque blue
Corroded and pitted
7
Rim fragment of small flask
Light blue
Durable
8
Piece of glass jewelry
Deep blue
Corroded
9
Rim fragment of small bottle
Greenish blue
Corroded
10
Rim fragment of cut beaker
Light blue
Semi stable
11
Base fragment of round vessel
Opaque blue
Completely corroded
12
Base fragment of round plate vessel
Light blue
Completely corroded
13
Base fragment of round plate
Yellowish green
Corroded and pitted
14
Body fragment of small cylindrical flask
Yellowish green
Durable
15
Rim fragment of large bottle
Yellowish green
Semi corroded
16
Body fragment of thin-walled vessel
Colourless
Corroded and pitted
17
Base fragment of round vessel
Light blue
Completely corroded
18
Rim fragment of small flask
Colourless
Semi durable
19
Body fragment of thin-walled vessel
Light blue
Corroded
20
Body fragment of thick-walled vessel
Opaque blue
Completely corroded
A – IR.4535
Complete glass bottle
Yellowish green
Semi corroded
B – R.4537
Complete glass vessel
Light blue
Semi corroded
C – IR.4538
Complete glass bottle
Light green
Durable
2.1.2. Burial condition of samples Table 2 summarizes the characteristic properties of soil (burial environment), which related certainly to the deterioration aspects of the glasses uncovered from the archaeological sites of Dohaleh, Waggas and Gadara. However, the primary investigation indicated that all the glasses are deteriorated, corroded and covered with several dry layers of soiled deposits corresponding to calcareous and clay remains during long-term burial in the soil. Optically, in most cases, the deposits are strongly adhered to the surfaces and are not greasy whereas weathering encrustations are very brittle, fragile and tended to flak away. These observations are coinciding with the excavation reports since those glasses were mostly buried in a moisture-containing environment and uncovered from slightly acidic and alkaline contexts. Jackson et al. (2012) [6] affirmed that when glass is buried in a water-containing environment, the surface of the glass reacts chemically with the water. Additionally, the amount and type of reaction is affected by the composition of the glass and the pH of the surrounding liquid. It is presently thought that a reaction is brought about by the diffusion of water (mainly through H+ cation exchange) into the glass and the migration of the alkali cations from the glass, leading to a silica-rich layer that is also reduced in density. This reaction is especially effective in glasses that are rich in potassium. This newly formed silica-rich layer acts partly as a protective layer to the glass, slowing the rate of decay [18–22]. It was stated that acid soils are aggressive to glass and alkaline soils are benign. Even so, with the exception of peat, a reasonable relationship appears to exist between the pH of the soil and the state of the glass. In acid media, there is an inter-diffusion of hydrogen-bearing species (H+, including H2 O) and alkaline or alkaline earth ions (Na+, K+, Ca2+ , Mg2+ ) and an alkaline earth/alkali
depleted layer and a hydrogen-enriched surface layer is formed through ion exchange. This is influenced by the strength of the acid, a more vigorous reaction taking place in strong acids. In soda–lime–silica glasses, increased acidity of the burial solution leads to a loss of alkali, and especially calcium, when the latter is present in higher concentrations (> 10 mol%) in the glass. The type of acid medium is also known to affect the leaching of alkalis and chemical changes at the glass surface [6,23,24]. In an alkaline medium, a slightly different reaction takes place: network dissolution, where OH- ions attack the silicate network, particularly the bridging oxygen atoms. This process is self-perpetuating, as new OH- ions are produced, which accelerates the process. At above pH 9, the rate of reaction of removal of silica is increased, but is constant within any given pH. At lower pH values, the rate of alkali removal is increased, the rate being determined by the diffusion of the alkali within the leached layer. Calcium ions do not pass into solution in alkaline media unless they are present at high concentrations in the glass. The temperature of the reaction can also affect the type of reaction and the rate. In alkali solutions and at warmer temperatures, more silica is lost in a glass that contains potash and low calcium, so the glass is depleted in alkali and silica. In acidic solutions, the glass will lose alkali but much of the silica will remain [18,22]. The loss of alkali manifests itself as iridescent laminar layers, which form on the surface of glasses that are silica-rich. These layers are produced with fluctuating environmental conditions, whereby the corrosion takes place in distinct phases. These are often seen in ancient and historical glasses and are particularly apparent in many Roman glasses (such as the present case study here). Furthermore in acid and alkaline soils opportunities for the formation of saline solutions are certain. Chlorides, carbonates and sulphates are the common salts present
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Table 2 Characteristic properties of soil in the three archaeological sites (Burial environment). Site
Soil properties (Burial environment) Morphological texture
Moisture content %
PH value
Salinity (EC) m.mohs/cm−1
Densitygm/cm3
Porosity %
Gadara
Calcareous clay
2.8
48
Waggas
Clay loam
3.5 Slightly saline 2.6 Slightly saline 3.2 Slightly saline
60
Calcareous clay
8.2 Alkaline 6.8 Acidic 8.5 Alkaline
2.4
Dohaleh
5.3 Moist 2.9 Semi-moist 4.4 Moist
2.6
58
in soiled deposits remains on glass. Since the buried or corroded glass is mostly porous, any increase in soil moisture will result in greater salt mobilization and crystallization during drying, leading to damage [18,23,25–27]. 2.2. Analytical techniques The chemical composition of the glass samples was determined by a Philips Magix pw2424 X-ray fluorescence spectrometer (XRF), which uses the high-purity silica BCS-CRM 313/1 standard certified reference material from the Bureau of Analyzed Samples LTD, UK and works under vacuum, voltage 20–60 KV, current 5–150 mA and a Power limit of 4050 watt. A 6000-Shimazu X-ray powder diffractometer (XRD) with Cu K␣ radiation (1.543 Ao) operating at reflection mode was used to determine the mineralogical composition of dirt and deposits on the glass surfaces. Furthermore, microscopic and optical assessment was carried out before and after cleaning process. A scanning electron microscopy attached with Energy dispersive X-ray microanalysis unit (SEM with EDX model FEI Quant 200) operated in a secondary electron mode was used to examine the surface morphology and structure of deposits or encrustations and the underlying glass surface. 2.3. Cleaning agents Chemical solutions of different concentration and pH values were prepared as cleaning agents in this study. Calgon (Sodium hexametaphosphate) (NaPO3 )6 is the polyphosphate compound used. Aqueous solutions of the sodium salt of disodium ethylenediaminetetraacetic acid (EDTA) [Na2 EDTA] were used as the aminocarboxylic compound. Citric and tartaric acids were the hydroxycarboxylic compounds used. Piranha solution (solution of sulphuric acid and hydrogen peroxide) is also tested. Sepiolite poultices (absorbing alumina) soaked by chemical agents were the most suitable methods used for applying chemical solutions on the glass surface. 2.4. Cleaning methodology The three groups of glass fragments underwent different chemical cleaning protocols: fragments in group A were invidiously cleaned with Calgon solution for 40 min; fragments in group B were treated for 40 min with 5% EDTA solutions, respectively, at 5, 7, and 9. pH values; fragments in group C were treated invidiously for 40 min with 5% citric acid and tartaric solutions, respectively, at 2, 4, 6 and 9 pH values. The lowest pH values correspond to the pure EDTA or citric acid solutions. Moreover, Piranha solution (conc. H2 SO4 /120 volH2 O2 with 3:1 volumetric ratio) was used to remove calcareous crusts on glass fragments in the three groups. The final applications were performed on the complete glass objects from Waggas site with only the appropriate chemical agents elected. Sepiolite poultice (absorbing alumina) was the most suitable methods used for applying cleaners’ solutions on the glass
Fig. 4. A secondary electron image of glass sample (1) showing the soiled deposits covered glass surface, deterioration and pitting proceeding from the surface to the interior.
surface since it is more easily removed, is more humid and can be reused several times. During cleaning, the poultices were not allowed to dry out. It should be noticed that, in some cases when needed, mechanical method using scalpels and dry brushes, was initially carried out to remove part of the very thick crusts and deposits, whereas dry cotton swaps were finally used to remove the dissolved deposits and soft crusts away from the glass surface. All the cleaning process has been carried out in the room temperature between 25 to 30 ◦ C. Glasses were carefully then washed many times with deionised water and dried under nitrogen. It should be noticed that, after cleaning process, further treatments such as consolidation and surface coating were performed in order to ensure the stability of these glass objects. 3. Results and discussion 3.1. Investigation observations From SEM observation of the glass samples covered with coherent deposits and encrustations, it can be seen that all the glass surfaces seem to be inhomogeneous pitted, curviplanar, surfaceplanar and highly fractured forms (Figs. 4 and 5). Large areas of the weathering crusts were destroyed and rich in dissolution voids and micro-cracks. Addition to that, other aspects of sugar-like surface, flaking, and highly fissured nature of decayed crusts were also observed (Figs. 6 and 7). Furthermore, Figs. 8 and 9 show secondary
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Fig. 5. A secondary electron image of glass sample (8) showing large areas of the weathering crusts be destroyed and rich in dissolution voids.
Fig. 7. A secondary electron image of glass sample (19) showing the aspects cracking and pitting of corroded surface.
electron images of magnified sections through corroded crusts from glass samples 2 and 11, showing deterioration proceeding from the surface to the interior. On the other hand, dirty layers, soil deposits and encrustations on the glass surface appeared to be inhomogeneous, differ in their thickness and strongly adhered to the glass surface and pitted areas (Fig. 10). Salty grains were observed between dirty crust and inside deep pits (Fig. 11). In all that cases, no presence of crystals or devitrification was observed. The compositions of 20 glass fragments from the three Roman sites as provided by the aid of XRF are shown in Table 3. The results of the analyses indicate that the major components of the glass samples are: silica (SiO2 Avg. 69.65%), soda (Na2 O Avg. 14.63%), lime (CaO Avg. 8.76%) and alumina (Al2 O3 Avg. 2.95%). They were also characterised by low contents of potash (K2 O Avg. 0.71%) and
magnesia (MgO Avg. 0.53%). Therefore, these glasses can be classified as soda–lime–silica (Na2 O–CaO–SiO2 ) glass, and correspond to the previously defined Levantine I glass group (Fig. 12), the common type of ancient glass for more than 3000 years [28–32]. This composition revealed that the main raw materials from which these glasses were manufactured were Levantine coastal sand as a source of silica, natron (from Wadi Natrun in Egypt) as a source of alkali soda, and lime (which is already present as impurity or shell fragments in the Levantine coastal sands) as a source of calcium [28]. However, no evidence for a local primary production of raw glass at these sites has been found to date. It was stated that glass production in the first millennium AD was divided between a relatively small number of workshops that made raw glass and a large number of secondary workshops that fabricated vessels. During the Roman
Fig. 6. A secondary electron image of glass sample (9) showing the weathered layers be destroyed, and losses its glassy nature.
Fig. 8. A secondary electron image of glass sample (2) showing the surfaces seem to be inhomogeneous pitted, curviplanar, surface-planar and highly fractured forms.
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Table 3 Chemical composition of the selected glasses obtained by X-ray fluorescence spectroscopy (XRF). Sample No.
1
Oxides (wt.%)
Total %
SiO2
Na2 O
K2 O
CaO
Al2 O3
MgO
MnO
Fe2 O3
PbO
TiO2
P2 O5
SO3
Cl2 O
67.99
13.33
0.86
10.17
3.12
0.89
0.04
0.57
0.07
0.13
0.17
0.14
0.84
99.62
2
76.37
6.02
0.17
11.04
2.74
0.62
0.03
0.59
0.04
0.11
0.24
0.13
0.90
99.60
3
68.95
13.92
0.64
9.55
2.67
0.54
0.02
0.53
0.04
0.09
0.11
0.14
0.73
97.93
4
68.69
14.04
0.68
9.81
3.33
0.49
0.04
0.54
0.11
0.08
0.34
0.09
0.69
98.93
5
68.83
13.53
0.70
10.07
2.95
0.47
0.03
0.48
0.08
0.11
0.16
0.11
0.65
98.17
6
69.77
15.97
0.69
8.82
2.74
0.50
0.03
0.48
0.07
0.12
0.13
0.10
0.94
100.36
7
69.48
15.09
0.76
8.85
3.10
0.48
0.04
0.53
0.05
0.09
0.11
0.08
0.92
99.58
8
69.44
15.65
0.71
8.72
2.82
0.45
0.05
0.52
0.02
0.09
0.17
0.18
0.73
99.55
9
68.94
14.81
0.74
9.04
3.19
0.56
0.02
0.44
0.03
0.10
0.12
0.12
0.85
98.96
10
69.69
15.72
0.68
8.37
2.83
0.49
0.04
0.46
0.04
0.09
0.11
0.08
0.83
99.34
11
70.24
15.54
0.69
9.03
2.75
0.52
0.03
0.47
0.07
0.03
0.14
0.11
0.82
100.44
12
69.63
14.80
0.70
8.84
2.90
0.49
0.02
0.48
0.09
0.11
0.16
0.13
0.77
99.12
13
68.95
15.27
0.75
8.69
3.47
0.47
0.06
0.53
0.06
0.13
0.13
0.12
0.82
99.45
14
69.48
14.46
0.69
8.55
3.09
0.61
0.03
0.56
0.05
0.10
0.12
0.09
0.71
98.54
15
69.71
14.23
0.61
8.32
2.81
0.45
0.05
0.49
0.10
0.11
0.17
0.09
0.82
97.96
16
68.99
14.76
0.70
8.78
2.94
0.58
0.04
0.51
0.08
0.13
0.19
0.12
0.83
98.65
17
69.86
14.56
0.69
8.89
3.02
0.51
0.03
0.52
0.06
0.07
0.11
0.13
0.86
99.31
18
69.45
15.22
0.73
8.35
2.93
0.55
0.04
0.49
0.04
0.10
0.28
0.12
0.84
99.14
19
69.98
14.35
0.82
8.86
3.04
0.52
0.04
0.45
0.03
0.06
0.16
0.10
0.80
99.21
20
68.62
14.37
0.74
8.76
2.72
0.49
0.03
0.53
0.07
0.08
0.13
0.14
0.82
97.50
Avg.%
69.65
14.63
0.71
8.76
2.95
0.53
0.03
0.50
0.06
0.09
0.16
0.11
0.80
98.06
and later periods, glass was produced from its raw materials in massive tank furnaces in a limited number of glass production centres (potentially in the Levantine area). The unformed chunks of raw glass originating from these furnaces were then re-melted to produce glass vessels at a larger number of glass working centres [28,30,31]. According to Abd-Allah (2010) [28] raw glass chunks
were imported to secondary production centres in Northern Jordan (such as Beit Ras) for local reworking in order to produce glass vessels in large quantities. However, the high content of soda (Na2 O, Avg. 14.63%) leads to the process of leaching and corrosion of those glasses during burial in wet environment [33–36]. In the completely corroded sample 2,
Fig. 9. A secondary electron image of glass sample (11) showing soil deposits and encrustations on the glass surface and between corroded areas.
Fig. 10. A secondary electron image of glass sample (6) showing dirty layers, soil deposits and encrustations on the glass surface appeared to be inhomogeneous and strongly adhered to the glass surface and pitted areas.
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Fig. 12. Levantine1 glass from Beit Ras compare with glasses from Gadara, Dohaleh and Waggas in Northern Jordan. Data of Abd-Allah 2010 [28].
Fig. 11. A secondary electron image of glass sample (16) showing salty grains dispread between dirty crust and inside deep pits.
there is an obvious change in the compositions in comparison with the durable sample 3, i.e., sodium and potassium content decreases (Na2 O 6.02% and K2 O 0.17%), whereas silica and calcium content increases (SiO2 76.37% and CaO 11.04%). Fig. 13 shows the XRD mineralogical analysis of soiled deposits separated from the surface of the three complete or intact glass objects. Accordingly, the surface deposits collected from object A from Gadara and object B from Dohaleh, mainly composed of calcite (CaCO3 ) and quartz (SiO2 ) which suggests that these deposits are corresponding to calcareous soil remains during burial, while
those collected from object C from Waggas site, mainly composed of calcite, quartz, dolomite [Ca Mg (CO3 )2 ] and the clay mineral quantinite [Mg4 Al2 (OH) 12CO3 .3H2 O], which suggests that these deposits are corresponding to clay loam soil remains during burial. EDX microanalysis of soiled deposits adhered to the glass fragments from those sites also revealed the presence of the compositional elements Si, Na, K, Fe, S, Cl, P, Mn addition to the earth alkaline elements Ca, Mg, and Al (Figs. 14 and 15). The presence of those elements indicate that they can be composed by insoluble salts, such as calcareous concretions, sulphated encrustations, aluminosilicate crusts also containing soluble salts (chlorides, phosphates and nitrates) as well as other inorganic species (iron, manganese, copper and black sulphides) [4]. As mentioned before, since the buried or corroded glass is mostly porous, any increase in soil moisture will result in greater salt mobilization and crystallization during drying, leading to damage.
Fig. 13. X-ray powder diffraction patterns showing the mineralogical composition of soiled deposits from glass objects a, b and c (Q = Quartz, C = Calcite, D = Dolomite, Qu = Quantinite).
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Fig. 14. Energy dispersive X-ray (EDX) microanalysis pattern of soil deposits on the glass object A.
3.2. Chemical cleaning assessment As previously mentioned, initial chemical cleaning was carried out on the experimented glass fragments to evaluate the activity of the chemical agents, whereas the complete glass objects were finally cleaned by the most reactive and combatable chemical agents. The most important observations during cleaning can be summarised that all chelating agents succeed in the removal of soiled deposits and encrustations thanks to a surface complexation mechanism, whereby the chelating agent is adsorbed onto the surface, weakens lattice sites, dissolves specific metal ion species and releases the chelated complex into solution. Nevertheless, the decay of glass surface due to cleaning is mostly differ from one agent to another, i.e. EDTA is generally more effective and safe at neutral pH with low concentrations around 5 to 7% (Applied on object A), but at alkaline pH with high concentrations above 7% it cause obvious decay of weak weathered glass surfaces. A
hydroxycarboxylictartaric and citric acids appeared a moderate safety on cleaning weathered glass but are very safe on cleaning stable glass as on object B. However, the use of citric acid and tartaric acid should be carried out under control, though are weak acids they attack severely on the presence of calcium stabilizer the structure of glass. Polyphosphate compounds (Calgon) have a tendency to damage corroded surfaces resulting in flaking of glass crusts and destroying iridescent layers or patina. On the other hand, a badcleaning procedure can result in a degradation of the glass surface by the formation of a silica gel layer, therefore the use of those solutions are excluded on corroded glass object. These results to be or become identical with the previous true that calgon (sodium exometaphosphoric) is very aggressive and especially on a very erosive glass because it is formed on a relatively low pH that weathers much more the glass with phenomena such as lamination and iridescences, especially on calcium which is stabilizer on glass lattice network [16,19]. Piranha solution (a solution of sulphuric acid and
Fig. 15. Energy dispersive X-ray (EDX) microanalysis pattern of soil deposits on the glass object C.
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Fig. 16. Scanning Electron Microscopy-Energy dispersive X-ray microanalysis (SEM-EDX) of glass samples cleaned with ethylenediaminetetraacetic acid (EDTA) solutions, show the etching of the glassy surface and the Si/Na and Si/Ca atomic concentration ratios increases with the pH: a-5, b-7and c-9.
hydrogen peroxide) was successfully used to remove calcareous crusts on object C. This solution is also highly useful for destroying biological species and organic layers. The risk of contaminating glass with sulphate ions is extremely low because of the lack of porosity of glass material. However, cleaning by this solution must be carried out within the scope of the deposits only, without prejudice to the glass surface. Caution must be taken since a strong acid such as hydrogen peroxide, which could oxidize metals-colorants of glass, and could provide change of colour in the object [16,37]. To compare the cleaning results, each piece was investigated under the optical microscope to evaluate the aggressiveness of the chemical agents used on the glass. It is possible to observe that, for both EDTA and citrate solutions, the etching of the glassy surface increases with the pH. In fact, the higher the pH the higher
the Si/Na and Si/Ca atomic concentration ratios as shown in the SEM-EDX images arranged in Fig. 16. Moreover, in alkaline environment, EDTA solutions seem to be more aggressive than the citrate ones. Of course in alkaline conditions the corrosion processes, caused by the nucleophic attack of the OH- ion on the silicon sites, can also happen and the glass surface is completely transformed into silica gel layers. Therefore, in using EDTA or citrate protocols for cleaning the surface of archaeological glasses alkaline environments have to be avoided in order not to damage the glassy surface with irrecoverable loss of material and irreversible transformation of the glass into silica gel [4]. However, Figs. 17 and 18 obviously reported the results of all chemical cleaning process carried out for both the glass fragments and complete objects respectively.
Fig. 17. Glass fragments before and after chemical cleaning with: (4) ethylenediaminetetraacetic acid (EDTA), (8) citric acid, (19) piranha solution.
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Fig. 18. Glass objects before and after chemical cleaning with: (A) ethylenediaminetetraacetic acid (EDTA), (B) citric acid, (C) piranha solution.
4. Conclusions In this work, the efficiency of some chemical agents on archaeological glass samples was studied. The pH environment was varied in order to verify the best conditions to avoid damage to the glassy surface. It was observed that all cleaning solutions were active in soften the solid deposits and encrustations but vary in their safety ratio on glass surface. However, cleaning by those solutions must be carried out within the scope of the deposits only, without prejudice to the glass surface to avoid the attack of unstable surface. EDTA and citric acid solutions are active in soften the solid deposits, and not etch the glassy surface at low pH values. Tartaric acid solution seems to be less aggressive than calgon (sodium hexametaphosphate) which has a tendency to damage corroded surfaces. From these observations we can suggest some easy rules when a piece of archaeological glass must be cleaned. Firstly, if not essential, it is not advisable to use chelating agents for cleaning corroded glass. The lime crusts can be removed by using a solution of sulphuric acid and hydrogen peroxide. This solution is also highly useful for destroying biological species and organic layers. Though the risk of contaminating glass with sulphate ions is extremely low because of the lack of porosity of glass material, caution must be taken since a strong acid such as hydrogen peroxide, which could oxidize metals-colorants of glass, and could provide change of colour in the object. In the case where a chelating agent must be used, for example to remove gypsum or salts not soluble in acid, a citrate solution can be used at a pH value lower than 7. A bad-cleaning procedure can result in a degradation of the glass
surface by the formation of a silica gel layer. The latter, most conveniently, a combination of different cleaning methods should be used in the conservation practice, after a preliminary, essential diagnostic study of the material nature and conservation state performed by physical and chemical investigations. Acknowledgement The author would hereby like to acknowledge the Archaeologists in the Department of Antiquities of Jordan for their constructive cooperation. Gratitude is also to Hamdi Mango Center for Scientific Research (HMCSR) team for their support among the JOCHERA project 2012. Thanks also are to the anonymous referees for their critical reviews and constructed suggestions. References [1] J. Cronyon, The elements of archaeological conservation, TJ Press, Cornwal, UK, 1990. [2] R. Abd-Allah, Z. Al-Muheiesn, S. Al-Howadi, Cleaning strategies of pottery objects excavated from Khirbet ed-Dharieh and Hayyan al-Mushref/Jordan: Four case studies, Mediterranean Archaeology and Archaeometry 10 (2) (2010) 97–110. [3] C. Altavilla, E. Ciliberto, S. LaDelafa, S. Panallero, A. Scandurra, The cleaning of early glasses: investigation about the reactivity of different chemical treatments on the surface of ancient glasses, Applied Physics A 92 (2008) 251–255. [4] M. Casaletto, G. Ingo, C. Riccucci, T. De Carro, G. Bultrini, I. Fragala, M. Leoni, Chemical cleaning of encrustations on archaeological ceramic artefacts found in different Italian sites, Applied Physics A 92 (2008) 35–42. [5] C. Sease, A conservation manual for the field archaeologist, 3rd edition, The University of California press, Los Angeles, 1994.
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[6] C. Jackson, D. Greenfild, L. Howie, An assessment of compositional and morphological changes in model archaeological glass in acidic matrix, Archaeometry 54 (3) (2012) 489–507. [7] T. El-Shamy, J. Lewins, R. Douglas, The dependence on the pH of the decomposition of glasses by aqueous solutions, Glass Technology 13 (1972) 81–87. [8] G. Cox, G. Cooper, Stained glass in York in the mid-sixteenth century: analytical evidence for its decay, Glass Technology 36 (1995) 129–134. [9] T. Rehren, A review of factors affecting the composition of early Egyptian glasses and faience: alkali and alkali earth oxides, Journal of Archaeological Science 35 (2008) 1345–1354. [10] M. Bani-Hani, R. Abd-Allah, L. El-Khouri, Archeaometallurgical finds Barsinia, Northern Jordan: Microstructural characterization from and conservation treatment, Journal of Cultural Heritage 13 (2012), doi:10.1016/j.culher.2011.12.005, in press. [11] F. Ernesperger, Attack of glass by chelating agents, Journal of American Ceramic Society 42 (1959) 373–375. [12] S. Calt, S. Spare, Smear layer removal by EDTA, Endodontology 26 (2000) 459–461. [13] S. Davison, Conservation and restoration of glass, 2nd edition, ButterworthHeinemann, Oxford OX2 8DP, UK, 2003. [14] F. Cotton, G. Wilkinson, C. Murillo, M. Bochmann, Advanced in organic chemistry, 6th ed, Wiley-VCH, New York, 1999. [15] D. Hamilton, Glass conservation, Conservation Research Laboratory, Texas A&M University, College Station, TX, 2000. [16] A. Paul, Influence of complexing agents and nature of the buffer solution on the chemical durability of glass. Part I, Theoretical discussion, Journal of Glass Technology 19 (1978) 162–165. [17] M. Pollard, C. Heron, Archaeological chemistry, The Royal Society of Chemistry, Cambridge, 1996. [18] T. El-Shamy, R. Douglas, Kinetics of the reaction of water with glass, Glass Technology 13 (1972) 77–80. [19] R. Newton, S. Davison, Conservation of glass, Butterworth, London, 1989. [20] J. Sterpenich, G. Libourel, Water diffusion in silicate glasses under natural weathering conditions: evidence from buried medieval stained glasses, Journal of Non-Crystalline Solids 352 (2006) 5446–5451. [21] M. Gulmini, M. Pace, G. Ivaldi, M. Ponzi, P. Mirti, Morphological and chemical characterization of weathering products on buried Sasanian glass from central Iraq, Journal of Non-Crystalline Solids 355 (2009) 1613–1621. [22] M. Pollard, C. Heron, Archaeological chemistry, RSC, Cambridge, 1996. [23] M. Melcher, M. Schreiner, K. Kreislova, Artificial weathering of model glasses with medieval compositions-an empirical study on the influence of
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Original article
Chemical cleaning of soiled deposits and encrustations on archaeological glass: A diagnostic and practical study Ramadan Abd-Allah a,∗,b,c a b c
Conservation Department, Faculty of Archaeology, Cairo University, Orman 12613, Giza, Egypt Institute of Archaeology, The University of Jordan, Amman 11942, Jordan Hamdi Mango Center for Scientific Research (HMCSR), The University of Jordan, Amman, 11942, Jordan
a r t i c l e
i n f o
Article history: Received 10 December 2011 Accepted 26 March 2012 Available online 30 April 2012 Keywords: Archaeological glass Soiled deposits Encrustations Examination Chemical cleaning
a b s t r a c t The aim of this study is practically to establish a chemical strategy for cleaning soiled deposits and encrustations on archaeological glasses. Investigations were performed on a series of Roman glass samples (Fragments and complete objects) coming from different excavations in northern Jordan. The chemical composition of the glass samples was determined by X-ray fluorescence spectrometer (XRF) analysis technique, whereas X-ray powder diffractometer (XRD) and Energy dispersive X-ray (EDX) methods were used to determine the mineralogical and elemental composition of the soiled deposits and encrustations on the glass surfaces. Furthermore, Scanning Electron Microscopy (SEM) examination and optical assessment were performed before and after cleaning glass. The glass samples were subjected to different cleaning protocols such as Calgon (Sodium hexametaphosphate), ethylenediaminetetraacetic acid (EDTA) at different pH values, citric and tartaric acids and piranha solution (a solution of sulphuric acid and hydrogen peroxide). Sepiolite poultices soaked by chemical agents were the most suitable methods used for applying chemical solutions on the glass surface. It can be concluded that EDTA is generally accepted as the most effective chelating agents recommended for cleaning encrustations on durable glass. It was more effective and safe at neutral pH with low concentrations around 5 to 7%. The calcareous crusts can safely be removed by using a piranha solution. Citric and tartaric acids appeared a moderate efficiency on cleaning weathered and stable glass. Calgon has a tendency to damage corroded and iridescent surfaces, and should be avoided when cleaning weathered glass. © 2012 Elsevier Masson SAS. All rights reserved.
1. Introduction A major objective of all conservation treatment is to increase the chemical stability of the object being treated. Cleaning often forms an important part of the stabilizing process. This is because dirt on an object can be a potent source of deterioration [1,2]. The cleaning of materials constituting artistic and/or archaeological handicrafts, such as glasses, pottery, frescoes, metals and woods, is a very important operation, because it takes off encrustations, deposits and dirt, rendering handcrafted articles and masterpieces more accessible from an aesthetic and a functional point of view. However, this kind of operation must be accomplished with great care in order to avoid any damage to the surfaces with an irrecoverable loss of material and information. Therefore, the cleaning procedures require the elimination of the extraneous substances present on the surface of the artefacts, these exogenous substances also being able to interact with the material by forming alteration products, and the respect of
∗ Tel.: +962 0 78 82 55 435; fax: +962 6 53 55 511. E-mail address: [email protected] 1296-2074/$ – see front matter © 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2012.03.010
the polychromy and the natural patina of the objects, by avoiding irreversible alteration of the surface [3]. A complex problem faced in the conservation of ancient artefacts is represented by the cleaning of thick encrustations from the surfaces of historical and artistic artefacts (ceramics, glasses, frescoes, stone monuments and statues, metals and wood). The removal of encrustations and hard-soiled deposits is an essential intervention, but yet controversial because the encrustations hinder significant decoration details and obscure the historical, social and artistic values of an ancient object but their removal could be harmful to the object itself. The commonly adopted procedure is mainly based on a mechanical cleaning that must be carried out in a very gentle and careful way, avoiding any surface damage that may produce material and/or information loss [4]. Sease (1994) [5] emphasised that in situ, it is usually desirable to remove surface dirt sooner rather than later in order to avoid the danger of it becoming drawn into the body or into cracks. In the case of wet or damp excavated objects, it may be important to remove surface dirt before it dries, both because it is often easier to do so before the dirt has hardened and because the dirt may shrink and cause damage as it dries. It was observed that the heavy carbonate crusts on wet glass objects are mechanically removed
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Fig. 1. Location map indicating the archaeological sites of Gadara, Dohaleh and Waggas in Northern Jordan.
before they allowed being dry out. Where these crusts are harder than the underlying surface, it is not possible to remove them completely by these methods. Resort to chemical removal of carbonate and sulphate crusts has to be made [1]. It was generally accepted that several parameters influenced the archaeological glasses pathology, like type of glass, burial conditions and the thermal (kiln) history. Jackson et al. (2012) [6] stated that the chemical durability of glasses is influenced by their composition, the conditions under which they were manufactured and their environment. It is the interplay between these factors that goes some way to explaining the results of the corrosion tests, in acid media, of two different glass compositions. Some work has been conducted on attack in alkaline environments, as this is the dominant surrounding matrix once the glass starts to corrode, and in the long term many commercial soda–lime–silica glasses are resistant to corrosion by weak mineral acids [7,8]. On the other hand, Rehren (2008) [9] assumed that the loss of alkali earth compounds during the preparation of ancient faience may explain why so little of the faience glazes are well preserved. Small objects, in particular, often made from crushed clean quartz rather than calcareous sand will not have enough alkali earth oxides in the glass to act as a stabilizer, resulting in rapid and complete corrosion of the glass phase once the object is buried. According to Abd-Allah et al. (2010) [2], before cleaning, it is essential to identify the type of glass, its composition as well as the nature of dirt and deposits. It is also important to understand that cleaning means the removal of soil or deposits and encrustation but not removal of original material or any opaque weathering crust or a patina, which has a protective action and archaeological feature. The choice of the cleaning procedure is crucial, since it is an irreversible process and it could induce irrecoverable damage. Essential for the conservator is the knowledge of the chemical nature and structure of the encrustations to be removed. They can be composed by insoluble salts, such as calcareous concretions, sulphated encrustations, alumino-silicate crusts also containing soluble salts (chlorides, phosphates and nitrates) as well as other inorganic species (iron, manganese, copper and black sulphides) or organic stains [4]. Mechanical cleaning, merely breaking the adhesion of dirt and moving it away, contrasts with chemical cleaning,
accomplished either by dissolving the dirt or by causing something to react with it. Although mechanical cleaning method have certain advantages for the object since nothing is added during them might cause further deterioration, such as solvents carrying the dirt further into porous materials, in most of cases the mechanical cleaning is not enough to remove all dirt and spots from the object, so we try to use wet and chemical cleaning methods for this purpose [10]. Chemical cleaning procedures generally consist of the direct use of chemical solutions or the application of poultices soaked by cleaning chemical and/or biological solutions and/or enzymatic systems. Traditionally, diluted acids or bases, ammonium carbonate, hydrazine hydroxide, hydroxylamine chloride and sodium hexametaphosphate are the most common reagents used for the cleaning of glass and ceramic materials [4]. In practice, water is the most important liquid cleaning agent, with the triple advantages of being very cheap, easily available and without hazard to the conservator, but it is rarely used alone as a solvent, and all kinds of additives are used to modify its properties. Wet cleaning may involve chemical reactions in the case of non-water soluble soils, sequestering or chelating agents, acids, alkalis, oxidation and reducing agents, and enzymes may be employed to improve the solubility and/or dispersion of soiling [1]. In the last few years, these procedures have been accomplished, in several cases, by using chelate systems. These consist of chelating species in a water solution that allow the elimination of the crusts coming from corrosion processes, being used both for cleaning metal materials and for the removal of crusts containing calcium salts (sulphate, oxalate, concretions or scialbo constituted by carbonate), and also being used for plaster, stone or glass supports [4]. These compounds are chemical species that succeed in eliminating the dirt and the encrustations present on the surface, by exploiting their polyelectrolyte nature and their ability to remove metal ions by chelating reactions. In fact, they are multi-dentate ligands that form very stable complexes that are, in particular, more stable than the alteration or incrustation products [11–13]. Moreover, in alkaline environments, they induce more wettable surfaces (having lower surface tension) and improve the electrostatic neutralization, facilitating the detachment of the dirt and the disintegration of the deposit that easily moves in the aqueous phase because of the reduction of aggregation, flocculation and deposition phenomena [14–15]. However, particular care has to be devoted to avoid the aggressive actions of chelating agents beneath the surfaces that can be attacked and partially dissolved because of the formation of complex species with uncontrolled and/or inappropriate pH conditions [16]. In this work, a detailed investigation of archaeological glass shards and objects found in three Jordanian sites, some of them also hardly encrusted were performed in order to identify the nature of the alteration deposits grown during the long-term archaeological burial. The morphological, chemical and structural characterization results were used for the setting up of a novel chemical cleaning experimental procedure for the removal of deposits and calcareous encrustations from glass artefacts to be used in the conservation practice.
2. Experimental 2.1. Glass artefacts 2.1.1. The selection of samples In order to evaluate the activity of the cleaning procedures, twenty glass fragments and three complete objects dated back to Roman period (1st–4th century AD) were selected. These glasses were uncovered during excavation works carried out by the Department of Antiquities of Jordan at the archaeological sites of Gadara
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Fig. 2. A view of glass fragments selected for the experimental cleaning.
(season 2009), Dohaleh (season 1994) and Waggas (season 2006) located in northern Jordan (Fig. 1). Since the excavations, these glasses are stored in uncontrolled condition storage at the National Museum of Irbid (Figs. 2 and 3). The choice of an ancient sample was made for two different reasons: in the early glasses there were no stabilizer elements, and Ca2+ is often contained in variable concentration with an uncontrolled Na/Ca ratio, so they are more sensitive to water-solution aggressiveness [17]. From a theoretical point of view, we believe that real samples are essential
to set a restoration protocol. In fact, in designing a new procedure, it is not easy to predict all the real problems that originate when old materials suffer chemical treatments. To evaluate the efficiency or aggressiveness of each cleaning agent, the glass fragments were classified into three groups, each experimentally cleaned by using only one chemical agent, whereas the complete or intact objects were finally cleaned by the appropriate cleaning agents and elected technique. Table 1 illustrates the description of these glasses.
Fig. 3. Complete glass objects selected for the final application of cleaning.
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Table 1 Description of the glasses selected for the analytical and practical study. Site
Sample No.
Colour
Deterioration
Gadara
1
Body fragment of thin-walled vessel
Light blue
Corroded and pitted
2
Body fragment of thin-walled vessel
Yellowish green
Completely corroded
3
Rim fragment of small flask
Colourless
Durable
4
Base fragment of round vessel
Yellowish green
Semi corroded
5
Neck and shoulder fragment of Small bottle
Greenish blue
Corroded
Dohaleh
Waggas
6
Base fragment of round plate
Opaque blue
Corroded and pitted
7
Rim fragment of small flask
Light blue
Durable
8
Piece of glass jewelry
Deep blue
Corroded
9
Rim fragment of small bottle
Greenish blue
Corroded
10
Rim fragment of cut beaker
Light blue
Semi stable
11
Base fragment of round vessel
Opaque blue
Completely corroded
12
Base fragment of round plate vessel
Light blue
Completely corroded
13
Base fragment of round plate
Yellowish green
Corroded and pitted
14
Body fragment of small cylindrical flask
Yellowish green
Durable
15
Rim fragment of large bottle
Yellowish green
Semi corroded
16
Body fragment of thin-walled vessel
Colourless
Corroded and pitted
17
Base fragment of round vessel
Light blue
Completely corroded
18
Rim fragment of small flask
Colourless
Semi durable
19
Body fragment of thin-walled vessel
Light blue
Corroded
20
Body fragment of thick-walled vessel
Opaque blue
Completely corroded
A – IR.4535
Complete glass bottle
Yellowish green
Semi corroded
B – R.4537
Complete glass vessel
Light blue
Semi corroded
C – IR.4538
Complete glass bottle
Light green
Durable
2.1.2. Burial condition of samples Table 2 summarizes the characteristic properties of soil (burial environment), which related certainly to the deterioration aspects of the glasses uncovered from the archaeological sites of Dohaleh, Waggas and Gadara. However, the primary investigation indicated that all the glasses are deteriorated, corroded and covered with several dry layers of soiled deposits corresponding to calcareous and clay remains during long-term burial in the soil. Optically, in most cases, the deposits are strongly adhered to the surfaces and are not greasy whereas weathering encrustations are very brittle, fragile and tended to flak away. These observations are coinciding with the excavation reports since those glasses were mostly buried in a moisture-containing environment and uncovered from slightly acidic and alkaline contexts. Jackson et al. (2012) [6] affirmed that when glass is buried in a water-containing environment, the surface of the glass reacts chemically with the water. Additionally, the amount and type of reaction is affected by the composition of the glass and the pH of the surrounding liquid. It is presently thought that a reaction is brought about by the diffusion of water (mainly through H+ cation exchange) into the glass and the migration of the alkali cations from the glass, leading to a silica-rich layer that is also reduced in density. This reaction is especially effective in glasses that are rich in potassium. This newly formed silica-rich layer acts partly as a protective layer to the glass, slowing the rate of decay [18–22]. It was stated that acid soils are aggressive to glass and alkaline soils are benign. Even so, with the exception of peat, a reasonable relationship appears to exist between the pH of the soil and the state of the glass. In acid media, there is an inter-diffusion of hydrogen-bearing species (H+, including H2 O) and alkaline or alkaline earth ions (Na+, K+, Ca2+ , Mg2+ ) and an alkaline earth/alkali
depleted layer and a hydrogen-enriched surface layer is formed through ion exchange. This is influenced by the strength of the acid, a more vigorous reaction taking place in strong acids. In soda–lime–silica glasses, increased acidity of the burial solution leads to a loss of alkali, and especially calcium, when the latter is present in higher concentrations (> 10 mol%) in the glass. The type of acid medium is also known to affect the leaching of alkalis and chemical changes at the glass surface [6,23,24]. In an alkaline medium, a slightly different reaction takes place: network dissolution, where OH- ions attack the silicate network, particularly the bridging oxygen atoms. This process is self-perpetuating, as new OH- ions are produced, which accelerates the process. At above pH 9, the rate of reaction of removal of silica is increased, but is constant within any given pH. At lower pH values, the rate of alkali removal is increased, the rate being determined by the diffusion of the alkali within the leached layer. Calcium ions do not pass into solution in alkaline media unless they are present at high concentrations in the glass. The temperature of the reaction can also affect the type of reaction and the rate. In alkali solutions and at warmer temperatures, more silica is lost in a glass that contains potash and low calcium, so the glass is depleted in alkali and silica. In acidic solutions, the glass will lose alkali but much of the silica will remain [18,22]. The loss of alkali manifests itself as iridescent laminar layers, which form on the surface of glasses that are silica-rich. These layers are produced with fluctuating environmental conditions, whereby the corrosion takes place in distinct phases. These are often seen in ancient and historical glasses and are particularly apparent in many Roman glasses (such as the present case study here). Furthermore in acid and alkaline soils opportunities for the formation of saline solutions are certain. Chlorides, carbonates and sulphates are the common salts present
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Table 2 Characteristic properties of soil in the three archaeological sites (Burial environment). Site
Soil properties (Burial environment) Morphological texture
Moisture content %
PH value
Salinity (EC) m.mohs/cm−1
Densitygm/cm3
Porosity %
Gadara
Calcareous clay
2.8
48
Waggas
Clay loam
3.5 Slightly saline 2.6 Slightly saline 3.2 Slightly saline
60
Calcareous clay
8.2 Alkaline 6.8 Acidic 8.5 Alkaline
2.4
Dohaleh
5.3 Moist 2.9 Semi-moist 4.4 Moist
2.6
58
in soiled deposits remains on glass. Since the buried or corroded glass is mostly porous, any increase in soil moisture will result in greater salt mobilization and crystallization during drying, leading to damage [18,23,25–27]. 2.2. Analytical techniques The chemical composition of the glass samples was determined by a Philips Magix pw2424 X-ray fluorescence spectrometer (XRF), which uses the high-purity silica BCS-CRM 313/1 standard certified reference material from the Bureau of Analyzed Samples LTD, UK and works under vacuum, voltage 20–60 KV, current 5–150 mA and a Power limit of 4050 watt. A 6000-Shimazu X-ray powder diffractometer (XRD) with Cu K␣ radiation (1.543 Ao) operating at reflection mode was used to determine the mineralogical composition of dirt and deposits on the glass surfaces. Furthermore, microscopic and optical assessment was carried out before and after cleaning process. A scanning electron microscopy attached with Energy dispersive X-ray microanalysis unit (SEM with EDX model FEI Quant 200) operated in a secondary electron mode was used to examine the surface morphology and structure of deposits or encrustations and the underlying glass surface. 2.3. Cleaning agents Chemical solutions of different concentration and pH values were prepared as cleaning agents in this study. Calgon (Sodium hexametaphosphate) (NaPO3 )6 is the polyphosphate compound used. Aqueous solutions of the sodium salt of disodium ethylenediaminetetraacetic acid (EDTA) [Na2 EDTA] were used as the aminocarboxylic compound. Citric and tartaric acids were the hydroxycarboxylic compounds used. Piranha solution (solution of sulphuric acid and hydrogen peroxide) is also tested. Sepiolite poultices (absorbing alumina) soaked by chemical agents were the most suitable methods used for applying chemical solutions on the glass surface. 2.4. Cleaning methodology The three groups of glass fragments underwent different chemical cleaning protocols: fragments in group A were invidiously cleaned with Calgon solution for 40 min; fragments in group B were treated for 40 min with 5% EDTA solutions, respectively, at 5, 7, and 9. pH values; fragments in group C were treated invidiously for 40 min with 5% citric acid and tartaric solutions, respectively, at 2, 4, 6 and 9 pH values. The lowest pH values correspond to the pure EDTA or citric acid solutions. Moreover, Piranha solution (conc. H2 SO4 /120 volH2 O2 with 3:1 volumetric ratio) was used to remove calcareous crusts on glass fragments in the three groups. The final applications were performed on the complete glass objects from Waggas site with only the appropriate chemical agents elected. Sepiolite poultice (absorbing alumina) was the most suitable methods used for applying cleaners’ solutions on the glass
Fig. 4. A secondary electron image of glass sample (1) showing the soiled deposits covered glass surface, deterioration and pitting proceeding from the surface to the interior.
surface since it is more easily removed, is more humid and can be reused several times. During cleaning, the poultices were not allowed to dry out. It should be noticed that, in some cases when needed, mechanical method using scalpels and dry brushes, was initially carried out to remove part of the very thick crusts and deposits, whereas dry cotton swaps were finally used to remove the dissolved deposits and soft crusts away from the glass surface. All the cleaning process has been carried out in the room temperature between 25 to 30 ◦ C. Glasses were carefully then washed many times with deionised water and dried under nitrogen. It should be noticed that, after cleaning process, further treatments such as consolidation and surface coating were performed in order to ensure the stability of these glass objects. 3. Results and discussion 3.1. Investigation observations From SEM observation of the glass samples covered with coherent deposits and encrustations, it can be seen that all the glass surfaces seem to be inhomogeneous pitted, curviplanar, surfaceplanar and highly fractured forms (Figs. 4 and 5). Large areas of the weathering crusts were destroyed and rich in dissolution voids and micro-cracks. Addition to that, other aspects of sugar-like surface, flaking, and highly fissured nature of decayed crusts were also observed (Figs. 6 and 7). Furthermore, Figs. 8 and 9 show secondary
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Fig. 5. A secondary electron image of glass sample (8) showing large areas of the weathering crusts be destroyed and rich in dissolution voids.
Fig. 7. A secondary electron image of glass sample (19) showing the aspects cracking and pitting of corroded surface.
electron images of magnified sections through corroded crusts from glass samples 2 and 11, showing deterioration proceeding from the surface to the interior. On the other hand, dirty layers, soil deposits and encrustations on the glass surface appeared to be inhomogeneous, differ in their thickness and strongly adhered to the glass surface and pitted areas (Fig. 10). Salty grains were observed between dirty crust and inside deep pits (Fig. 11). In all that cases, no presence of crystals or devitrification was observed. The compositions of 20 glass fragments from the three Roman sites as provided by the aid of XRF are shown in Table 3. The results of the analyses indicate that the major components of the glass samples are: silica (SiO2 Avg. 69.65%), soda (Na2 O Avg. 14.63%), lime (CaO Avg. 8.76%) and alumina (Al2 O3 Avg. 2.95%). They were also characterised by low contents of potash (K2 O Avg. 0.71%) and
magnesia (MgO Avg. 0.53%). Therefore, these glasses can be classified as soda–lime–silica (Na2 O–CaO–SiO2 ) glass, and correspond to the previously defined Levantine I glass group (Fig. 12), the common type of ancient glass for more than 3000 years [28–32]. This composition revealed that the main raw materials from which these glasses were manufactured were Levantine coastal sand as a source of silica, natron (from Wadi Natrun in Egypt) as a source of alkali soda, and lime (which is already present as impurity or shell fragments in the Levantine coastal sands) as a source of calcium [28]. However, no evidence for a local primary production of raw glass at these sites has been found to date. It was stated that glass production in the first millennium AD was divided between a relatively small number of workshops that made raw glass and a large number of secondary workshops that fabricated vessels. During the Roman
Fig. 6. A secondary electron image of glass sample (9) showing the weathered layers be destroyed, and losses its glassy nature.
Fig. 8. A secondary electron image of glass sample (2) showing the surfaces seem to be inhomogeneous pitted, curviplanar, surface-planar and highly fractured forms.
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Table 3 Chemical composition of the selected glasses obtained by X-ray fluorescence spectroscopy (XRF). Sample No.
1
Oxides (wt.%)
Total %
SiO2
Na2 O
K2 O
CaO
Al2 O3
MgO
MnO
Fe2 O3
PbO
TiO2
P2 O5
SO3
Cl2 O
67.99
13.33
0.86
10.17
3.12
0.89
0.04
0.57
0.07
0.13
0.17
0.14
0.84
99.62
2
76.37
6.02
0.17
11.04
2.74
0.62
0.03
0.59
0.04
0.11
0.24
0.13
0.90
99.60
3
68.95
13.92
0.64
9.55
2.67
0.54
0.02
0.53
0.04
0.09
0.11
0.14
0.73
97.93
4
68.69
14.04
0.68
9.81
3.33
0.49
0.04
0.54
0.11
0.08
0.34
0.09
0.69
98.93
5
68.83
13.53
0.70
10.07
2.95
0.47
0.03
0.48
0.08
0.11
0.16
0.11
0.65
98.17
6
69.77
15.97
0.69
8.82
2.74
0.50
0.03
0.48
0.07
0.12
0.13
0.10
0.94
100.36
7
69.48
15.09
0.76
8.85
3.10
0.48
0.04
0.53
0.05
0.09
0.11
0.08
0.92
99.58
8
69.44
15.65
0.71
8.72
2.82
0.45
0.05
0.52
0.02
0.09
0.17
0.18
0.73
99.55
9
68.94
14.81
0.74
9.04
3.19
0.56
0.02
0.44
0.03
0.10
0.12
0.12
0.85
98.96
10
69.69
15.72
0.68
8.37
2.83
0.49
0.04
0.46
0.04
0.09
0.11
0.08
0.83
99.34
11
70.24
15.54
0.69
9.03
2.75
0.52
0.03
0.47
0.07
0.03
0.14
0.11
0.82
100.44
12
69.63
14.80
0.70
8.84
2.90
0.49
0.02
0.48
0.09
0.11
0.16
0.13
0.77
99.12
13
68.95
15.27
0.75
8.69
3.47
0.47
0.06
0.53
0.06
0.13
0.13
0.12
0.82
99.45
14
69.48
14.46
0.69
8.55
3.09
0.61
0.03
0.56
0.05
0.10
0.12
0.09
0.71
98.54
15
69.71
14.23
0.61
8.32
2.81
0.45
0.05
0.49
0.10
0.11
0.17
0.09
0.82
97.96
16
68.99
14.76
0.70
8.78
2.94
0.58
0.04
0.51
0.08
0.13
0.19
0.12
0.83
98.65
17
69.86
14.56
0.69
8.89
3.02
0.51
0.03
0.52
0.06
0.07
0.11
0.13
0.86
99.31
18
69.45
15.22
0.73
8.35
2.93
0.55
0.04
0.49
0.04
0.10
0.28
0.12
0.84
99.14
19
69.98
14.35
0.82
8.86
3.04
0.52
0.04
0.45
0.03
0.06
0.16
0.10
0.80
99.21
20
68.62
14.37
0.74
8.76
2.72
0.49
0.03
0.53
0.07
0.08
0.13
0.14
0.82
97.50
Avg.%
69.65
14.63
0.71
8.76
2.95
0.53
0.03
0.50
0.06
0.09
0.16
0.11
0.80
98.06
and later periods, glass was produced from its raw materials in massive tank furnaces in a limited number of glass production centres (potentially in the Levantine area). The unformed chunks of raw glass originating from these furnaces were then re-melted to produce glass vessels at a larger number of glass working centres [28,30,31]. According to Abd-Allah (2010) [28] raw glass chunks
were imported to secondary production centres in Northern Jordan (such as Beit Ras) for local reworking in order to produce glass vessels in large quantities. However, the high content of soda (Na2 O, Avg. 14.63%) leads to the process of leaching and corrosion of those glasses during burial in wet environment [33–36]. In the completely corroded sample 2,
Fig. 9. A secondary electron image of glass sample (11) showing soil deposits and encrustations on the glass surface and between corroded areas.
Fig. 10. A secondary electron image of glass sample (6) showing dirty layers, soil deposits and encrustations on the glass surface appeared to be inhomogeneous and strongly adhered to the glass surface and pitted areas.
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Fig. 12. Levantine1 glass from Beit Ras compare with glasses from Gadara, Dohaleh and Waggas in Northern Jordan. Data of Abd-Allah 2010 [28].
Fig. 11. A secondary electron image of glass sample (16) showing salty grains dispread between dirty crust and inside deep pits.
there is an obvious change in the compositions in comparison with the durable sample 3, i.e., sodium and potassium content decreases (Na2 O 6.02% and K2 O 0.17%), whereas silica and calcium content increases (SiO2 76.37% and CaO 11.04%). Fig. 13 shows the XRD mineralogical analysis of soiled deposits separated from the surface of the three complete or intact glass objects. Accordingly, the surface deposits collected from object A from Gadara and object B from Dohaleh, mainly composed of calcite (CaCO3 ) and quartz (SiO2 ) which suggests that these deposits are corresponding to calcareous soil remains during burial, while
those collected from object C from Waggas site, mainly composed of calcite, quartz, dolomite [Ca Mg (CO3 )2 ] and the clay mineral quantinite [Mg4 Al2 (OH) 12CO3 .3H2 O], which suggests that these deposits are corresponding to clay loam soil remains during burial. EDX microanalysis of soiled deposits adhered to the glass fragments from those sites also revealed the presence of the compositional elements Si, Na, K, Fe, S, Cl, P, Mn addition to the earth alkaline elements Ca, Mg, and Al (Figs. 14 and 15). The presence of those elements indicate that they can be composed by insoluble salts, such as calcareous concretions, sulphated encrustations, aluminosilicate crusts also containing soluble salts (chlorides, phosphates and nitrates) as well as other inorganic species (iron, manganese, copper and black sulphides) [4]. As mentioned before, since the buried or corroded glass is mostly porous, any increase in soil moisture will result in greater salt mobilization and crystallization during drying, leading to damage.
Fig. 13. X-ray powder diffraction patterns showing the mineralogical composition of soiled deposits from glass objects a, b and c (Q = Quartz, C = Calcite, D = Dolomite, Qu = Quantinite).
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Fig. 14. Energy dispersive X-ray (EDX) microanalysis pattern of soil deposits on the glass object A.
3.2. Chemical cleaning assessment As previously mentioned, initial chemical cleaning was carried out on the experimented glass fragments to evaluate the activity of the chemical agents, whereas the complete glass objects were finally cleaned by the most reactive and combatable chemical agents. The most important observations during cleaning can be summarised that all chelating agents succeed in the removal of soiled deposits and encrustations thanks to a surface complexation mechanism, whereby the chelating agent is adsorbed onto the surface, weakens lattice sites, dissolves specific metal ion species and releases the chelated complex into solution. Nevertheless, the decay of glass surface due to cleaning is mostly differ from one agent to another, i.e. EDTA is generally more effective and safe at neutral pH with low concentrations around 5 to 7% (Applied on object A), but at alkaline pH with high concentrations above 7% it cause obvious decay of weak weathered glass surfaces. A
hydroxycarboxylictartaric and citric acids appeared a moderate safety on cleaning weathered glass but are very safe on cleaning stable glass as on object B. However, the use of citric acid and tartaric acid should be carried out under control, though are weak acids they attack severely on the presence of calcium stabilizer the structure of glass. Polyphosphate compounds (Calgon) have a tendency to damage corroded surfaces resulting in flaking of glass crusts and destroying iridescent layers or patina. On the other hand, a badcleaning procedure can result in a degradation of the glass surface by the formation of a silica gel layer, therefore the use of those solutions are excluded on corroded glass object. These results to be or become identical with the previous true that calgon (sodium exometaphosphoric) is very aggressive and especially on a very erosive glass because it is formed on a relatively low pH that weathers much more the glass with phenomena such as lamination and iridescences, especially on calcium which is stabilizer on glass lattice network [16,19]. Piranha solution (a solution of sulphuric acid and
Fig. 15. Energy dispersive X-ray (EDX) microanalysis pattern of soil deposits on the glass object C.
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Fig. 16. Scanning Electron Microscopy-Energy dispersive X-ray microanalysis (SEM-EDX) of glass samples cleaned with ethylenediaminetetraacetic acid (EDTA) solutions, show the etching of the glassy surface and the Si/Na and Si/Ca atomic concentration ratios increases with the pH: a-5, b-7and c-9.
hydrogen peroxide) was successfully used to remove calcareous crusts on object C. This solution is also highly useful for destroying biological species and organic layers. The risk of contaminating glass with sulphate ions is extremely low because of the lack of porosity of glass material. However, cleaning by this solution must be carried out within the scope of the deposits only, without prejudice to the glass surface. Caution must be taken since a strong acid such as hydrogen peroxide, which could oxidize metals-colorants of glass, and could provide change of colour in the object [16,37]. To compare the cleaning results, each piece was investigated under the optical microscope to evaluate the aggressiveness of the chemical agents used on the glass. It is possible to observe that, for both EDTA and citrate solutions, the etching of the glassy surface increases with the pH. In fact, the higher the pH the higher
the Si/Na and Si/Ca atomic concentration ratios as shown in the SEM-EDX images arranged in Fig. 16. Moreover, in alkaline environment, EDTA solutions seem to be more aggressive than the citrate ones. Of course in alkaline conditions the corrosion processes, caused by the nucleophic attack of the OH- ion on the silicon sites, can also happen and the glass surface is completely transformed into silica gel layers. Therefore, in using EDTA or citrate protocols for cleaning the surface of archaeological glasses alkaline environments have to be avoided in order not to damage the glassy surface with irrecoverable loss of material and irreversible transformation of the glass into silica gel [4]. However, Figs. 17 and 18 obviously reported the results of all chemical cleaning process carried out for both the glass fragments and complete objects respectively.
Fig. 17. Glass fragments before and after chemical cleaning with: (4) ethylenediaminetetraacetic acid (EDTA), (8) citric acid, (19) piranha solution.
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Fig. 18. Glass objects before and after chemical cleaning with: (A) ethylenediaminetetraacetic acid (EDTA), (B) citric acid, (C) piranha solution.
4. Conclusions In this work, the efficiency of some chemical agents on archaeological glass samples was studied. The pH environment was varied in order to verify the best conditions to avoid damage to the glassy surface. It was observed that all cleaning solutions were active in soften the solid deposits and encrustations but vary in their safety ratio on glass surface. However, cleaning by those solutions must be carried out within the scope of the deposits only, without prejudice to the glass surface to avoid the attack of unstable surface. EDTA and citric acid solutions are active in soften the solid deposits, and not etch the glassy surface at low pH values. Tartaric acid solution seems to be less aggressive than calgon (sodium hexametaphosphate) which has a tendency to damage corroded surfaces. From these observations we can suggest some easy rules when a piece of archaeological glass must be cleaned. Firstly, if not essential, it is not advisable to use chelating agents for cleaning corroded glass. The lime crusts can be removed by using a solution of sulphuric acid and hydrogen peroxide. This solution is also highly useful for destroying biological species and organic layers. Though the risk of contaminating glass with sulphate ions is extremely low because of the lack of porosity of glass material, caution must be taken since a strong acid such as hydrogen peroxide, which could oxidize metals-colorants of glass, and could provide change of colour in the object. In the case where a chelating agent must be used, for example to remove gypsum or salts not soluble in acid, a citrate solution can be used at a pH value lower than 7. A bad-cleaning procedure can result in a degradation of the glass
surface by the formation of a silica gel layer. The latter, most conveniently, a combination of different cleaning methods should be used in the conservation practice, after a preliminary, essential diagnostic study of the material nature and conservation state performed by physical and chemical investigations. Acknowledgement The author would hereby like to acknowledge the Archaeologists in the Department of Antiquities of Jordan for their constructive cooperation. Gratitude is also to Hamdi Mango Center for Scientific Research (HMCSR) team for their support among the JOCHERA project 2012. Thanks also are to the anonymous referees for their critical reviews and constructed suggestions. References [1] J. Cronyon, The elements of archaeological conservation, TJ Press, Cornwal, UK, 1990. [2] R. Abd-Allah, Z. Al-Muheiesn, S. Al-Howadi, Cleaning strategies of pottery objects excavated from Khirbet ed-Dharieh and Hayyan al-Mushref/Jordan: Four case studies, Mediterranean Archaeology and Archaeometry 10 (2) (2010) 97–110. [3] C. Altavilla, E. Ciliberto, S. LaDelafa, S. Panallero, A. Scandurra, The cleaning of early glasses: investigation about the reactivity of different chemical treatments on the surface of ancient glasses, Applied Physics A 92 (2008) 251–255. [4] M. Casaletto, G. Ingo, C. Riccucci, T. De Carro, G. Bultrini, I. Fragala, M. Leoni, Chemical cleaning of encrustations on archaeological ceramic artefacts found in different Italian sites, Applied Physics A 92 (2008) 35–42. [5] C. Sease, A conservation manual for the field archaeologist, 3rd edition, The University of California press, Los Angeles, 1994.
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