Reyhane Mirabootalebi, Conservation of A Highly Degraded Egyptian Limestone Stele, e-conservation Journal 3, 2015, pp. XX-XX
Available online 06 November 2015
Conservation of A Highly Degraded Egyptian Limestone Stele
Reyhane Mirabootalebi
Abstract
Through a study of the deterioration phenomena of ancient limestone artefacts in indoor environments, this research investigates viable conservation measures to retard deterioration of an ancient Egyptian limestone funerary stele, now at the University of Melbourne, Classics and Archaeology collection. This work discusses the results of analytical techniques including chemical spot test for identifying soluble salts, optical microscopy, environmental scanning electron microscopy with an attached X-ray energy dispersive system (ESEM-EDS) and powder X-ray diffraction(XRD)applied to identify the causes of rapid deterioration believed to have occurred mainly in an indoor environment. The paper also discusses various methods for the stabilization of fragile areas including using Japanese paper and using limestone fragments in combination with an adhesive. The challenges encountered during the treatment and the outcomes are also discussed. The results emphasize the significance of ongoing preventive conservation for safeguarding archaeological artefacts in indoor environments.
1. Introduction
The Classics and Archaeology collection of the University of Melbourne holds an ancient Egyptian funerary stele, known as the Oxyrrhynchus stele which is thought to date from the Ptolemaic period (332-330 BC) (Figure 1). The object was kept in storage facilities since its purchase in 1957, according to available documents. When examined in 2010, the stele was in extremely poor and unstable condition with extensive structural and surface losses, and it was assumed at that time that no effective conservation measures could salvage the artefact.
It is known that limestone archaeological artefacts can actively deteriorate indoors even though they were removed from the major destructive agents of their burial sites [1-4]. Studies have shown that the structural and surface deterioration that these artefacts demonstrate are due to a combination of intrinsic and extrinsic factors such as the limestone’s mineralogical and physical properties and fluctuations in relative humidity and temperature [1-4]. Often these artefacts are kept in storage areas without appropriate environmental control. The resulting damage may go unnoticed for long time and can be quite extensive and irreparable.
A conservation project was defined to gain an understanding of the causes of the rapid and extensive deterioration of the stele and to investigate viable conservation options. This paper presents the results of the analyses carried out to understand the causes of deterioration as well as the conservation interventions which were undertaken to stabilise the artefact's condition and to slow down the deterioration. Despite the challenges encountered during the long treatment process, the results show that the conservation measures have been successful in stabilising the object and greatly improving its structural and textural integrity.
2. Indoor Deterioration of Limestone
It is known that limestone can go through extensive and relatively rapid degradation while indoors when the stone is exposed to uncontrolled changes in relative humidity and temperature. Limestone artefacts in such conditions show similar patterns and degree of deterioration which commonly occur in a short time [1-4].
Research has shown that there are several major intrinsic and extrinsic destructive factors predisposing ancient limestone to deterioration indoors [1-8]. Some of the intrinsic factors include high concentration of soluble salts, high presence of clay minerals in stone composition, and petrographic characters of limestone such as high micro-porosity. These factors, once combined with adverse conditions of indoor environments, can cause irreversible damage to limestone.
Soluble salts are known to be the major destructive factor in porous materials like stone degradation. These salts are chlorides, sulphates, and nitrates of sodium, potassium, ammonium, magnesium and calcium. The typical surface and structural deterioration patterns attributed to salt behaviours in stone are surface scaling, deep cracking, micro-cracking, expansion, surface powdering, pitting and granular-disintegration [1, 2, 8, 9].
Soluble salts are known to damage porous materials through a range of mechanisms, including physical pressure caused by crystallization, hydration and differential thermal expansion [5, 10]. Salts also are known to contribute to swelling of clay contents of clay-rich stones accelerating deterioration process. The changes in relative humidity can cause salts to go through repeated cycles of dissolution and crystallisation. These effects can take two different forms: efflorescence, which is a harmless but unattractive deposit, occurs when the salts’ crystallisation is formed on the stone surface; if the precipitation of salts occurs beneath the surface, subflorescence is formed, which is highly damaging.
Other factors can contribute to salt-induced damage on porous materials. It has been found that the presence of a mixture of salts within a porous structure can intensify the damaging effects [1, 11]. According to Charola [1], the presence of a mixture of salts within a porous structure can influence the solubility of any individual salt in the mixture as a mixture of various salts has a range of equilibrium relative humidity at a given temperature instead of one. As a result, an increase in the cyclic crystallization/dissolution of the salts can intensify the damaging effects. Other intrinsic factors can also contribute to the salt deterioration process including the degree of porosity of stone [12]. The pore system within the stone structure provides an area where deterioration processes can occur. It has been found that the limestone with a high proportion of micro-porosity (pore size less than 5 µm) tend to be more affected by salt deterioration mechanisms than those with less large pores.
The presence of clays as compositional compounds in stones is recognised to be a principal contributor to the decay of stone indoors [3, 4, 6, 7, 13]. The clay components within limestone go through cycles of swelling and shrinkage as a result of wet/dry conditions caused by wild changes in relative humidity. This produces tensile stresses within the stone matrix sufficient to damage the internal structure of limestone. In addition, the presence of soluble salts is known to be contributing to the swelling of clay [3, 6].
3. The Oxyrrhynchus stele
The Oxyrrhynchus stele was purchased by the University in 1957 from the Metropolitan Museum of Art, New York. The accompanied documents suggested that the piece was acquired as a gift from the Egyptian Exploration Fund in 1897 through Flinders Petrie (1853-1942), an English Egyptologist and archaeologist during his investigations in Oxyrrhynchus in the modern city of el-Bahnasa, in 1896-1897 (letter from P.F Dorman to the Metropolitan Museum of Art, 16 July 1984).
The stele has a rectangular shape with a rounded-top, depicting low-relief inscriptions and imageries on the top surface. The surface is divided into four parts: there is a remain of a winged sun-disk on top, beneath which there is a line of text. In the lower part, the deceased stands before a table of offerings facing Anubis, the jackal-headed deity of the dead. On the floor beside the table, jugs of wine and beer can be seen. In the central part there are remains of three text lines. The bottom section depicts figures of two jackals facing each other with a Horus eye above each.
3.1. Conservation Condition
An examination in 2010 showed that the stele was in an extremely poor and unstable condition (Figure 1). A comparison between an early photo taken from the stele prior to 1957 at the Metropolitan Museum and the current state of the stele and suggested that a great deal of deterioration and losses had occurred since (Figure 2). According to the records, the stele was kept most of its post-excavation life in storage areas without appropriate environmental control (letter from C.A. Hope to the Melbourne University, 19 February 1990). This evidently had a major destructive impact resulting in extensive and irreparable losses to the surface and structure that occurred in a relatively short time period.
The stele exhibited various patterns of deterioration compatible with soluble salts activities and presence of clay minerals. The stone body was crumbling and scaling actively (Figures 3-4). This is known to be due to loss of inter-granular cohesion and poor particle cementation in limestone caused by destructive activities of soluble salts. Granular disintegration has resulted in extensive loss of surface decorations and structural losses. There were numerous cracks, mostly occurring parallel to the bedding plane, a phenomenon known to be due to swelling and shrinkage of clay components induced by relative humidity cycles, and thermal expansion and contraction of deteriorated stone caused by temperature cycles. The degree and depth of deterioration of the limestone suggested that the object might have been subject to water damage through desalination treatment by immersion method as part of previous conservation attempts. Whitish powdery deposits were apparent all over the surface (Figure 5).
Left to right:
Figure 1. The state of preservation of the Oxyrrhynchus, before treatment, 2010.
Figure 2. An early photo of the stele taken prior to 1957 at the Metropolitan Museum of Art. A comparison between Figures 1 and 2 shows an extensive damage had occurred during the stele's storage indoors.
Figure 3. Details of the condition of the upper part, showing scaling and crumbling led to significant structural and surface losses.
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3.2. Previous Restorations
According to the available records, the stele was subject to a number of restoration attempts. The major structural fracture across the middle was restored with a thick milky adhesive of unknown origin. The attachment was reinforced further by inserting a brass metal rod to hold the broken parts together (Figure 6). A thin grey-coloured surface coating has been applied all over the front surface and edges, probably as a hydrophobic protective layer, concealing a great deal of surface decoration.
Extensive presence of wax residues scattered all over the top surface and the edges suggested that wax was used most likely as a hydrophobic surface coating, consolidant or infilling material.
Left to right:
Figure 4. Details of the stele showing cracks, extensive scaling and the disintegration of the stele body, before treatment.
Figure 5. Details of the stele, showing cracks, losses and salt deposits on the surface.
Figure 6. Details of previous restorations procedures: a brass rod inserted for structural strengthening and wax residues applied on surface, before treatment, 2010.
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4. Experimental
The composition of limestone plays an important role in the material’s behaviour in deterioration processes. Therefore, the limestone characterisation could contribute to understanding the stele’s causes of deterioration. The techniques used in this research were chemical spot test for soluble salts, optical microscopy, environmental scanning electron microscopy with an attached X-ray energy dispersive system (ESEM-EDS) and powder X-ray diffraction (XRD). The required samples were collected from abundant detached insignificant pieces.
Chemical spot tests were performed to investigate the presence of soluble salts including chlorides, sulphates and nitrates following the methods described by Odegaard [14].
A cross-sectioned sample was prepared by embedding a small limestone sample in resin (Cray Valley Clear Casting Resin, Catalyst: Methyl ethyl ketone peroxide) and then polished. The observation of the cross-section for direct observation of the material’s porosity, grain morphology and micro-cracks was performed using an Olympus BX51 microscope equipped with an Olympus DP70 digital camera.
Electron microscopy was used to analyse the morphology of the stone and to identify its elements. ESEM-EDS analyses were carried out using a Phillips (FEI) XL30 ESEM TMP coupled with EDS, an Oxford INCA-300. The analyses were carried out at an accelerating voltage of 20 kV operating in a pressure of 0.7 Torr, in BSE mode.
The sample for XRD analyses (~1g) was grounded in a mortar and pestle and then analysed using a Bruker D8 Advance diffractometer. The phases were identified using the MDI Jade software package and the PDF-4 Minerals 2010 version of the Powder Diffraction File.
Electron microscopy was used to analyse the morphology of the stone and help to detect chemical composition of its elements. ESEM-EDS analyses were carried out using a Phillips (FEI) XL30 ESEM TMP coupled with EDS, an Oxford INCA-300.The analyses were carried out at an accelerating voltage of 20 kV operating in a pressure of 0.7 Torr, in BSE mode.
5. Results and Discussion
5.1. Chemical spot tests results
The results of chemical spot tests showed that all samples were positive for chloride and negative for nitrates and sulphates salts.
5.2. Optical Microscopy
The Optical microscopy revealed details such as porosity, grain morphology and micro-cracks. As illustrated in Figure 7, the grains formed a powdery non-solid fragmentary matrix which is non-uniform in colour, with a considerable amount of microcracks.
5.3. ESEM-EDS analysis
The back scattered electron (BSE) images (Figures 8, 10, 12) show details of the surface morphology, particularly of small round white particles. Elemental analysis detected the presence of calcium (Ca), carbon (C) and oxygen (O) in most particles suggesting the particles may be calcium carbonate (Figure 9). The particles were non-uniformly distributed within the matrix. EDS analysis also detected high levels of chloride (Cl) and sodium (Na), which may indicate the presence of sodium chloride (NaCl) (Figures 12). Also, silicon (Si) and aluminium (Al) were detected which indicate the presence of aluminosilicate minerals (clay), present in forms of fluffy areas between the white particles. The ESEM images also revealed the presence of fossil shells and mollusc fragments distributed throughout the matrix, pointing to the origins of limestone (Figures 8, 10, 13).
Left to right:
Figure 7. Cross section sample of limestone.
Figure 8. Microphotograph image of the limestone sample shown in Figure 7.
Figure 9. The presence of calcium in the white areas of Figure 8 suggests the presence of calcite.
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Left to right:
Figure 10. Microphotograph image of a limestone sample.
Figure 11. The presence of calcium and silicon suggests the presence of clay particles in Figure 10.
Figure 12. The presence of chlorine (Cl) and sodium (Na) in a small spot in Figure 10 shown as white suggests it is sodium chloride.
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5.4. XRD analysis
XRD results show that calcite (CaCO3) is a major component of the limestone, with gypsum (CaSO4.2H2O) and quartz (SiO2) as minor components (Figure 14). Unexpectedly, XRD failed to identify the presence of aluminosilicates or clay minerals. This was incompatible with the results achieved by ESEM-EDS analyses, as the latter could detect aluminosilicates within the polished sample. Nevertheless, this could be due to the presence of clay compounds in an amount below the detection level of the instrument in the sample (around 5%).
XRD also confirmed the presence of soluble salts in a small amount consisting only of halite (NaCl). The presence of chloride salts in a small amount (~0.1% w/w) is known to be sufficient to render limestone prone to degradation [3, 6].
6. Conservation Plan
The aim of the conservation plan for the stele was to stabilize the object by reducing the impact of environmental destructive agents and to prevent or slow down further damage. The results of the analyses have shown that the stele had a high concentration of soluble salts. Studies show that objects with high content of soluble salts continue to be affected by destructive salt activities even though they are kept in a controlled environment [1, 8, 16]. To tackle this problem, it was essential to reduce the salt content. Although the only viable option was to undertake a water-based technique or poulticing [9, 17-20], the stele was in an advanced state of disintegration and it was crucial to improve the object’s structural integrity prior to any water-based treatment to minimize the risk of damage from direct contact with water.
The challenge thus was to find a consolidant which, in addition to its desired properties (such as chemical and light stability, causing surface minimal impact, and re-treatability), would have high penetration depth, good adhesion to the limestone and would allow water-based treatments to be performed. Research in the conservation literature suggested that, for limestone, none of the available consolidants could meet all the above requirements [20-24].
There is, however, one group of materials known as alkoxysilanes (or silanes) which seem to be most suitable due to their depth of penetration, re-treatability and hydrophobicity [25-29]. The main limitation of using alkoxysilanes on any calcareous stone is its poor chemical binding. The bond it creates within the stone matrix is mainly physical as the consolidant penetrates irregularities in the surface of the grains. Although the physical bond may be strong, it is not as strong as a chemical bond [5, 9]. Nonetheless, alkoxysilanes have been used for consolidation of highly deteriorated limestone and other calcareous artefacts and successful outcomes have been reported [9, 10, 27, 28, 30, 31].
A conservation plan was then established as follows: in-depth consolidation to improve structural integrity, salt-reduction through poulticing, and optimization of the environmental storage and display conditions. It had to be borne in mind that conservation of highly deteriorated stone materials remains problematic, since objects in poor condition have lost structural and textual integrity, and most standard conservation interventions yield less effective results, or can cause further irreversible damage [4, 9, 23, 29]. The following sections describe the treatment steps and the challenges encountered.
6.1. Surface Cleaning
The stele’s surface was extensively obscured with detached fragments, past restoration materials and residues, dust and dirt. The removal or reduction of the previous surface coating and wax residues was desirable to assist with enhancing the effects of new treatments, as the exposed limestone has a higher surface attraction for new conservation materials than a coated surface [23]. In addition, this could enhance the surface visibility and reveal details of surface decorations which were covered with coatings applied in previous restorations.
The front surface was divided into a number of sections using a grid system. Each section was cleaned by removing loose debris and dislodged pieces with tweezers and soft brushes. These pieces were collected in glass containers allocated to each grid square. These pieces were very fragile and had to be carefully handled. While most pieces could be allocated to their original locations, some were damaged beyond recognition. The wax residues were reduced mechanically with a scalpel and dental pick.
6.2. Pre-consolidation
Pre-consolidation of fragile areas on the top surface was essential prior to reduction of previous surface coatings through wet cleaning. Pre-consolidation was performed through local application of a solution of Paraloid B72 (5% to 10% w/v in acetone, or in acetone and ethanol 1:1). Paraloid B72 was chosen mostly because of its good adhesion property and its compatibility with tetraethoxysilane (an alkoxysilane that was planned to be used for in-depth consolidation) as tetraethoxysilane is immiscible in acrylic resins [28].
6.3. Reduction of previous coatings
The result of the solubility tests on the old grey coating showed that it was soluble in ethanol. The wet-cleaning technique involved gentle rolling of cotton swabs dampened with ethanol (100%) on the sound surface areas. Alternatively, poultices of cotton wool dampened with ethanol were placed onto surface areas and covered with plastic sheets for several minutes to prolong solvent exposure. The outcome was satisfactory since it revealed decoration details that had been hidden under the coating (Figure 15).
Left to right:
Figure 13. Microphotograph image showing a mollusk fragment.
Figure 14. XRD difractograms of limestone samples.
Figure 15. Details of the condition of the lower part of stele before and after wet cleaning. These images show the reduction of surface coat applied in previous restoration improved the surface visibility, during treatment, 2011.
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6.4. Full-consolidation
Tetraethoxysilane (TEOS) from the silane family was chosen for full-consolidation treatment of the stele due to its hydrophobicity, allowing water-treatment to be performed subsequently. SilresBS OH100, known as WackerOH100, is a commercial stone strengthener based on partially polymerized TEOS, with a neutral catalyst (MSDS 2008).
The application method involved preparation of the front surface by drying the area using acetone since the presence of water can negatively affect the polymerisation process [5, 28]. It was decided to use the consolidant in a concentrated form to impart greater quantity of the material into the degraded stone. Disposable micropipette was employed to apply the consolidant as it provided good control during the process. It is known that the application schedule can influence the quantity of silane absorbed by the stone [28]. The applications were grouped in three, referred to as cycles. There was a 10 minutes interval between applications in each cycle, and 30 minutes to one hour intervals between cycles. In order to control the application schedule, the front surface was divided into five sections, and the time of each application was recorded.
The penetration of the consolidant into the stone was visible from the side edges as the wetness lines were slowly going down during the application process. About 1.6 kg (approximately 4 litters) was consumed.
It should be noted that the detached pieces that could be located were subject to full-consolidation with silane. Once the application was completed, the excess material on the surface was removed with tissues and dabbing gently with cloths. Relative humidity and temperature were measured at 53% and 20°C, which were well in the optimum ranges required for the silane to harden successfully (RH>40% and 10°C<T<20°C according to the material MSDS, 2008). The object was left for seven weeks for the polymerisation to be complete.
6.5. Salt-reduction treatment
Salt-reduction treatment was performed in week eight, one week after the silane polymerization process was complete. At this stage, the object appeared to be structurally strong enough to withstand water-based treatment (although the structure integrity was improved significantly, total immersion was to be avoided since it was too aggressive). For salt-reduction techniques, poulticing method was used though only partial desalination can be achieved by this technique. The objective was to reduce the salt content as much as possible. Therefore, in order to inhibit the damaging effects of the remaining salts, it was crucial to implement environmental control measures for storage/display.
For the poultice material, blotting paper pulp (Canson, medium thickness) was used because of its good absorbing qualities [2]. The process of making paper pulp involved first soaking shredded blotting paper in water overnight before making into pulp using a blender.
The surface was moistened slightly for better adhesion of poultice to the surface, and then covered with a layer of Hollytex as a barrier layer. The surface was divided into ten sections using grid system in order to be able to measure and compare the amounts of salt extracted in various locations. Wet poultices were placed by hand on the top surface in about 1cm thickness, and they were replaced with new ones once dried (Figure 16).
To monitor salt-reduction rate, a sample of dried poultice (10×10cm) was removed from each section, put individually in 500mL distilled water and the conductivity of the dispersion was measured using a Labchem conductivity meter (results illustrated in Table I). The average conductivity measurement was monitored for the magnitude of change. Although a few sections demonstrated irregular behaviour, the average salt extracted in each application showed a decreasing trend (Figure 17). Only a slight increase was observed in the amount of salt extracted in the fifth application because the poultice remained for a longer duration. The process was stopped after the sixth application, since the average conductivity was no longer changed dramatically; meaning a negligible amount of salts was removed [4, 5, 9, 19].
Those detached fragments which could be relocated were desalinated by immersion. They were placed into a plastic sieve and immersed in a container filled with deionised water. The water was changed every 24 hours for three times when the measured conductivity reduced considerably. Although the process could continue to remove more salts, it was decided to stop to prevent potential damage to the pieces due to lengthy soaking in water.
Left to right:
Figure 16. Desalination treatment by poulticing techniques using blotting paper as salt absorbing material, during Treatment, 2011.
Figure 17. The diagram shows the average conductivity measurements during the salt-reduction treatment step.
Figure 18. Infilling of surface losses using Japanese paper with methyl cellulose, before and after treatment 2011.
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6.5.1. Re-appearance of Salts
Once the treatment was completed and the object was left to dry in an air-conditioned environment, salt crystallisation reappeared on a few areas on the surface, either as white thin fluffy deposits or as crystalline encrustations, predominantly on the top surface where salt extraction measures were relatively high in the last application. This phenomenon was due to the drying process and indicated that the salts were still present. As water moved towards the surface to evaporate, the salt was drawn to the surface and precipitated. Salt deposits were loosely attached to the surface and could be removed easily by dental picks and brush vacuuming. This phenomenon was observed to reoccur over the next few days till the object was completely dry.
6.6. Re-adhesion of detached fragments
While some detached pieces were damaged beyond recognition, many pieces could be located successfully using the early photograph of the stele before any major damage occurred.
The majority of the detached pieces had lost their body volume and required infilling to increase their bulk before being re-adhered back to their location. For infilling/bulking materials, it was decided to make use of insignificant limestone debris and powdery deposits that had been collected from the stele during cleaning treatment. These fragments did not have any indication to suggest being part of the stele’s surface. By using these insignificant pieces as infilling materials, the pieces have become part of the object again, and less foreign materials were introduced to the stele. It should be noted that only a small proportion of the detached pieces was used for this purpose, and a large amount of these fragments has been kept uncontaminated for future research and/or use.
Several bulking materials and adhesives were tested for infilling and re-attaching the detached pieces, including various combinations of marble flour and glass micro-balloon with limestone powder in Paraloid B72 solution in acetone [32]. The outcome of the trial showed that a mixture of limestone powder and glass micro-balloons with Paraloid B72 solution in acetone was the most appropriate, due to moderate strength of the fill, and material and appearance compatibility with limestone.
Limestone powder was prepared by grinding fragments after desalination. A thick paste was made by mixing limestone powder, glass micro-balloons (1:1) with a solution of Paraloid B72 in acetone (40% w/v) in a small glass jar. The fill was applied partly on the back of fragments and partly on their locations by using a fine spatula after the areas treated with a 10% solution of Paraloid B72 in acetone (w/v) to improve adhesion. To minimise shrinkage, the infilling was applied in layers and in several stages.
6.7. Use of Japanese Papers for Stabilizing Powdery Surface Areas
An experiment was undertaken with Japanese tissue and methyl cellulose adhesive (in water) to stabilise powdery surface areas. Most of the affected surface were on areas where the top had been lost and underlying stone became powdery again after water treatment
Japanese tissue was used in two different methods according to surface condition. First, it was made into a paste using a thick mixture of methyl cellulose (7% w/v in water) to fill the small holes/gaps on the surface. To reduce shrinkage upon drying, filling was performed in several stages. In the second technique, thin Japanese tissue strips with methyl cellulose solution were applied on crumbling and powdery surfaces. Long fibrous structure of Japanese tissue was acting as a netting to hold onto the powdery areas and to encapsulate and stabilise crumbling effect. Due to translucency of Japanese tissues, no tinting was required and the treatment was discrete (Figures 18-19).
6.8. Overview of conservation outcomes
The main objective of the conservation attempt during the entire treatment process was to slower the degradation mechanisms and to improve the structural and surface integrity. Any infilling or loss compensation was to be avoided unless it was essential for stability of the stele. The techniques and materials used in this conservation treatment proved to be very effective in stabilising fragile surface areas and improving the artefact’s structural and surface integrity.
It was noticed that during the full-consolidation treatment stage, the silane could penetrate into the old surface coating in the areas where it had not been removed completely in the previous treatment stage. Interestingly, the removal of the old coating was still possible using ethanol once silane was hardened. This indicated that wet-cleaning could have been performed after full-consolidation when the surface was strong enough to stand the treatment. In addition, silane slightly solubilised the wax residue facilitating its mechanical removal.
The consolidation treatment was successful in improving the stele’s structural and textural integrity. Signs of the structural and surface instability such as crumbling and powdering that were evident before the treatment were noticeably reduced. In addition, consolidation had minimal impact on the appearance. The darkening effect that had been observed at the beginning was gradually reduced over a few weeks. No tidemarks or glossiness were observed on the stone surface.
Despite the water-repellence effect that the consolidated surface showed at the beginning of the process, the treatment was successful in removing considerable amounts of salts. The results showed that contrary to what was stated by Scherer [5], the temporary hydrophobicity of the consolidated surface does not prevent the removal of salts. Nevertheless, it was observed that the water-treatment had affected some of the previously stabilized areas as they become slightly unstable and minor crumbling was observed on surface.
The materials and techniques used for re-adhering the detached pieces were effective in improving the structural and surface integrity of the stele, as well as aesthetic. For instance, the use of Japanese tissue has proved to be an excellent versatile material for stabilising superficial powdering. It was due to good adhesion property of its long fibres, subtle visual appearance, easy workability and incredible strength.
The stele has retrieved its structural and textural strength to a great degree; however, it seems still vulnerable to the damaging activities of the remaining salts which can be induced by fluctuations in humidity and temperature in an uncontrolled environment (Figures 20-21).For future preservation and to arrest deterioration mechanisms as much as possible, it is recommended to keep the artefact in a controlled environment, for instance inside a micro-environment, and maintaining constant relative humidity at 45% ±0.2 and temperature at 19°C±1 [2].
Left to right:
Figure 19. Details of repair a loss, using Japanese paper, before and after Treatment, 2011.
Figure 20. Detail of the preservation state of the stele top right after treatment, 2011.
Figure 21. General aspect of the preservation state of the Oxyrrhynchus stele, after treatment, 2011.
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7. Conclusion
Visual examination of the stele and its degradation patterns suggested a high level of salt activities demonstrated through patterns such as granular disintegration, crumbling/powdering, cracks, and presence of salts crystals visible with naked eyes.
The initial view has been confirmed analytically. The results of chemical spot tests, Optical microscopy, ESEM-EDS and XRD showed that the stele contains soluble salts and aluminosilicate compounds. In addition, factors such as a high content of calcite, as shown by XRD results, and morphological features such as the presence of micro-cracks in stone matrix and non-solid and fragmentary distribution of what is thought to be calcite particles could render stone more prone to internal disintegration caused by salt activities.
The results of this research emphasize the importance of ongoing preventive conservation in safeguarding archaeological artefacts as they inherently are vulnerable to deterioration processes induced by inappropriate environmental conditions. The artefact of this case study was a representative example of the extent and degree of damage that can occur to ancient limestone objects as a result of a combination of internal factors and poor environmental conditions in a short period of time.
Conservation of highly deteriorating limestone artefacts is problematic and available conservation materials and techniques have serious limitations, partly due to a presence of gap in stone conservation science and partly due to the fact that an advanced state of deterioration may render treatments less effective. However, the results of this research show that, for the stele, the benefits of undertaking conservation interventions well outweigh possible negative or ineffective consequences. The conservation materials and techniques used in this project have some acknowledged drawbacks. For instance, the salt-reduction treatment through poulticing technique was successful in reducing salt content significantly; however, the introduction of water caused some fragile surface areas to become crumbly again even though they had been stabilised at an earlier stage. Nevertheless, despite these effects, the treatments were reasonably effective in improving structural integrity and, as a result, the stele is now in considerably better condition.
Moreover, maintaining adequate storage and display conditions is vital for the long-term preservation of this artefact. The stele is still very vulnerable to destructive agents, particularly soluble salts that can be activated by fluctuations in temperature and relative humidity of an uncontrolled environment.
8. Acknowledgments
I’d like to thank Ms. Marcelle Scott and Ms. Holly Jones-Amin for their kind support and invaluable advice.
9. References
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Reyhane Mirabootalebi
Objects and textiles conservator
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Reyhane earned her Master degree in conservation of cultural materials from the University of Melbourne in 2011. She has worked in a number of institutions including the Centre for Cultural Materials Conservation the University of Melbourne, the National Museum of Australia and Heritage Conservation centre, Singapore. Her area of interest in conservation is the arts and cultural materials from the Near East and Central Asia. Reyhane currently works as an objects and textiles conservator in the Centre for Cultural materials Conservation, the University of Melbourne.
