Moisture Adsorption Mechanism of Earthen Plaster Containing Soluble Salts in the Mogao Grottoes of China

ABSTRACT

In the Mogao Grottoes of Dunhuang, China, the ambient relative humidity (RH) fluctuation in the caves is the main cause for the deterioration that has been observed, especially the deterioration of the earthen plaster and wall paintings linked to the presence of NaCl and Na₂SO₄ salt contaminants. To help identify the mechanism responsible for this salt-related deterioration and provide a theoretical basis for the development of a preventive conservation regime, this study addressed the basic moisture adsorption mechanisms of earthen plaster with soluble salts by taking an experimental approach. It is shown that, as ambient RH increases, the moisture adsorption process of the plaster containing the soluble salts can be regarded to have three basic stages, as evidenced by the results of the moisture adsorption tests performed on earthen plaster samples with different NaCl or Na₂SO₄ contents at various humidity levels. The moisture adsorption mechanism and its determining factors at each stage are also analysed. In addition, it is suggested that the hygroscopicity of the plaster structure and the interaction between the different kinds of salts in the plaster, which makes the dissolution or hydration of the salt in the plaster pores much easier, should be considered more seriously regarding increases in the ambient RH in the caves.     

Conservation and restoration of five coffered wooden ceilings

Abstract

The Palacio del Almirante in Cuzco, Peru, was badly damaged by an earthquake in 1950. Restoration work was interrupted due to lack of funds and various decorative elements, including the coffered wooden ceilings which had been dismantled, were dispersed. When conservation work resumed in 1975, the pieces had to be located and reassembled, with the help of photographs taken before the earthquake. The separate pieces were cleaned, disinfected and consolidated, missing pieces were replaced and the polychromy was reintegrated. Since the original beams could no longer support the weight of the ceilings, a system was devised whereby the ceilings were suspended from above by iron bars.     

Investigation of acetic acid bacteria isolated from the Kitora tumulus in Japan and their involvement in the deterioration of the plaster of the mural paintings

Abstract

The Kitora tumulus, which is thought to have been built around the late seventh to early eighth century, has beautiful mural paintings which were executed directly on a very thin layer of plaster in the stone chamber. When the paintings were found, the plaster was partly detached from the stone wall and the murals were therefore considered to be likely to fall off the wall with a casual touch or stimulation. Therefore, it was decided to detach the mural paintings and to store them flat and under controlled environmental conditions. This was initiated immediately after the excavation of the tumulus in 2004. However, fungal growth and biofilm development were observed within the stone chamber during the relocation work. In 2005, small holes containing black substances were observed on certain areas of the ceiling plaster, and following investigation an acetic acid bacterium, Gluconacetobacter sp., was isolated from the black substances. The bacterium was also isolated from the ceiling, floor, and east wall in the stone chamber in 2008 after the relocation of most of the paintings had been completed. These bacteria were shown to decompose calcium carbonate (CaCO₃), one of the primary components of the plaster, and to produce organic acids such as acetic acid. Additionally, they were observed to decrease the pH of the culture media significantly in the presence of ethanol and glucose. This is the first example of the characterization of acetic acid bacteria isolated from decayed plaster paintings, and it is likely that microbes such as these bacteria have been involved in the deterioration of the plaster. Chemicals to treat microbes in the Kitora tumulus during the relocation work were selected on the basis of their antimicrobial efficacy, low potential to cause adverse effects on the paintings, and low level of toxicity to humans, depending on the condition of the plaster or stone in each area. However, some chemicals, especially ethanol, may act as a carbon source, which could encourage the growth of microbes and thereby the production of acids by the microbes when diluted to a low concentration or in a degraded state. Moreover, prior contamination by other microbial species in the form of a biofilm could also encourage the growth of the acetic acid bacteria by providing low-molecular-weight organic materials as a nutrient source.     

Mestre Piranga - study and conservation of Brazilian religious sculptures from the eighteenth century

Abstract

Known today only by the name of the region where he lived, Mestre Pirang-a, an eighteenth-century santeiro (saint-maker), is considered a great artist by afficionados of Brazilian sacred art. Part of the present work consisted in a survey and cataloguing of the surviving religious imagery in the region of the Piranga river valley. A considerable number of these images could be attributed to the master. Three sculptures were analyzed with regard to form, style, iconography and technique of execution (support and polychromy), using scientific methods. Their state of preservation was investigated and a proposal for conservation was drawn up.     

The Matriz of Catas Altas,Minas Gerais,Brazil: techniques,materials and style

Abstract

The region of gold deposits in Brazil was discovered by pioneers at the end of the seventeenth century. The richness and the demographic explosion of the new General Mines region (Minas Gerais) gave rise to strong Portuguese political control: religious orders and missionary activities were forbidden there. Consequently, Minas Gerais saw the development of Third Orders and brotherhoods of laymen. The competition between the various religious groups prompted the construction of many churches, the richness of their interior decoration based on the baroque aesthetic. The mother church of Nos sa Senhora da Conceição in Catas Altas is a magnificent example of the contribution of both the Portuguese and the indigenous people to the construction of artistic monuments. The church dates from 1738 and the architectural construction is complete but the internal decoration remains unfinished. This made it possible to study the techniques and materials used. In the areas where the ground has been applied but no polychromy, there is a first layer of kaolin followed by a gesso layer. This unusual technique may have been prompted by economy: kaolin is a mineral easily found in the metallurgical zone of Minas Gerais.     

THE CONSERVATION AND TECHNIQUE OF THE HERLIN ALTARPIECE (1466)

Abstract

The restoration of the Hedin altar in Rothenburg o.d.T., dated 1466, is described. The polychromed sculptures were cleaned and loose paint was fixed. Methods and materials used in the restoration of the paintings, the areas polychromed in various techniques and the brocade reliefs are mentioned. The altar was protected from direct sunlight. The construction of the shrine and the technique of carving are extensively discussed. Sculptures made from one piece of lime wood were hollowed out and closed with a plank. Traces of rough finishing are visible. Shrine and predella were polychromed after a preliminary assembly of the sculptures. The various techniques used in the undisfigured polychromy are described as they are observed with the unaided eye. It can be shown that the working methods were rational. The fabrication of relief brocades and the various patterns used are discussed in detail. The treatment of the various paintings on the wings, the predella and the reverse of the shrine is reported. The question is raised to what extent the painter Herlin can also have been responsible for the polychromy.     

The conservation and restoration of a processional image: The Christ of the Expiration

Abstract

This image is the work of the sixteenth-century sculptor Marcos Cabrera and belongs to the Brotherhood of the Expiration in Seville. It was made to be carried in procession during the Holy Week celebrations. When the statue came to the ICRBC in Madrid for treatment it presented serious problems due to the unusual material of which it is made — polychromed paper-pulp — and the unfortunate treatments to which it has been subjected. Metal strengthening rods had been inserted in the figure and the cross, as well as a polyester consolidating material which had caused further damage. The polychromy was covered with a thick layer of paint which had plasticized. A complete scientific analysis was carried out by an interdisciplinary team both before and during the treatment which had the aim of stabilizing the sculpture to enable it to fulfil its ritual function.     

A Multi Proxy Investigation of Moisture,Salt, and Weathering Dynamics on a Historic Urban Boundary Wall in Oxford,UK

ABSTRACT

Boundary walls are neglected but important parts of historic urban environments, and they are often prone to serious deterioration. Understanding moisture and salt dynamics within boundary walls can help infer the causes and dynamics of deterioration. This investigation investigates the patterns of moisture, salt, and deterioration on a 300 year old limestone boundary wall in Worcester College, Oxford. Multiple methods to assess moisture and salts within and across the wall (electrical resistivity tomography, handheld resistivity-based moisture meter, paper pulp poultices, scanning electron microscopy, inductively coupled plasma optical emission spectrometry, and ion chromatography of small scale sampling of deteriorated limestone) were used in conjunction with decay mapping of both sides of the wall. The salt weathering strongly correlates with severely weathered zones at the wall and salts, mainly sulfates, seem to be the main agent of decay processes. The combined results demonstrate that the environmental influences driving stone decay can differ on a very small scale even at a comparatively simple structure like a boundary wall, and that repairs can have adverse effects if the patterns of salt and moisture dynamics are not sufficiently known.     

Scanning electron microscope hypsometry as a tool for evaluating the impact of surface treatments: A case study on sub-fossil bone

Abstract

Plaster of Paris field jackets are commonly employed for lifting fragile archeological and paleontological material during excavation. When the plaster has been applied directly to the object surface its removal poses a severe risk of damage and loss. This study documents the development of an experimental method to test the suitability of various techniques for removing plaster of Paris from sub-fossil bone. The use of digital microscopic elevation models, already in use in other disciplines, was tested as a method for quantifying surface loss resulting from conservation treatments. The study concludes that citric acid is not suitable for use in close proximity to sub-fossil bone. Air abrasion and laser ablation proved extremely damaging, but ultrasonic cleaning caused no detectable damage. This study has relevance to the removal of plaster of Paris from other materials, such as carbonaceous statuary. The analytical technique itself has a far wider application and is recommended for evaluation of all surface treatments that have the potential to cause loss on a microscopic scale.     

Technique and restoration of three sculptures by Pedro de Mena

Abstract

This paper describes the examination and treatment of three polychrome wooden sculptures by Pedro de Mena, an important seventeenth-century Spanish sculptor. These were three interesting cases, involving not only the usual conservation processes but also special situations such as the recovery of a glass eye, the replacement of missing parts and polychromy with the help of old photographs, and the analysis of a sculpture where the flesh colours varied depending on whether or not they had been exposed to sunlight. Our understanding of polychrome sculpture technique is enriched with new information concerning the blues of the cloaks, coloured glazes, and materials added to the paint to achieve different effects.     

Technological Study of the Decoration on an Erard Harp from 1818

ABSTRACT

Historical pedal harps from the early nineteenth century are highly adorned objects, decorated with various techniques such as polychromy, gilding, decoupage, and gilt composition ornaments. Even though a large number of such instruments are kept in private and public collections, until now their decorative features have not been examined systematically. Moreover, little has been written about the motives behind the choice of various decorative techniques by harp makers. This article presents results from the investigation of a double-action harp made in 1818 by Erard, the leading manufacturer of harps in Regency London, focusing on the technological aspects of its decorative features. Various analytical methods, such as visual examination, microscopy, SEM-EDX, FTIR, and Py-GC-MS, were used to identify the inorganic and organic components in the coating of the harp and to analyse its stratigraphy. Inspection with ultraviolet light revealed details of the original layers and of later additions, while radiography made the internal structure as well as damaged areas and repairs visible. The outcome of this object-based examination was complemented by archival research on the Erard firm, providing new insight into Erard’s production and marketing strategies.     

Examination and treatment of a seventeenth-century Spanish polychrome sculpture by Jose Caro

Abstract

The sculpture of St Gines de la Jara in the collection of the J. Paul Getty Museum, dated 1692, is an outstanding example of the estofado technique of imitating brocade in gold leaf and paint. The life-size figure retains most of its original polychromy. In previous restoration treatments, there had been an unsuccessful attempt to simulate the estofado. The present treatment included a comprehensive technical examination, using X-radiography to identify construction techniques and cross-sectional analysis for the layer structure and materials. The decoration of the sculpture was carefully studied with the goal of understanding more clearly the technique of the estofador. A cleaning strategy was then devised. Where there were losses in the estofado, it was decided to aim for a chromatic compensation using watercolours, rather than attempting to duplicate the original process.     

Technique and treatment of the St Michael Weighing Souls in the Royal Convent of Santa Clara,Toledo, Spain

Abstract

The history, materials and technique of an early fifteenth-century wall painting, St Michael Weighing Souls, are described. The pigments are typical of mediaeval Spain but the methods by which they were applied show a sophisticated understanding of their optical properties. Complex mechanisms of deterioration in some areas of colour are discussed, together with the rationale, materials and methods used in the conservation treatment.     

Study of the deterioration products,gilding, and polychromy of the stones of the Scuola Grande Di San Marco's façade in Venice

The Scuola Grande di San Marco is one of the most outstanding Renaissance buildings in Venice. Destroyed by fire in 1485, the Scuola was reconstructed between 1487 and 1495 by Pietro Lombardi (and sons), together with Giovanni Buora and Mauro Codussi. The façade is considered one of the finest and most original exteriors of Venetian buildings and in particular features refined sculptural and painted decorations and a remarkable pictorial effect created by the extensive use of polychrome marbles and the false perspective view between the pilasters of the lower order. Most of the stone materials (Istrian and Verona red limestones, nero di Roveré, pavonazzetto toscano, Carrara marble and various reused antique marbles) present evident decay phenomena such as scaling, exfoliation, powdering, fissuring, salt efflorescence, black crusts, and artificial patinas of various kinds. The present study reports the results of petrographic and chemical laboratory analyses carried out in preparation for the conservation treatment of the building, in order to characterize the main micro- and macro-morphologies of decay and to identify the causes and mechanisms of deterioration as well as the nature and application technique of the polychromy, gilding, and patinas present on parts of the monumental Renaissance façade.     

A DESIGN FOR LARGE PLASTER PIECE-MOULDS MADE IN THE FIELD

Abstract

An improved technique for the accurate reassembly of large plaster piece-moulds made in the field is described. The proposed method relies on the construction, within the main mould, of supporting detachable plaster arches which are accurately surveyed and planned in situ and subsequently easily reassembled prior to the making of the final cast.     

Diagnosing Decay Mechanisms at the Porta Nocera Necropolis,Pompeii: The First Step Towards Effective Preventive Conservation

ABSTRACT

Aspects of preventive conservation are central to the long-term preservation of archaeological sites such as Pompeii. Mitigating moisture ingress and its manifold effects on decay mechanisms such as salt degradation are of critical importance when planning durable conservation strategies. A suite of non-destructive techniques was used to diagnose decay mechanisms at an ancient tomb monument at the Porta Nocera Necropolis. Rising damp and salt contamination are shown to be actively contributing to on-going deterioration of the plaster surfaces. An existing shelter is shown to be inadequate in its protection. These initial results are used to inform a more in-depth, long-term survey as well as the design of a holistic preventive strategy.     

Evaluation of disinfection techniques for the conservation of polychrome sculpture in Iberian museums

Abstract

Insect infestation is a serious problem in museum collections in Spain and Latin America. This paper investigates the efficacy of modified atmospheres as an anoxant for museum pests, using wooden objects including polychrome sculptures infested by Cerambycidae and Anobiidae species. Insects of long life cycle have been found to be very resistant to carbon dioxide atmospheres. However, inert gases such as argon and nitrogen used in plastic barrier bags effectively eliminated insects in all stages of development. No chemical alteration was observed in the treated materials, including polychromy. These treatments are in progress in a project for biodeterioration control in countries with both tropical or subtropical and Mediterranean climates.     

Oryza sativa L. (Rice) in the Ancient Earthen Plasters of Painted Fragments from Bezeklik,China

ABSTRACT

This research reports for the first time the inclusion of Oryza sativa straw, root, and husk as a vegetal additive in the fifth century CE decorative mud plaster of the Buddhist cave of Bezeklik. The investigation is based on the morphological, anatomical, and silica bodies study of the vegetal additives accomplished through a light microscope, stereomicroscope, Fourier-transform infrared spectrophotometer, and scanning electron microscope from the isolated fragments of the painted plaster, transported to India in the nineteenth century and presently housed in National Museum, New Delhi. Studies indicate that loamy sand soil sourced locally for the earthen plaster was mixed with paddy straw in the proportion of 25–30% by volume for cohesion and strength. The excess of sand and absence of clay-size particles in plaster is, however, causing disaggregation of the grains due to lack of cohesion. As locally available materials are preferably used for any plaster works, this study also raises the question – was ancient Bezeklik, now a desert of Northwest China, once a rice-growing area in addition to the crops of Triticum aestivum (wheat), Setaria italica (Foxtail millet), and Hordeum vulgare (barley) reported by many researchers, based on hypothetical environment recreation. This study will help prepare a compatible plaster for the restoration of fragile-painted plaster fragments.     

Problems of adhesion between support and ground layers in polychrome wooden sculptures in Guatemala

Abstract

Bibliographical research and scientific analysis of samples obtained from five polychrome wooden sculptures of the colonial tradition in Guatemala were used to study the techniques and the materials used. The methodology involved qualitative analysis of the samples and microscopic examination of cross-sections. Scanning electron microscopy (SEM) was used to identify pigments and to ascertain the composition of the gilding. It was hypothesized that the deterioration of the sculptures, particularly the separation of the ground layers from the wooden support, was due to the action of secondary products of Cedrela odorata L. The loss of adhesion of the animal glue used in the preparation may be partly related to the precipitation of the protein of the gelatin due to the action of the tannins present in the wood.     

A Non-Destructive Analytical Study of a Recently Discovered Roman Wall Painting

Summary

A Roman wall painting of the first century AD, recently discovered in Rome, has been studied extensively, in situ, by non-destructive X-ray fluorescence (XRF) analysis. The coloured pigments, the medium and the plaster were all examined. The presence of potassium and calcium in all coloured layers demonstrates that the painting technique is of a type very similar to that used in Pompeian wall paintings, and described in the 1950s by Selim Augusti. In this type of wall painting, the pigments were dissolved in a lime/soap/water solution, the function of the soap being to mitigate the negative effects of lime on alkali-sensitive mineral-based colours.