Monitoring Acoustic Emission in an Epidemiological Pilot Study of a Collection of Wooden Objects

ABSTRACT

This experimental program, implemented by the Managing Collection Environment Initiative at the Getty Conservation Institute, has provided data about the response of historic objects to changes in relative humidity (RH). Alongside other less sensitive documentation techniques (visual observation, physical measurements, photography, and 3D scanning), acoustic emission (AE) was monitored for six historic, museum-like wooden objects exposed to gradually increasing RH variations. Quantitative calibration of AE sensors and an anti-correlation measuring scheme to reduce environmental noise allowed the AE system to effectively trace micro-damage development caused by environmentally induced stress. Using an epidemiological approach, with respect to sample selection and experimental protocol, the obtained results can be used to explore the acceptable range of climate fluctuations for museum collections. The study’s well-controlled environmental protocol also allowed for the examination of the concept of ‘proofed fluctuation’ by the analysis of object response to reoccurring conditions.     

Climatological analysis in relation to the conservation of a sixteenth-century spanish polychrome wooden ceiling

Abstract

A sixteenth-century Spanish wooden ceiling was installed in the Morning Room of Hearst Castle in the early 1920s. Since that time, the adhesion between the ground/paint layer and its wooden support has weakened considerably. As a first step in designing an effective conservation program, it was necessary to assess the role of the environment in this deterioration process. Air movement patterns, humidity and temperature parameters, air pollutants, and the impact of approximately one million visitors per year on the microclimate were studied over a one-year period. Correlations between the various components of the study were mapped, including CO₂ concentrations vis-à-vis the number of visitors, and humidity and temperature changes in relation to tourists, air circulation patterns and variations in the external climate. As a result of this study, temporary measures were taken to improve the conditions of the immediate environment and proposals were made for more permanent solutions.     

Prediction of the relative humidity response of backboard-protected canvas paintings

Abstract

simple mathematical model is presented that predicts the decay time of the internal relative humidity (RH) of backboard-protected canvas paintings when subjected to external RH fluctuations. The model takes into account the permeance of the canvas and the hygroscopicity of the enclosed materials. Predictions from the model are tested experimentally. The wooden stretcher releases moisture more slowly than expected, providing less buffering than predicted by the model. The practical implications of these findings are discussed.     

Dimensional Stability and Moisture Properties of Gap-Fillers Based on Wood Powder and Glass Microballoons

ABSTRACT

Conservation of wooden objects exposed outdoors is extremely difficult and complex due to their constant exposure to fluctuating weather conditions and biological degradation. Filling the gaps in such objects requires the use of specific materials that in particular can adapt to changes in wood dimensions in response to humidity variations. Various materials have been used so far for filling voids in wooden artefacts but none of them was entirely suitable. Therefore, the authors have attempted to provide the basic characteristics of the selected commonly used filling materials based on pine wood powder or glass microballoons as fillers and Paraloid® B-72, Klucel® G, or glutin glue as binders, as the first step in research aiming at developing new and more appropriate gap-fillers. Special attention was paid to the dimensional stability of filling materials upon drying and exposure to water vapour and liquid water. The overall results indicate that among the examined fills those consisting of Paraloid® B-72 and glass microballoons revealed best properties as they were relatively dimensionally stable upon drying and exposure to moisture – despite high absorption of water – and were quite easy to finish. Generally, better properties were observed for filling materials containing higher concentrations of adhesives and these materials will be modified in the next step of the research. Since the main drawbacks of the examined gap-fillers were low water resistance, poor dimensional stability upon drying or exposure to water vapour or liquid water, and fragility or friability, future research into their modification will head towards improvement of the above-mentioned properties.     

Relating relative humidity fluctuations to damage in oak panel paintings by a simple experiment

ABSTRACT

Panel paintings are essentially wooden boards painted on one side. Due to the vapor resistance of the paint layer, changing ambient conditions lead to exchange of moisture on only one surface. Subsequently, a non-uniform moisture content profile is formed across the thickness of the board. As a result, differential expansion causes the board to bend in case of no mechanical restriction, or it leads to a build-up of stresses inside the material if restrained. Experiments with oak boards sealed on one side and exposed to a change in the ambient relative humidity (RH) were performed. By scaling, the response of any board with different thickness can be predicted. Since the bending of the board can be described as a linear system behavior, the frequency response can be predicted based on the step response. In combination with critical strains for wood and gesso from the literature, this gives insight into allowable RH fluctuations in terms of frequency and amplitude for different board thicknesses.     

Climate control for the passive conservation of the romanesque painted wooden ceiling in the church of Zillis (Switzerland)

Abstract

Extensive investigations of the deterioration of the romanesque painted wooden ceiling in the church of Saint Martin in Zillis (Switzerland) have revealed three major active deterioration phenomena: (1) tented flaking of paint layers; (2) flaking of the wheat starch fixative together with parts of the paint layer; and (3) fungal growth. These deterioration processes correlate with the dynamics of the prevailing room climate. Both forms of flaking are mainly driven by fluctuations of the relative humidity (RH), whereas the fungal attack can be related to high average RH and stagnant air. Frequent daily fluctuations in RH of up to 20% in summer are due to visitors leaving the doors open, while fluctuations of up to 25% are related to heating in winter. Given the risks of new consolidation treatments, the aim is to slow the decay processes by influencing the climate in the room. Installation of automatic door-closers reduced fluctuations but led to a rise in RH and a reduction of air movement, causing greater fungal growth. Subsequently, an automatic air-exchange system triggered by absolute humidity values was installed to reduce excess humidity and to exchange stale air without producing major fluctuations in the relative humidity. This kind of 'climatisation' has the advantage of being highly flexible, allowing variation of the rate and scale of exchange; it is, moreover, truly reversible.     

Allowable microclimatic variations for painted wood

Abstract

Environmental standards for cultural heritage collections have been much debated in recent years. The interest in the issue has been driven by the growing movement towards green museums, that is, managing indoor museum environments in a responsible and efficient manner, especially in terms of reducing energy consumption and carbon emissions but at the same time maintaining high standards of collection care. Painted wood is among the category of heritage objects most vulnerable to relative humidity and temperature fluctuations. Therefore, scientific understanding of how changes in environmental conditions ultimately affect painted wood is crucial to the development of rational guidelines for the control of climate in museums and historic buildings. This review provides a systematic progression through two fundamental approaches to establish the allowable ranges of climatic variations – an analysis of the mechanical response of painted wood as a complex, multilayer system to climate variations, and an analysis of the historic climate to which the objects have acclimatized. The climate specifications and standards based on both these approaches are reviewed.     

The conservation of artworks and hot air heating systems in churches: are they compatible? the case of rocca pietore,italian alps

Abstract

Most artworks require a stable microclimate and a field study has been undertaken to understand the environmental risks in a mountain church that is heated once a week. The church was selected on the basis of the artworks present and the existing heating system: a hot-air system which is popular due to its low cost and rapid response. The heating system is switched on only during the liturgical services and generates rapid temperature and humidity changes which, in the long run, are very damaging. The temperature stratification leaves people with their feet in cold air, whereas the air in the upper levels is too hot and dry. Moisture condenses on the cold walls. Deliquescent salts in masonry and frescoes undergo dangerous crystallization cycles. Wooden artifacts contract and expand in line with humidity cycles. Deposition of candle and incense smoke is increased, darkening the artworks. The paper discusses these problems, some common errors, and possible methods to mitigate the risk factors.     

POLYCHROMED SCULPTURES IN JAPAN

Abstract

Japanese sculptures are made of wood, dry lacquer, clay, gilt bronze and stone. Buddhistic statues, made of wood, are the most abundant. The painting techniques of the wooden sculptures, including ground coating, pigments, and application of Japanese lacquer, are described, and typical examples of Buddhistic statues are given. Specific case studies on a clay statue and a wooden sculpture are made.     

The Influence of Hygroscopic Materials on the Fluctuation of Relative Humidity in Museums Located in Historical Buildings

ABSTRACT

The preservation of artefacts in museum collections is profoundly affected by fluctuations in temperature and, especially, relative humidity (RH). Since the late nineteenth century, many studies have been carried out on the best way to control hygrothermal conditions. In old buildings located in maritime temperate climate zones (such as Portugal) with strong thermal inertia, and which have low ventilation rate (relative to the volume and number of visitors), daily and seasonal hygroscopic inertia may help to assure the maintenance of RH stabilization conditions. The use of expensive active systems may be minimized through the passive behaviour of internal finishing building materials. This work presents the results of an experimental laboratory study conducted in a flow chamber to demonstrate the enormous potential of hygroscopic materials in stabilizing interior relative humidity. Based in these results and in-situ monitoring in a museum housed in a building, located in Porto, with a typical construction of the 1950s (granite masonry and reinforced concrete slabs), a numerical analysis was done to quantify the influence of hygroscopic materials in stabilizing the interior relative humidity.     

Conservation of Iron with Tannin

Abstract

Ivory can undergo severe cracking and splitting when exposed to adverse environmental conditions. The anisotropic dimensional properties of ivory have been investigated under varied temperature and relative humidity conditions. Migration of moisture into and out of ivory was found to play the principal role in its expansion and contraction. To control the resultant swelling and shrinking, the use of polymer coatings and waxes to act as moisture barriers was examined. A particularly effective polyvinylidene chloride emulsion was further tested for facility of application and aesthetic appearance. The incorporation of plasticizers into ivory to reduce brittleness and thereby relieve stress was also investigated.     

Guideline in Jeopardy: Observations on the Application of the ASHRAE Chapter on Climate Control in Museums

ABSTRACT

This paper provides the climate specifications of several museums in the Netherlands that have been renovated. These climate specifications developed in the last two decades are very similar, and hence demonstrate that not much has changed in 20 years. Since the use of the climate classes presented by ASHRAE often plays a major role in developing climate specifications in Dutch museums, an analysis was made of its pros and cons. Issues concerning the use of the ASHRAE table are presented: the table is used as a list, the meaning of short fluctuations is unclear, and translating the numbers to a control algorithm is troublesome. An alternative table is presented for temperature and relative humidity specifications to overcome these issues.     

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.     

Curators and Conservation

Abstract

Prehistoric decorated caves, witness to man's artistic activity many thousands of years ago, have survived till now thanks to favourable conditions of conservation. Since their discovery, as a consequence of changes brought about by various developments and heavy visitor traffic, their precarious equilibrium has been threatened. Before we attempt to strengthen or to restore this state of equilibrium, it is desirable to understand the mechanisms governing the current in situ conditions in relation to the circulation of air, variations of humidity and temperature, the relationship between the cave and its surroundings, and the role of microorganisms.     

Cross-sections and Chemical Analysis of Paint Samples

Abstract

Consolidation of decayed wooden artifacts can be achieved by impregnation with epoxy prepolymer dissolved in a mixture of toluene and glycidyl ether and subsequent polymerization in situ. The principles of selecting the successful method of application are discussed, and the details of the procedure given. Impregnation with the described solution causes insignificant expansion of the wood in contrast to the action of polyvinyl acetate in alcohol solution.     

Preliminary tests for mechanical properties of wooden ‘buttons’ used for attaching auxiliary supports behind panel paintings

Abstract

When wooden supports of panel paintings have been severely altered or damaged and the original cross beams are missing, new cross beams or other types of auxiliary supports are connected to the panel's back-face in order to achieve desired effects including panel strengthening and control of deformations. Some conservation laboratories use small wooden blocks, each glued on the back face of the panel and holding freely the head of a screw connected (often by means of springs) to the auxiliary support; such blocks being called in Italian ‘piedini’ or ‘bottoni’, the English term ‘buttons’ is proposed here. This paper reports research aiming to characterize the mechanical behavior of three types of ‘buttons’ having different geometries and made of various wood types (oak, walnut, lime, beech, and a medium density fiberboard). Short-term mechanical tests (～1 minute duration) were performed with a universal testing machine by axially pulling out the 4 mm diameter steel screw from the button, glued onto a dummy board. Load and deformation were recorded, and the load–deformation curves were analyzed. The results show that the deformability and the load-carrying capacity of the buttons were influenced primarily by the manufacturing geometry and secondarily by the wood density.     

Radiography of Paper

Abstract

Absorbent materials such as wood, paper and cotton stabilize the atmosphere of show-cases against the relative humidity changes caused by temperature variation and by exchange of air with the surroundings.

There are several exchange processes between the air in the case and the outside air: diffusion through porous COllstruction materials, air flow caused by temperature and pressure changes and airflow by convection in a show-case whose relative humidity differsfrom that of its environment. The air changes about once a day in show-cases made by conventional joinery techniques. The rate of exchange can be reduced to less than five volumes a year by sealing a show-case so that only one small hole is left for pressure equalization. A dust and sulphur dioxide filter can then be fitted to maintain a pure atmosphere within.

In heated rooms without air-conditioning the RH may fall dangerously low in winter. A static method of RH control for a show-case is described. This is a saturated solution of sodium bromide covered by a silicone rubber membrane.     

Risk Assessment and Preservative Measures for Volatile Organic Compounds in Museum Showcases

ABSTRACT

The first step in preservative measures for artworks in airtight showcases is the exclusion of outgassing from construction materials or, at least, limitation of the emissions to a minimum acceptable level if the volatile organic compounds (VOCs) generated may be qualified as harmless. Once it is ascertained that the objects in the showcases are responsible for VOC emissions, a step forward becomes necessary, implying showcase ventilation with air flowing through active sorbent materials. Solid phase micro-extraction (SPME) followed by GC separation and MS identification has been applied for checking the outgassing characteristics of different types of materials used for the construction of airtight showcases, and the method was found particularly attractive in terms of speed and sensitivity, suitable for measurements in presence of low-emitting materials. Sampling could also be done in the case interiors, thus making possible detection and recognition of emissions from art objects within a case. A comprehensive preventive conservation scheme has been further devised by means of active air circulation within the showcase and functional sorbent materials along the flow, for complete suppression of gaseous compounds. Control of the compounds in the air flow and of their filtration efficacy was achieved by direct sampling, also using SPME fibers. Continuous monitoring of the air quality in the showcases has been developed with insertion of a photoionization detector capable of detecting the VOCs in the air by inducing molecular ionization, and measuring the generated photocurrent. The system is not selective, but gives the total concentration, reaching ppb sensitivity with high-quality data. In addition it includes temperature and humidity sensors, making the whole a useful instrument for environment control.     

Microwave Treatment for Pest Control: Coleoptera Insects in Wooden Objects

By far the most common species of woodworms perceived to be the main cause of damage to wood, and in particular to wooden art objects, are Anobium punctatum de Geer and Hylotrupes bajulas of Coleoptera family. In search of non-chemical methods for eradicating these wood pests, microwave (MW) heating can be an effective tool to help control the insects in their different life stages (eggs, larvae, and adult). The effect of 2.45?GHz frequency MW exposure on wooden art objects is described and discussed. MW heating time is reported in connection with lethal temperature of the pest and moisture content of the treated wooden object. The treatment was carried out using Microwood 12, especially conceived for portability, easy operation, and control. Microwood 12 consists of a switch mode high-voltage power supply and an industrial continuous wave magnetron connected to a rectangular-shaped horn antenna. The measure of temperature and control of operation is done via a built-in infrared thermometer.     

Long-term changes in climate and insect damage in historic houses

Abstract

Insect pests are an important source of concern in historic houses as the larval stages in particular can feed on a variety of important heritage materials, causing significant and sometimes irreparable damage to collections. Damage to wood and textiles is a special problem. The lifecycles of insects are sensitive to climate and require relatively warm conditions. There has been a significant increase in the presence of insect pests within historic houses in the early twenty-first century. The reasons may include: warmer winters, widespread use of natural fibres, less potent insecticides, and occupation of new niches indoors. The interior climate, especially increasing warmth, offers the potential for greater insect growth and survivability. Modelling changes in the temperature and humidity within the Cartoon Gallery at Knole, southern England, for the period 1770–2100 suggests a dramatic increase in favourable temperature conditions through the current century.