The recent availability of portable, hand- held video microscopes could encourage conservators and scientists to overcome the difficulties presented by in situ microscopy, helping to refine and reduce sampling, and providing information relevant to the wall painting surface in context. The video microscope produces a magnified colour image display on a monitor, allowing on-site group viewing and discussion. Its potential is to reveal fine surface details (1x000x magnification), to record and store images (using video, digital and/or photographic processes) for reference and analysis; and finally to allow the comparison of carefully referenced images recorded at different times to assist monitoring of surface change. However, these three distinct applications - examination, recording and monitoring - become progressively more demanding in application, both in planning and equipment requirement.

Hand- held examination exploits the flexibility and freedom of the video microscope allowing the surface investigation of relatively large areas of painting. Steady hand- held imaging is possible using low magnification lenses (<20x). For recording purposes, steady images with variable lighting conditions and magnification (7x- 400x in this study), are essential. Considerable planning and extra equipment is required: video recorder, and/or hard copy printers, text generator, camera supports and stands. Images can be referenced by labelling video tape with relevant information (superimposed text or soundtrack). Final tape editing can improve presentation, but tape- to- tape copying will result in some loss of image quality. While monitoring - requiring still more equipment and planning - is the most challenging application, it is also potentially very rewarding, by showing how decay mechanisms occur using micro- monitoring to record the dynamics (rate, stages) of surface change (eg the growth of salt crystals).

In situ technical and practical limitations were assessed and evaluated at four sites with wall paintings using an MS- 500 micro- Scopeman (Moritex) video microscope and other various equipment. The study was sponsored by English Heritage, which kindly provided the equipment. Considerations for the selection of the electronic equipment were compatibility, picture quality, portability and cost, and for the support equipment, portability, flexibility, stability, durability and cost. Individual specification of video microscope systems, extra features, system packaging (eg portability), and prices were found to vary widely depending on manufacturer. Video microscopes - essentially closed- circu it television systems - are subject to the constraints imposed by present television technology therefore the advantages and limitations of the video signal, and the solid- state CCD camera are described. Both optical and electronic magnification, and the factors limiting resolution are discussed. Surfaces of most interest from a conservation point of view are also very demanding in terms of depth of field requirement (eg cracking, flaking). The reduction of the depth of field at magnifications >40x is problematic, as only part of the surface can be focused in each image. Lenses from different manufacturers show notable differences in depth of field and working distance specification. The fragility of wall paintings necessitates the adoption of non- contact lenses, thus forfeiting the immediate focusing and steady image reproduction offered by contact- type lenses. Fixed- magnification, zoom and endoscopy lenses are evaluated. Lens calibration in reference to monitor size - to ascertain the actual magnification - is described, and both manual and automatic screen measurement systems are outlined. The integral fibre- optic (direct) illumination of the video microscope was often used in combination with an external light source, since raking light proved an essential addition for wall painting applications. Various types of lighting are evaluated, a main consideration being not to heat or irradiate the painting surface. Constraints associated with on- site microscopy (eg transport, storage and security), are assessed together with an evaluation of equipment necessary to provide access to the wall, and to support the equipment.

The application of video microscopy is given far greater value if complemented by both contextual video recording (camcorders) and reference to previous investigations, particularly cross- sectional analysis. It is envisaged that the addition of on- screen analytical information (eg time, relative humidity, temperature) will further enhance the technique in the future. The analogue signal generated by the video microscope and recorder is degradable (particularly magnetic tape). Therefore to preserve the quality of the captured image particularly over the long- term, digitisation would be preferable.

67/94
AN INVESTIGATION INTO THE MONITORING OF THE SURFACE TEMPERATURE OF WALL PAINTINGS (COURTAULD INSTITUTE OF ART DISSERTATION)
Robyn Pender
Number of pages - 182

The environmental monitoring of microclimate promises a better understanding of the decay mechanisms operating on any particular wall painting, and can be used to design strategies of preventative conservation and of treatment that are both most effective and least invasive in the longer term. In designing monitoring systems and in interpreting the data from them, however, it is necessary to understand the major sources of error in the measurements taken and their minimisation.

The particular case of the measurement of surface temperature is discussed in detail here, with the problems peculiar to wall paintings - a delicate and inhomogeneous surface, of low thermal conductivity and of a temperature very similar to that of the ambient air - being closely examined. The importance of measuring surface temperature is presented in terms of associated deterioration phenomena, alongside a selection of case studies of its monitoring from the recent conservation literature.

A series of experiments designed to test various methods of attachment for contact surface temperature sensors is described, and the results reported in detail. These suggest in particular that thermal insulation of the sensors is highly desirable. Non- contact surface temperature measurement using infra- red thermometers is also examined, with tests being made to determine the impact of emissivity on the readings.

In conclusion, it is argued that - since the greatest source of error comes from measurements which are insufficiently well-related to the site and to the patterns of damage and deterioration present on the painting - the final design of any system of environmental monitoring is most properly the province of the conservator rather than of specialised consultants, and to make this possible the wider discussion within the conservation field of its mechanics is highly desirable.

68/94
LASERS: A PRELIMINARY STUDY OF THEIR POTENTIAL FOR THE CLEANING AND UNCOVERING OF WALL PAINTINGS (COURTAULD INSTITUTE OF ART DISSERTATION)
Lisa Shekede
Number of pages - 114

A pilot study sponsored by English Heritage was conducted which set out to observe and record the mechanisms by which radiation from three different laser types interacted both with coatings on polychromed surfaces and with the pigment layers themselves.

To provide a framework for evaluating the potential of lasers for the removal of coatings in a conservation context, the basic principles governing laser operation were first outlined. The various means by which laser radiation acts upon surfaces was also described, and the principles and problems of establishing an approach to cleaning and uncovering were also briefly discussed.

Three terracotta panels, each prepared using a different medium (fresco, tempera and oil) were coated with three types of material which are commonly found as surface contaminants (limewash, a wax- based, and a soot- based coating). A wide range of typical wall painting stratigraphies was thus provided on which to conduct laser trials. In order to differentiate between the effects of laser radiation on pigments, media and coating materials, two ceramic tiles were also prepared, one with a limited palette of pigments applied without binding media; the second with two types of medium (egg tempera and linseed oil) and one type of coating material (wax).

Trials were conducted using three different types of laser (Nd- YAG, excimer and dye) operating respectively in the infrared, ultraviolet and visible wavelength ranges. By increasing energy densities or number of pulses, four threshold levels (minimum cleaning, optimum cleaning, damage and loss of pigment layers) were established. The Nd- YAG and dye lasers achieved limited layer by layer uncovering of limewash from the fresco panel, but not without some disruption and loss of the paint layer. The Nd- YAG removed most of the wax coating from the fresco panel with much less physical damage to the paint layer, but some pigment alteration was observed. None of the lasers removed either wax or limewash coatings from the oil panel without pigment alteration, disruption and penetration of the paint layers. All three lasers removed the soot- based coating from the tempera panel, particularly from lime white, however, many of the pigments were again observed to discolour, and loss of the paint layer occurred with some pigments before optimum cleaning levels could be reached.

Though separation of coatings from the paint layer were observed during some of the trials, success was often compromised by the problem of pigment alteration. Alteration (usually darkening) of many of the pigments was observed using all three laser types on all three panels. By subjecting the pigment- only tile to laser radiation it was possible to demonstrate that the same types of pigment alteration occurred, and were therefore independent of the media. Following the laser irradiation of the media- only tile it was observed that egg tempera, linseed oil and wax discoloured at higher energy densities than those required to induce alteration in most of the pigments examined on the pigment- only tile.

The results of these trials provided useful indicators of some of the potential hazards and advantages of developing lasers as conservation tools. The conclusions which can be drawn from these preliminary studies, however, are limited and should be regarded primarily as the basis for further research.

69/94
DETERIORATION OF THE ROMANESQUE WALL PAINTINGS AT HARDHAM CHURCH, WEST SUSSEX: A STUDY OF THE INFLUENCE OF SALTS AND ORGANIC COATINGS
Alison Sawby
Number of pages - 180

The early Romanesque wall paintings at Hardham Church, Sussex, have been the subject of concern due to the continual decline in their condition. A two-year monitoring programme of the building's climate has been carried out by the Courtauld Institute which the present study, sponsored by English Heritage, was intended to complement. The two principal deteriogens responsible for the severe decay were identified, on the basis of a phenomenological survey, as salts and previous restoration materials (organic coatings). The combined destructive action of these agents is due to their susceptibility to physical changes (salts undergoing deliquescence and crystallisation cycles of salts, and the hygroscopic expansion and contraction of coatings) within a fluctuating environment. Deterioration phenomena, salt efflorescences, and added materials were mapped to assess both their extent and their possible interrelation. The primary deterioration phenomenon observed was the disruption of the paint layer by salt efflorescences, exacerbated by the organic coatings, which tear the paint layer from its support. Two types of organic coating were identified visually (using normal and ultra violet light), and were analysed by Fourier Transform Infrared Spectroscopy and Pyrolysis- Gas Chromatography- Mass Spectroscopy. One coating, identified as a natural wax, was shown by stratigraphic and documentary evidence to have been applied in 1934. The other coating gave spectra consistent with those of modern synthetic polyamides, and may have been applied in the 1960s; relative humidity cycling showed this coating to undergo dramatic dimensional alteration.

A variety of sampling methods was employed to investigate both the global and the local stratigraphic distribution of salts (efflorescences, micro- cores, aqueous extracts and cross- sections), with the intention of correlating those results with the observed deterioration phenomena, and to compare an indirect, non- invasive method of sampling (aqueous extraction) with direct, invasive methods (micro- cores and cross- sections). Analysis of efflorescences was conducted using a combination of mineralogical (Polarising Light Microscopy) and chemical (micro- chemical tests and Energy Dispersive X- ray analysis) identification methods, and the crystal morphology studied using Scanning Electron Microscopy. Ion analysis of aqueous extracts and micro- cores was carried out using

Inductively Coupled Plasma Atomic Emission Spectrometry for elemental cations, and Ion Chromatography for anions. Quantitative results from micro- cores were used to make a tentative calibration of the information yielded from aqueous extracts, to investigate the potential for minimising invasive sampling by combination with non- invas ive sampling.

Analyses identified gypsum as the primary crystalline salt at the time of the investigation; the presence of sodium, potassium, chloride and nitrate ions was also confirmed, however under the prevailing environmental conditions (95% relative humidity and 5oC) the corresponding solid salts were not observed. From global and stratigraphic distribution profiles an air- borne source of sulphur compounds was postulated, and presumed to be due to a coal- fired stove used during the earlier part of this century. It was therefore concluded that the gypsum efflorescences are extremely slow- growing, and unlikely to increase in quantity. In the light of environmental data (arising from a two- year monitoring programme of the building from 1989- 1991 conducted by the Courtauld Institute of Art), it was concluded that the destructive impact of this salt is due to environmental fluctuations. Recent studies of the environmental response of salt mixtures are discussed. The role of microorganisms, observed during Scanning Electron Microscopy, and also the formation of deliquescent salt efflorescences during periods of lower relative humidity were not possible to investigate under the scope of this project, but would be suitable topics for further research.

Summaries 27/94, 30/94 and 56/94 are grouped elsewhere but are also 'Keyworded' here.