BCD 2018

162 C AT H E D R A L COMMU N C I AT I O N S C E L E B R AT I N G T W E N T Y F I V E Y E A R S O F T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 1 9 9 3 – 2 0 1 8 In addition, there may be a need for sampling of salt efflorescence, paint, mortar, plaster and substrates generally for analysis, so that the materials are fully understood and any contaminants or peculiarities identified. The results may indicate the most suitable methods and techniques for testing, monitoring, cleaning and conserving/repairing. Mortar and paint analysis may also include calculations of their water vapour permeability and porosity, to give a clearer picture of their performance and suitability in the environmental conditions found in the building. Moisture measurement Hand-held ‘moisture meters’ (such as those made by Protimeter) measure either electrical resistance or capacitance to correlate relative moisture content. Capacitance meters measure the impedance of a surface (to a depth of up to 4cm) to an alternating current, and resistivity meters measure electrical resistance to a direct current. As impedance and resistance decreases with moisture content, and resistance decreases with higher salt concentrations, these readings can provide useful information on both moisture and salt movement. However, they are also affected by the type of material and careful interpretation is required. Carbide meters provide accurate measurement of moisture, but require core samples to be taken from the wall at different depths. Each sample is mixed with calcium carbide in a sealed container and the percentage of moisture is calculated from the pressure of acetylene gas produced (more moisture = more gas). Core samples from different depths and locations provide the most accurate picture of moisture movement, but it is time-consuming. The number of samples required can be reduced substantially by using this method to calibrate hand-held moisture meters for a specific material or material type. Thermal imaging cameras are quick to use and cover large areas from a remote distance, including areas which may otherwise be inaccessible. They record variations in infra-red radiation from the surface and substrate which generally correspond with variations in material and temperature. When correctly calibrated, images range from blue (coldest) to red (warmest), and generally damp areas are colder (blue) than dry areas (red) during the day, with the reverse mostly true at night. The information can be very useful provided the effect of the material type and time of day are understood. Further verification by other analytical methods may also be necessary. Salt measurement Soluble salt and pH test strips or ‘salt meters’ may be used to provide a semi- quantitative measure (mg/L or ppm) of the concentrations of anions (sulphates, chlorides, nitrates and others) from any soluble salts present on the surface. Salt meters use the increased electrical conductivity of salt solutions as a guide to salt concentrations. If moisture and salt readings are taken systematically across (and within) the walls in grid fashion, then moisture and soluble salt profiles/gradients can be plotted. This information helps to identify salt sources (historic and/or ongoing), intensity and relationship, enabling conservators to distinguish between rising damp and related capillary diffusion, between direct rain ingress and infiltration, and between condensation at the surface and within the pores. It may also help to identify hygroscopic salt activity. The proportion of soluble salts in the sample is often too small to be weighed, while still being significant in terms of its deleterious effect. By weighing and crushing the sample, dissolving the salts in the known volume of de- ionised water and then filtering, the percentage of sulphates, nitrates and chlorides can be semi-quantitatively determined using salt anion test strips. Strips are also available for some metal cations. Where detailed information is required on the salt types and mixes, their concentrations (cations and anions) and their degree of hydration, surface and core samples can be submitted for ion chromatography, SEM, or XRD analysis. This enables calculations to be made of the relative humidity (RH) and temperature at which the salts become active. Light readings Lux meters may be used to measure light levels and to assess the exposure of the wall paintings to ultra-violet light in particular. As UV can have adverse effects on certain paints/ pigments, measurements may indicate the need for UV protection methods. Environmental recording and monitoring Readings of RH and temperature outside the building and at different locations inside it may help put the moisture and salt readings in context for the day of the site visit. They can be correlated with the moisture meter readings, allowing a comprehensive understanding of the moisture dynamics. Furthermore, this allows the absolute humidity (AH or moisture density, g/m 2 ) of the air inside and outside the building to be calculated, given the RH and temperature (or inferred from dew point readings), the differences and variation of which relate to the water-tightness and buffering effect of the building envelope. If the AH remains greater inside than outside persistently (regardless of RH changes), this would indicate excessive residual moisture in the fabric. Continuous readings of environmental conditions may be recorded with data loggers placed inside and outside the building. These allow the changes in temperature, RH and AH/dew point inside the building to be correlated with changes outside during all four seasons. This gives more weight to inferences about residual moisture sources, and highlights the potential for condensation events and rain ingress. INTERVENTION OPTIONS It is only once all the data has been collated and correlated that the following decisions can be made: • What type of conservation treatment is required, active and/or passive, to eliminate or reduce further deterioration of the wall painting? • What are the most suitable methods and materials for conservation/ remedial treatment which will stabilise the wall painting and its environment, ensuring its longevity? • Will further moisture, soluble salt and environmental monitoring be required, not least to assess the success or otherwise of any conservation interventions? • What ongoing maintenance may be required to keep the wall painting secure in its environment? Preventive and passive interventions Periodical monitoring and appropriate maintenance of the entire structure according to the ‘little and often’ rule are simple but Consolidation of medieval wall paintings utilising injections of nanolime, All Saints Church, Little Kimble, Buckinghamshire Injection of lime-based grout was undertaken to stabilise the historic plaster, All Saints Church