The Problem Stone Progressive Survey Technique

The Problem Stone Progressive Survey Technique (ProSt-ProST). A Pilot Study at the Bell Tower, Tower of London. Martin Michette1, Heather Viles1, Constantina Vlachou2, Ian Angus3 1. School of Geography and the Environment, University of Oxford; 2. Historic Royal Palaces; 3. Carden & Godfrey Architects Introduction Stage 2: Stone Mapping The Problem Stone Progressive Survey Technique (ProSt-ProST). combines recent developments in the ield of building surveying in to an integrated approach. It is intended for use by architectural conservators at built heritage sites in order to diagnose the causes of building stone deterioration. Precise diagnosis will enable targeted conservation strategies. This will be particularly useful for building stones where causes of decay are poorly understood and past conservation has been inefective. The survey is broken down in to several stages, with stages progressively iltering interesting variation for investigation in the following stage. The following is a pilot study at the Bell Tower, Tower of London. The Bell Tower was built in the 12th century and contains a signiicant proportion of Reigate Stone. Stage 1: Metric Survey The objective of the metric survey is to produce orthographic elevations of the entire building or site under investigation. The purpose of the elevations is to record information obtained in later stages in a manner that facilitates numerical analysis and long term assessment of change. The level of detail must be able to display individual stones. Viable techniques for producing templates for CAD include laserscanning, structure-from-motion photography, tacheometry and rectiied photography. This study used existing AutoCad elevations produced from a laserscan carried out in 2015. SFM photography was also trialed and can be recommended as a low-cost alternative provided appropriate measures are taken to enable metric scaling (e.g. inclusion of scale bar in photoset). The objective of the stone mapping is to classify all extant stone types in to meaningful categories. The main aim is to locate the problem stone types to be surveyed in later stages by identifying mineralogical characteristics. A secondary aim is to uncover historical changes to the fabric that could suggest past replacement of problem stones or indicate historical decay mechanisms. This may necessitate supplementary observations of changes in mortar type and stone course and should be further supported by archival research. The categories should be marked on elevations using distinct colours, shades or patterns. It is not necessary to classify precise stone type other than the problem stone(s) under investigation. During a survey conducted in 2015 [1], 31 diferent lithotypes were identiied and recorded as separate layers in AutoCad elevations. These were reduced to four categories for the present study. Reigate Stone and Poor Quality Kentish Ragstone were categorised as distinct problem stone types. To investigate the historical extent of Reigate Stone, a further category was deined as Post-medieval Replacement. These were identiied as stone types introduced in to London after the 18th century and examples of Kentish Ragstone laid in post-medieval course patterns. All other Ragstone and any other stone types were categorised as medieval stone. Conident in-situ identiication of lithotype can be challenging and future surveys will beneit from glossaries to assist recognition and a lexible approach to problem stone deinition. Stage 3: Decay Mapping Stage 4: Initial Petrographic Analysis The objective of the initial petrographic analysis is to carry out rapid, NDT on a broad sample of the varying problem stone conditions identiied in the previous stages. The aim is threefold: • Quantify variations recorded in the condition assessment for future cross-referencing with samples tested in a controlled environment. • Inform environmental monitoring strategy, and enable assessment of response to environmental mechanisms via repeated implementation. • Enable selection of representative samples for detailed analysis in later stage(s). The objective of the condition survey is to identify deterioration patterns and record these to produce decay maps. The primary aim is to enable selection, examination and comparison of representative samples during the analytical stages. Classiication of deterioration patterns should be based on an appropriate selection from the ICOMOS Illustrated Glossary of Stone Deterioration Patterns [2]. This stage is intended to provide an overview of diferential condition rather than an exhaustive assessment. Recorded patterns should be limited to the most prominent. Any anomalies should be noted separately in case correlations emerge during future surveys. The selected patterns will form a specialised glossary for individual problem stone types. A reduced sample set focusing on two conditions was chosen for the pilot study. Powdering and laking were chosen as they represent the two most common patterns. A sample set of 26 stones (13 per pattern) across ive of the elevations was selected, with each pattern sampled across 3 elevations. Several techniques were employed to gather data on a range of characteristics. The choice of devices was informed by initial hypotheses on the factors afecting the stone deterioration, portability, speed and ease of use, and cost and availability to the practitioner. The Portable Petrographic Pilot Pack (PoPePiPa) included: • An Equotip Piccolo to measure surfce hardness. • A spectrophotmoter to measure colour, in order to proximate Glauconite content. • A Protimeter Surveymaster to measure moisture content. Surface Hardness green: powdering, blue: laking, red: fresh quarried Acknowledgements This project is funded by the EPSRC Centre for Doctoral Training in Science and Engineering in Arts, Heritage and Archaeology and Historic Royal Palaces. It will be carried out in collaboration with Historic Royal Palaces and Carden and Godfrey Architects at University College London and the University of Oxford. Results tended to conlict the initial hypotheses. Powdering stones were thought to have a higher Glauconite content for example, accounting for the more aggresive weathering pattern. Not only was surface hardness (left hand image) found to be lower, but powdering stones also measured less green with the spectrophotometer. The techniques did however record and quantify noticeable variation across diferent patterns. A reinedment of the PoPePiPa is suggested for future surveys. Conclusions The ProSt-ProST was efective in identifying a variety of distinct conditions and using portable, non-destructive techniques to classify these. Two steps are necessary before the technique can be widely implemented: • The creation of specialised glossaries of weathering patterns for prevalent problem stone types. This will facilitate onsite identiication and provide a resource for harmonised condition assessment. • The design of an efective PoPePiPa, assembled from portable devices that are afordable to general practice and capable of classifying key characteristics, whilst limiting time and intervention on-site. Blistering, powdering, laking and chipping were identiied on the Bell Tower. These were each categorised in to light and severe. Light was classiied as afecting between one third and two thirds of surface area and severe more than two thirds of surface area. Erosive features induced by material loss were deined separately in order to investigate correlations between form and rate of decay. These were sub-divided in to shallow, deep and alveolisation. Shallow was classiied as mostly uniform backward erosion of 1-3cm and deep was classiied as mostly uniform backward erosion of more than 3cm; both were judged by eye. Alveolisation implies the formation of cavaties. References [1] K. Hayward, C. Matthews, Assessment Report on the Archaeological Building Recording (including petrographic survey) and Watching Brief at the Bell Tower and adjacent Inner Curtain Wall, HM Tower of London, London Borough of Tower Hamlets, unpublished report (Feb 2016). [2] ICOMOS, Illustrated glossary of stone deterioration patterns, http://www. icomos.org/publications/monuments_and_sites/15/pdf/Monuments_and_ Sites_15_ISCS_Glossary_Stone.pdf, 2008 (accessed 24.08.16)