Abstract
This paper presents an examination of the timescale of phase transition behaviour of a series of salts known to cause damage to wall paintings and other cultural property. The rate of deliquescence and crystallisation of single salts (nitromagnesite and halite) under different RH regimes, and the extent to which this was affected when mixed with other salts (niter, nitratite and gypsum), was investigated. The study was conducted using simple conventional techniques (mass measurements over time) and also using an innovative new method: timelapse video imaging with online data annotation. The results demonstrate the synergy gained from combining video imaging with environmental data in reference to time in the study of salt phase changes: where it revealed new information concerning the kinetics of deliquescence and crystallisation. The implications of these results for the implementation of environmental control measures within historic buildings are discussed.
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Notes
The RH within the climate chamber was controlled using a micro climate technology (MCT) generator (model: 92MCG. TC), and MCG Monitoring Software via a laptop. The climate chamber had its own humidity sensor, however the RH and T data recorded was acquired from a higher quality Vaisala HMP233 transmitter.
While it would have been optimal to undertake the mass measurement and timelapse experiments simultaneously, this was not possible as the mass measurements necessitated the movement of samples on and off the balance, hence precluding the possibility for timelapse imaging. Moreover, the timelapse experiments were able to provide different, but complimentary information through the use of a four chamber sample holder, which allowed simultaneous imaging of four different combinations of salt, thus providing direct visual comparison between the behaviour of each.
The time lapse system comprised of an Infinivar™ Video Inspection Microscope attached to a Sony DX−930 video camera. Build around a multitasking Apple Macintosh computer, the system was required to automate procedures and fulfil the technical requirement of obtaining images at time intervals and mixing computer video with computer graphics. Thereby, adding to the captured images the environmental and time data that was simultaneously being received. Hardware included a Truevision NuVista + framegrabber for digitisation and overlaying graphics and video. Software for data acquisition, manipulation, and presentation included the following: ZTerm (a communication software for modems. It runs in the background and was used to log data readings to file independently of LabVIEW); LabVIEW for Macintosh (National Instruments) using VIs (Virtual Instruments: these software-created modules can be assembled to perform the same operations as physical instruments) for direct processing of the data; O’Clock (to provide an analogue clock face). Software for macro programming: Quickeys. Software for VCR and digital capture control: Video Online; Animaq. The Sony UVW-1800 Betacam edit/recorder was a compromise at the time in lieu of an affordable, practical digital solution. Nonetheless, together with the animation controller board it was capable of sequential recording of single or multiple frames, and frame-accurate retrieval and editing. The Sony UVW-1800 was computer-controlled via an RS-422 9-pin interface by the DiaQuest DQ-Animaq edit controller board and software. The NuVista + board handled all video imaging aspects, while LabVIEW’s role was to run in the background, updating data in real-time or at regulated intervals on to a computer graphics display. Precise live video overlay with computer graphics is only possible using genlock circuitry. Therefore the camera, VTR controller, frame buffer card, transcoder, VTR, and monitors need to be driven (or regulated) from a single source of sync.
During the mass measurement experiments it was found that while the rate of moisture uptake and loss of the samples differed dependent on the salt mixture composition, nevertheless, this rate did not vary throughout the course of the 2 h monitoring period (i.e. all plots of adsorption and desorption over time were linear). Consequently, the results of these experiments are presented here in terms of the total moisture exchange that took place over 2 h.
The results for the behaviour of the salts at the RHeq of the primary salt (see Figs. 5, 6) do indicate that some degree of adsorption takes place. However, the accuracy of RH control within the chamber was ±2.5% RH, and so this slight degree of moisture uptake could be attributed to temporary and slight fluctuations of RH above the RHeq, and therefore cannot be conclusively attributed to deliquescence behaviour below the RHeq. Indeed, it is important to note that in the case of the salt mixtures here described, once deliquescence has started the mixed salt solution formed will have a lower RHeq than that of the pure salt. Consequently, moisture uptake may proceed even if the RH once again falls below the RHeq of the single salt. As a preliminary to the timelapse experiments, the salt mixtures were held at RH values close to, but slightly below the RHeq of the primary salt (RHeq−5% RH). For the nitromagnesite mixtures, some slight degree of moisture deliquescence was observed for the nitromagnesite–halite combination after a period of about three and half hours. However, again, this might have been to temporary RH fluctuations above the RHeq of nitromagnesite. For the halite mixtures, no apparent deliquescence was observed. Consequently, the evidence indicates that deliquescence commences only when the RHeq of the primary salt is exceeded—albeit temporarily.
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Acknowledgments
This work was undertaken at, and financially supported by, the Getty Conservation Institute (GCI). Specific thanks to those at the GCI are due to Marta de la Torre (former Program Director, Training), Dr. Eric Doehne, Jim Druzik, Shin Maekawa, Michael Schilling and Herant Khanjian. Moreover, a great debt of gratitude is owed to Sharon Cather (Conservation of Wall Painting Department, Courtauld Institute of Art) who supervised this project, for her detailed and useful comments on all aspects of this work. Finally, grateful acknowledgement must be made of the perceptive observations and invaluable guidance generously given by Dr. Christine Bläuer Böhm and Dr. David Saunders.
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Sawdy, A., Heritage, A. Evaluating the influence of mixture composition on the kinetics of salt damage in wall paintings using time lapse video imaging with direct data annotation. Environ Geol 52, 303–315 (2007). https://doi.org/10.1007/s00254-006-0496-6
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DOI: https://doi.org/10.1007/s00254-006-0496-6