Temperature Monitors Works of Art Health as Human Beings

A review of different cases studies concerning heritage buildings, frescos, archaeological ceramic and wood paintings, shows as Thermal non Destructive Testing and Evaluation is particularly effective examining works of art. Nevertheless, in spite of fascinating images, a quantitative and reliable diagnosis is difficult. Hence, in many cases particular equipments and procedures must be arranged. Established techniques allows to monitor different pathologies affecting historical buildings and passive or active approaches are presented. The analysis of the temperature signal allows exploring new applications as the structural analysis or the comfort evaluation. Examples of algorithms and testing procedures are selected in order to give an overview of the thermographic method capabilities. Thermal properties as diffusivity or effusivity are extremely useful for the material characterisation. In addition, their mapping is an effective investigation tool for TNDE. Trends and suggestions for different applications are pointed out. Introduction: A large literature is growing about Thermal non Destructive Testing and Evaluation (TNDT/TNDE) [1]. Progress of IR thermography makes this method, alone or combined with others, a suitable tool. Today, highly resolute equipments or much cheaper ones are applied in combination either to sophisticated data reduction algorithms or simply looking at thermal images and exploiting the operator expertise. TNDT/NDE is a typical indirect measurement and temperature is a perfect informative parameter when shallow defects are investigated. Temperature monitors an incredible number of different phenomena because any physical and chemical process involves heat, at last. In this sense, imagination is necessary to take advantage of opportunities given by this method. Particularly, works of art are more and more issued for economic and cultural reasons. For this application the concept of “defect” is much broader than in industry, including object’s knowledge and its status monitoring. In fact, usually the real history of precious items has been lost along centuries and the NDT target becomes the discovery of hidden information, but fast, reliable and self-explicative results are needed. Furthermore, the surface temperature, moisture and airflow distribution are key factors for the comfort of people working inside a renewed historical building. New perspectives are now opening to thermography in these fields. The thermal method has been demonstrated extremely useful for works of art, mainly because of his optical nature, flexibility and imaging characteristic. Unfortunately, just to mention a few problems encountered to test unique pieces, consider actual strong limitations of heating or touching, the lack of any reference, a very complex structure and surface clutter. In practice, there is a great difference inspecting monuments, like historical buildings or much smaller objects, than can be moved into a lab. In the former case, portability of the equipment and productivity of algorithms are key points. Think about inspecting 10000 m frescoed walls, during the restoration activity, with scaffolds and very concentrated people around. Fortunately, portable items can be tested inside the workshop and very advanced techniques are now available bridging thermography and the photothermal method. Generally, a laboratory work is fundamental to set up the procedure, even for buildings. In fact, preliminary tests on samples of the inspected part, built by specialized workshops according to original recipes are used to optimize the experimental procedure. Mathematical modelling is widely used to test data reduction algorithms, to evaluate effects of different variables and to simulate real tests. Unluckily, the simulation by numerical or analytical models is not a trivial task, due to the complexity of the target and large unknown in thermal properties, geometry and boundary conditions. Finally, it must be noticed that there are so many different applications and possible uses of TNDT that a simple list is quite difficult. The presented selection is based mostly on the purpose to give a review of main application and processing algorithms. Each case study gives a different technique and data reduction, according to the particular goal. The mathematical bases and details of procedures will be found on annexed bibliography. The first part of this paper deals with historical buildings, reviewing well established techniques and also giving new trends. The following section gives some applications dedicated to ceramic and wood painting. Historical buildings monitoring: The fastest way to test a building is to work in steady state, with a passive approach. Even if a passive qualitative monitoring is chosen, mathematical modelling of the thermal problem allows deciding if and when thermography is appropriate to a particular case. The first presented example deals with the moisture mapping inside the historical arsenal of Venice [2]. The moisture excess within building is a major cause of damages, energy spending and discomfort. Moisture is dangerous even because activates biological or chemical attacks, if linked to pollution. Most of ancient buildings are affected by the presence of moisture, due to the high porosity of materials. Water capillary rising from the ground turns out into a characteristic almost horizontal frontier. Other moisture causes, as surface condensation, leakages from roof or piping give different patterns. Localized high water concentration get soon to saturation, while in many cases the extent of the moist zones will not appear visually and are not close to the failure. On the contrary, IR image shows on the surface the extent of moisture spread, but a careful analysis is needed Therefore, thermography is effective in founding the moisture source, because of temperature analysing both in space and time domain. There are many techniques suitable for the moisture detection by thermography [1,3]. Here is illustrated the simplest one, a passive technique working in steady state where data are processed by a statistical tool. A crucial point is due to changes of surface optical properties as a result of the moisture itself. Emissivity varying in the IR measuring band and absorptivity in the visual band could bring on false alarms. It is interesting to observe that water staining of the surface or mold and actual water content of each point are not proportional. Such a visual indication is symptomatic but sometime misleading, because it appears suddenly when moisture concentrates, but remains after the surface dried out. Time analysis is quite important for the moisture control, but it requires normally a long lasting observation time and a correlation on seasonal and weather events [4]. Generally, the choice of the right time to perform the test is of great importance, because various phenomena are activated by the moisture accumulation. In fact, building is submitted to slow varying but “noisy” boundary conditions and different heat fluxes may interact each other in destructive or additive sense. The Historical arsenal of Venice is nowadays passing from the Italian navy property to be converted for civil purposes. Most of buildings correspond to large hangars made in brick in different ages ranging XI-XVII centuries. The passive approach followed here allows mapping qualitatively the moisture distribution due to the cooling effect of water evaporation [5]. Thermal scanning of the internal surface of the Tezone 105 is reported in fig.1a (East side on the top and West on the bottom). The investigation of a nearly 1000 m surface requires the shot of several thermograms, which will be composed in a mosaic. Some removable markers are placed on the surface in order to know ambient temperature and locating recorded thermogram [6] (a visual image is taken for each field of view). The classification of the surface in homogeneous areas is achieved in natural, almost-steady, thermal state. The phase changing rate depends on air temperature and relative humidity levels. During the test, the best environmental conditions require a medium-high transpiration. It is mandatory a relative humidity (RH) lower then 80% in the boundary layer and air temperature not below 6-7 C°. The inspected surface has to be kept out of direct heating at least for 6 hours before the scanning, because different absorption coefficient of the surface causes effects contrasting the evaporative cooling. Hence, it is a good practice recording temperature and RH for 24 hours before and during the thermographic taken. The identification of the damp areas is achieved by the comparison between the temperature of the dry and moist surfaces. For instance, two sections of interest (marked on fig.1a) are combined in fig.1b and processed. The superimposed histogram of fig.1b clearly indicates with the first peak a colder and therefore moist area, corresponding to the west side.