Carmen Vazquez-Calvo

Overview of recent knowledge of patinas on stone monuments: the Spanish experience

Abstract The historic treatment of stonework has often been linked to the artificial application of patinas, mainly for aesthetic and protective reasons. Increasingly, however, researchers have identified a possible combined origin for patinas that has linked natural, biological processes to those associated with an artificial, man-made origin. This suggests that, although coatings may have been initially applied on purpose, they transform over time with the aid of micro-organisms and other chemical interactions. The original mixture applied to create a patina could include lime and/or gypsum, water, natural pigments and organic additives. However, their present-day mineralogy is varied and includes a wide range of minerals from calcium carbonates to calcium sulphates, calcium oxalates, calcium phosphates, silicates (quartz, feldspar, clay minerals) and iron oxides/hydroxides. Patinas have been studied in detail in Greece and Italy, but rarely in Spain. In this paper, existing knowledge on Spanish patinas is co-ordinated and previous and current research summarized. Emphasis is placed on artificial patinas initially applied to protect stone. These both appear to effectively protect the stone substrates on which they were applied and provide an insight into historical techniques of stone conservation. Because of this their preservation should be a strong consideration in restoration projects. Ongoing research focuses on the challenges of reproducing patinas, based on historical references.

The measurement of surface roughness to determine the suitability of different methods for stone cleaning

The roughness of stone surface was measured, before and after bead blasting-based cleaning methods, to select the most efficient one to be used in masonry and stonework of specific areas of the Cathedral of Segovia (Spain). These types of cleaning methods can, besides the removal of soiling and surface deposits, leave a rougher surface, which would mean higher and more rapid water retention and deposit accumulation due to a specific surface increase, therefore accelerating stone decay. Or, in contrast, the cleaning method can be so aggressive that it can smooth the surface by reducing its roughness, a fact that usually corresponds to excessive material removal—soot and deposits–-but also part of the stone substrate. Roughness results were complemented with scanning electron microscopy observations and analyses and colour measurements. Finally, it was possible to select the best cleaning method among the six that were analysed, for different areas and different stone materials. Therefore, this study confirms the measurement of surface roughness as a reliable test to determine the suitability of stone cleaning methods; it is a non-destructive technique, portable and friendly to use, which can help us to rapidly assess—together with other techniques—the efficacy and aggressiveness of the stone cleaning method.

Laser-induced fluorescence and FT-Raman spectroscopy for characterizing patinas on stone substrates.

AbstractThis article reports on a compositional investigation of stone patinas: thin colored layers applied for protective and/or aesthetic purposes on architectural or sculptural substrates of cultural heritage. The analysis and classification of patinas provide important information of historic and artistic interest, as their composition reflects local practices, the availabilities of different materials, and the development of technological knowledge during specific historical periods. Model patinas fabricated according to traditional procedures and applied onto limestone, and a historic patina sample from the main façade of the San Blas Monastery in Lerma (a village in the province of Burgos, Spain), were analyzed by laser-induced fluorescence and Fourier transform Raman spectroscopy. The results obtained demonstrate the ability of these two analytical techniques to identify the key components of each formulation and those of the reaction products which result from the chemical and mineralogical transformations that occur during aging, as well as to provide information that can aid the classification of different types of patinas. FigureCross section of model patina (left) and FT-Raman spectrum of historic patina from the façade of San Blas Monastery, Lerma, Burgos, Spain (right).

Evaluation of Portable Raman for the Characterization of Salt Efflorescences at Petra, Jordan

ABSTRACT The advantages of using portable Raman spectrometer equipment, such as avoiding sampling and providing a higher number of results, are contrasted with some of its shortfalls that make other analytical techniques necessary to characterize salt efflorescences on historic buildings. In-situ analyses of salt efflorescences were carried out with a portable Raman at both the so-called “Silk Tomb” and “Monastery” rock-cut façades at the Archaeological Park of Petra (Jordan). Samples were also taken to be analyzed in the laboratory with X-ray diffraction (XRD) and Environmental Scanning Electron Microscope with Energy-Dispersive X-ray spectroscopy and Cathodoluminescence (ESEM-EDS-CL). This research shows the pros and cons of these analytical techniques—and how they complement each other—to identify the occurrence and determine the origin of soluble salts, which are deeply damaging these rock-cut monuments by salt crystallization processes.

The Use of Portable Raman Spectroscopy to Identify Conservation Treatments Applied to Heritage Stone

ABSTRACT Nondestructive in situ analysis of materials is highly desirable in cultural heritage studies, because it precludes the need for intensive sampling. The present study focused on the usability of Raman spectrometers, which can provide such analysis, in the identification of conservation treatments applied to stone materials forming part of the architectural heritage. Two products commonly used to conserve stone monuments, an ethylmethacrylate copolymer and an ethyl orthosilicate, were analyzed with a Raman spectrometer, both as supplied and after application to limestone. The main conclusion was that portable Raman analyzers can detect the presence and—in some cases—identify the nature of products on stone substrates. The latter is not always possible due to product-stone substrate interaction. The study clearly showed that a product and substrate database is needed for portable Raman spectroscopy to be usable in the analysis of conservation products, both before and after application to stone substrates.

Limestone on the ‘Don Pedro I’ facade in the Real Alcázar compound, Seville, Spain

Abstract This paper discusses the research conducted prior to restoring the ‘Don Pedro I’ facade on the Real Alcázar or royal palace at Seville, Spain. The different types of stone on the facade were located and characterized, and their state of decay mapped. Although other materials (brick, rendering, ceramics, marble) are present on the facade, its main elements are made from two types of limestone: palomera and tosca, each in a different state of conservation and exhibiting distinct behaviour. Colour parameters, real and bulk densities, compactness, open porosity, water saturation coefficient and total porosity were determined to characterize the two varieties. In addition, ultrasonic techniques were used to map the various levels of decay on the facade, stone by stone, for future interventions. The findings show that owing to its petrographical and petrophysical properties, palomera stone is of a lower quality than tosca stone, and has undergone more intense deterioration.

Detection of calcium phosphates in calcium oxalate patinas

Calcium oxalate patinas have been studied on historical monuments for years. In some cases they are associated with calcium phosphate, the presence of which has received contrasted attention and may have been overlooked with the characterisation techniques used. This study addresses the state of the art, the most suitable characterisation techniques for identifying calcium phosphate – the most effective and reliable being SEM-EDS of thin sections – and the shortcomings of other methods such as XRD, FT-IR or optical microscopy. It also describes examples of the presence of calcium phosphate analysed in patinas on Spanish monuments. Calcium phosphates were found to appear in these patinas in different ways and shapes: in small aggregates, in a layer mixed with other patina components, as a uniform layer, or constituting a layer with filaments, or forming a porous layer with a sponge-like texture. These findings add new data for the debate about the origin of calcium oxalate patina. Moreover, a comparison of these with other findings is the basis for a proposal of a new denomination for such patinas.