Journal of Chromatography A, 1147 (2007) 79–84 Gas chromatography applied to cultural heritage Analysis of dark patinas on granite surfaces  N. Aira a , V. Jurado b , B. Silva a , B. Prieto a,∗ a Departamento Edafolox´ ıa e qu´ ımica agr´ ıcola, Facultad Farmacia, Universidad Santiago de Compostela, 15782 Santiago de Compostela, Spain b Instituto Recursos Naturales y Agrobiolog´ ıa, Consejo Superior de Investigaciones Cient´ ıficas, 41012 Sevilla, Spain Received 12 December 2006; received in revised form 7 February 2007; accepted 7 February 2007 Available online 11 February 2007 Abstract The formation of dark patinas has been mainly related with deposition of gases and particles, and sulphation mechanisms particularly in calcareous rocks. However, in granitic monuments located in Galicia (northwest of Spain), especially in rural areas, this origin is unlikely since granite is a very poor calcium-containing rock and the atmospheric pollution in this area is negligible. A biological origin seems to be most probable. In order to know the importance of biological and atmospheric factors in the formation of dark patinas on granitic monuments, a number of analyses using several techniques are needed. The characterization of fatty acids in patinas by gas chromatography has established clear differences among samples. Thus, all of the samples having a biological origin displayed a similar fatty acid pattern, with unsaturated fatty acids predominating, especially oleic and linoleic acids; these were followed in abundance by the branched fatty acid 18:00 anteiso, and palmitic acid. Different patterns of fatty acid allowed establishment of an anthropogenic origin of the some of the patinas analysed. © 2007 Elsevier B.V. All rights reserved. Keywords: Dark patinas; Decay; Gas chromatography; Granite 1. Introduction The blackening of materials exposed to outdoor environments has a negative effect on buildings, mainly historical buildings and monuments. This is a serious problem in historical cities, as the cleaning and maintenance of monuments is difficult and expensive. Information about the composition of dark patinas is essential to enable efficient cleaning of affected surfaces, and numerous studies have been carried out to identify their com- ponents. Black patinas and crusts are commonly attributed to atmospheric pollution resulting from deposition of gases and particles on the surfaces of buildings [1–4]. The main component of many patinas is gypsum, which forms by reaction of atmo- spheric SO 2 with the calcium carbonate present in limestones and marbles, the commonest stone material used in monumental buildings in Europe. Nevertheless, the formation of dark patinas, such as those appearing on monuments built with siliceous rocks  Presented at the 6th Meeting of the Spanish Society of Chromatography and Related Techniques, Vigo, 8–10 November 2006. ∗ Corresponding author. Tel.: +34 981563100. E-mail address: edprieto@usc.es (B. Prieto). and situated in non-polluted areas, cannot always be explained by these processes and a biological origin appears more likely. In Galicia (NW Spain) almost all ancient buildings and mon- uments are made of granite, a calcium-poor rock and most of them, whether located in urban or rural areas, are covered to a greater or lesser extent by dark patinas. Patinas are also very common on natural outcrops. Analysis of short chain fatty acids has for many years been routinely used in the identification of anaerobic bacteria. More- over, fatty acids in the range C 9–20 have also been used to characterise genera and species of bacteria. Microbial identifi- cation system (MIS) produced by microbial ID (MIDI; Newark, DE, USA) [5] is a commercially available system for identifi- cation of microorganisms by fatty acid. Gas chromatography is used for qualitative and quantitative identification of fatty acids and methyl esters, and pattern matching is then used to identify microorganisms based on their fatty acid profiles. Recently, this system was used for analysing biofilm and dark patinas in Ira- nian monuments, where a high proportion of fatty acids from photosynthetic organisms like cyanobacteria was found [6]. In the present study, dark patinas removed from buildings and monuments located in diverse areas of Galicia (cities, small 0021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2007.02.018