Acta Geoturistica volume 9 (2018), number 1, 1-8 doi: 10.1515/agta-2018-0001 Stone in architecture and sculpture – source material for reconstruction 1* 2 MAREK W. LORENC and H. LORENC 1 Institute of Landscape Architecture,Wrocław University of Environmental and Life Sciences, ul. Grunwaldzka 55, 50-357 Wrocław, Poland 2 Center of Physical Education and Sport,Wrocław University of Environmental and Life Sciences, ul. Grunwaldzka 55, 50-357 Wrocław, Poland (*corresponding author e-mail: [email protected]) ABSTRACT Stone is the oldest, natural material, which was (and still is) used as both building and sculptural material. The most commonly used for these purposes are: granites, marbles, limestones and sandstones, representing the three main genetic groups of igneous, metamorphic and sedimentary rocks. All of them are permanently being destroyed in result as well of natural weathering as microbiological activity and anthropogenic pollution of atmosphere, known as deterioration. The speed of such decay depends on both environmental conditions and mineral composition of the stone and it can lead to such intensive destruction that conservation may require partial replacement. Smaller damages are refilled with appropriate mineral masses, whereas in case of bigger damages refilling with natural stone is necessary. Professional conservation practice demands the selection and use of the same rock or the rock that is, in so far as is possible, identical to that originally used. It can be done only after previous detailed petrographical studies of the original material. Only then the stone material used for reconstruction will be appropriate and stonework performed properly will not (or almost not) leave marks. In many cases the ancient quarries do not exist and original source material is not available. Then petrographical studies of numerous rock-samples, which are recently available from other existing and/or working quarries, will allow the indication the most similar material. In many cases, unfortunately, the stone used for replacement is not identical to the original but only macroscopically similar. In such a case results might be visible sooner or later. These will be differences in colour, differences in structure and in some cases even crystallization of secondary minerals in the newly inserted fragments. Keywords: stone, architecture, sculpture, deterioration, petrography, reconstruction. INTRODUCTION types of stone were materials used to produce simple tools and weapons. And this Stone is a natural material accompanying has not changed in many places all around a man since the dawn of time. In different the world. Apart from its protective and climatic zones, people lived in caves which utility applications, stone was also used for are natural karst formations developing in decorative purposes which is best proven some types of rocks; and where there were by not only various design elements but no caves, people hid under great rock also, and most of all, numerous sculptures masses or overhangs. Later on, they originating from different periods of the collected loose rock debris and used it as a history. building material for different walls and Although stone in general is relatively simple habitable structures. What is more, durable material, its different types are appropriately processed fragments of some characterised with different reactions to © 2018 Marek W. Lorenc and H. Lorenc. This is an open access article licensed under the Creative Commons Attribution-NonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/). 1 Acta Geoturistica volume 9 (2018), number 1, 1-8 weather conditions, undergoing more or micas which are chemically complex iron less advanced destruction over time. The and magnesium alluminumsilicate. The most often, sharp edges become blunt and universal character of this vast group stems all tiny details fade away so that artistic from their crystal structure and relatively expression is gradually dying away. Stone high hardness (Lorenc, 2004; Lorenc & features located within great urban and Mazurek, 2007). industrial areas are especially exposed to The group with the commercial name of destructive conditions, where natural "granites" often includes equally nice, hard weathering processes are intensified with and crystalline rocks of totally different the impact of deterioration related to origin. The most often, these are different anthropogenic pollution of atmosphere and types of gneiss and some quartzites which poor ventilation of urban areas. The modern are metamorphic rocks formed as the result conservation techniques enable one to stop of mineral, structure and texture further destructive processes; however, in transformations of, among others, some cases, visible destructions require sandstone and granite. refilling in order to restore a sculpture or A second group of rocks widely applied architectural structure to its original state in architecture and arts are sandstones (this also concerns mechanical damages). In which occur in equally wide range of this case one should aim at obtaining such a colours as in the case of the above- stone material which will not differ from mentioned "granites". These are rocks of the original material of the reconstructed sedimentary origin, composed of single feature, and the close petrographical grains of sand joined with mineral binding analysis proves to be very useful here. material which composition determines the basic physical qualities of these rocks. If the binding material is silica (SiO2), these TYPE OF STONE types of sandstones are the most durable and weather resistant. There are also With regard to its durability and weather sandstones of calcite (CaCO3) or clay resistance, the best stone material is a vast matrix which have relatively low chemical group of crystalline rocks, commonly resistance; thus, rarely used in the stone known as "granites" when, in fact, it industry. Apart from quartz sandstones of includes igneous rocks of different mineral silica matrix, other types are quite compositions, starting with gabbro and susceptible to weathering and deterioration. diorite, through tonalite, monzodiorite and The details concerning a type of sandstones' syenite, and ending with proper granite. All grain framework and binding material may these are hard rocks with a wide range of be determined only during petrographical colours, from black to white, through all analysis. It should be emphasised that all shades of brown, red, yellow and even sandstones are porous rocks which makes green and blue. Their mineral composition them absorbents of water and moisture - the is various and their chemical structure is major factors facilitating decomposition dependent on proportional content of such and/or disintegration of a stone. major minerals as quartz (SiO2), potassium- Commonly used stone materials are feldspar (KAlSi3O8) and plagioclase carbonate rocks which, with regard to their (a continuous series of minerals from genetics, represent two completely different sodium albite NaAlSi3O8 to calcium groups. One of them are limestones which anorthite CaAl2Si2O8). Furthermore, in are sedimentary rocks of organic or relation to technical and chemical chemical origin, almost entirely composed parameters, of considerable significance is of calcium carbonate (CaCO3). Here, this the percentage of other rock-forming mineral does not have a crystal structure. minerals as pyroxenes, amphibole and The other group includes marbles of 2 Acta Geoturistica volume 9 (2018), number 1, 1-8 metamorphic origin, coming from structure of a rock due to rinsing calcium limestones transformed in high pressure and carbonate (CaCO3) out from a binding temperature conditions. Here, calcium agent and replacing it with insoluble carbonate takes a crystal form and this gypsum (CaSO4 x 2H2O). causes that a stone has slightly different What is more, rock materials undergo physical properties than a limestone. A destruction because of bacteria, algae, disadvantage of all carbonate rocks is the lichens and fungi, that is the process of fact that, as practically monomineralic biodeterioration. One of the metabolism rocks (composed almost exclusively of products of these microorganisms are calcite), they are much softer in comparison organic acids which, in reaction with some with granites and sandstones. minerals, cause precipitation of new salts, depending on the rock porosity, even 2-3 cm deep under an external surface. DETERIORATION Other indications of stone destruction as the result of the influence of microorganisms Airborne aggressive dusts and gases are changes in original colouring, and new cause considerable damages to stone colourful stain and discoloration. Another features, especially, in a poorly ventilated indications of biodeterioration is stone compact urban development. Sulphur spalling and dilapidating due to fungous dioxide (SO2) and nitric oxides (NOx), activity (Lorenc, 2003; Lorenc & Mazurek, mostly anthropogenic, are especially 2007). Methods of stone conservation in dangerous in this scope, which, when order to protect it against further destruction joined with water vapour, transform into have long been applied and the details very aggressive acids. Reaction with some concerning these activities may be found in rock minerals, especially those of porous many reference books (among others, nature, results in the acids causing Haber et al., 1988; Domasłowski, 2011; crystallization of new salts changing the Wilczyńska-Michalik