Stone in Architecture

Since prehistoric age men used stone for its unique durability to erect monuments of extraordinary, mostly religious importance. Due to lacking transportation facilities until the 19th century stones from nearby sources had to be chosen to build churches, castles and towns. Only for exceptional cases rare and decorative stones like marble were transported over long distances when stone of the same color and beauty was not available in the near vicinity. The design of building structures and elements must be adapted to the mineralogical, physical and mechanical properties of stone. The high compressive strength and the low tensile strength of stone require special techniques to overarch gateways and to erect vaults. Mediaeval builders succeeded in the erection of high and light structures like Gothic church choirs or spires could only with the help of hidden steel anchors to stabilize the construction. Only with the emergence of steel and reinforced concrete, the limits that stone properties pose to building structures are overcome and a new era of architectural building design began.

Most historic structures and many of our recent buildings have been constructed from natural stone. This chapter gives an overview of available natural stone resources and trends in building stone extraction. It documents the various uses of stone from an architectural point of view showing historic and recent examples on more than a hundred color photos depicting construction periods from prehistoric to recent times. Besides describing the uses of stone, the chapter also provides information on the main rock-forming minerals, their properties and classification, which enables an easier identification of the various stones. Wealthy illustrated sections outline the main rock groups from igneous, metamorphic to sedimentary rocks, allowing the reader to understand their origin; to recognize various rock types and compare their potential use. Fabric differences, colors, shades and tints of stones and their appearance on facades are also illustrated helping the reader to distinguish between various types of commercially sold “granites” and “marbles”. By providing detailed descriptions on most stone types with an explanation on their origin, mineralogy, fabric and their potential application, the chapter clarifies the misuse of commercial names and the improper use of stone in engineering and architectural practice. This is often derived from the misidentification of available stones, and limited knowledge of stone properties.

Since early antiquity dimension stones have been used as building materials due to their natural beauty and availability, and the diversity of their applications has increased ever since. As any other building material, dimension stones today have to fulfill the physical and technical requirements demanded by architects. This chapter focuses on the physical and mechanical properties of dimension stones, while emphasizing that stones are an old, but still modern building material. Among the parameters discussed here are water absorption, thermal conductivity and expansion, hygric and hydric properties, strength, abrasion, the more modern aspect of breaking load at the dowel hole, and ultrasonic wave velocities. Extensive data sets and a variety of case studies reveal relationships between the physical properties and the internal fabric elements of the dimension stones, such as sedimentary layering, metamorphic foliation, pores, and microcracks. In addition, these fabric elements are often responsible for the weathering behavior of the dimension stones, which not only affects the heritage but also the safety of modern buildings. This is illustrated through laboratory experiments and case studies.

It is generally assumed that stone is one of the most durable materials because it is compared to weaker building materials, such as wood or mud. But stone can deteriorate and many factors will affect it. The nature of the stone is critical in determining its resistance to the various deterioration factors. The most important one, salt, was already identified by Herodotus, nearly two and a half millennia ago. However, salt by itself is not damaging, it requires the presence of water for its aggressiveness to become evident. And water is needed for biocolonization to occur, for freeze-thaw phenomena and for wet-dry expansion. Control of this single factor can decrease significantly the deterioration potential of a stone and any structure built from it. The chapter aims to present a review of the most important deterioration processes and their effect on the various types of stones and rocks used by man. Among them are thermal effects, the influence of moisture, both as water vapor or in liquid state, the presence of salts and the damages that can be expected from biocolonization. The chapter also aims at identifying the areas where more research is needed to understand the actual deterioration mechanism of the various factors.

It has long been recognized that the environment exerts a wide range of pressures on building stone. These can be loosely thought of in terms of the physical pressures of climate, particularly water and temperature issues and the chemical attack by pollutants. Twentieth century concern was especially focused on attack by acid rain, but on longer time scales weathering is likely to be the most important mechanism of degradation. Normal meteorological variables of temperature and precipitation, while valuable in parameterizing climate impacts on stone, it is advantageous to combine or accumulate simple meteorological when assessing the effect of weather on building stone. Classic maps of climate offer some guidance about the geographic variability of pressures on stone, but these neglect air pollution, wind and relative humidity. Climate maps need to be tuned to the impacts on heritage issues. Air pollution has been present in cities for thousands of years, but the extensive use of coal and the sulfur dioxide produced when it was burnt was particularly aggressive towards stone. This sulphur is oxidized to sulphate and responsible for the disfiguring gypsum crusts that characterized stone buildings in the 20th century. Contemporary urban atmospheres have lower concentrations of sulfur dioxide, but nitrogen oxides and ozone are increasingly found. Along with these there are a range of organic acids and polycyclicaromatic compounds that potentially damage and discolor urban facades of the future. Climate change may also impose new threats to architectural stone.

In this chapter the main methods applied to the characterization of stone deterioration on buildings and objects of art are presented. The text begins with the classification of weathering forms. A short, illustrated glossary following the recent proposal of the ICOMOS (International Council on Monuments and Sites) is given, and the techniques and problems of mapping are discussed. On-site evaluation tests (non- and less-destructive) as well as laboratory investigations on material samples are explained. Their main goals, preconditions, side-effects and tools are demonstrated and illustrated with results from selected case studies. The focus is on routine methods such as moisture and salt analysis, and determination of mechanical, structural and hydric properties (drilling resistance, ultrasonic wave measurements, Karsten tube measurements etc.). Additional methods are briefly discussed and linked with the recent literature. A separate part is dedicated to classical biological methods of investigation as well as modern molecular techniques.

Since antiquity, replacement and repair of damaged stone has been practised to delay the deterioration of buildings and monuments. Today, the aim of stone conservation is the preservation of these historic and/or artistic objects for future generations in the state in which we have received them. The approach that has been taken to address this challenge has direct links to the emergence of chemistry in the 19th century. Chemicals such as water glass, fluorosilicates and ethyl silicate were tested as consolidants for stone shortly after their synthesis in the laboratory. After World War II, organic compounds, such as acrylic and epoxi resins found their way into conservation practice. This chapter deals with all the steps required in a conservation intervention, such as the problems presented by cleaning as well as those of desalination. It includes a review of conservation materials, such as consolidants, water repellents and biocides. Silicon organic compounds are given special attention since they are the main chemicals used in the formulation of both consolidants and water repellents. The requirements for the various mortars that may be needed, including renders, as well as that of other finishes such as paints and antigraffiti coatings are also discussed.