Lectures on Materials Science for Architectural Conservation
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Lectures on Materials Science for Architectural Conservation Lectures on Materials Science for Architectural Conservation Giorgio Torraca The Getty Conservation Institute Los Angeles ©2009 J. Paul Getty Trust The Getty Conservation Institute 1200 Getty Center Drive, Suite 700 Los Angeles, CA 90049-1684 United States Telephone 310 440-7325 Fax 310 440-7702 E-mail [email protected] www.getty.edu/conservation Production editor: Angela Escobar Copy editor: Dianne Woo Designer: Hespenheide Design ISBN: 978-0-9827668-3-5 (print on demand) The Getty Conservation Institute works internationally to advance conservation practice in the visual arts—broadly interpreted to include objects, collections, architecture, and sites. The GCI serves the conservation community through scientific research, education and training, model field projects, and the dissemination of the results of both its own work and the work of others in the field. In all its endeavors, the GCI focuses on the creation and delivery of knowledge that will benefit the professionals and organizations responsible for the conservation of the world’s cultural heritage. Contents vii Foreword Giacomo Chiari ix Preface Part 1 1 Electronegativity, Chemical Bonds, Crystals, Molecules, and Chemical Reactions 1 1.1 Electronegativity 2 1.2 Chemical Bonds 8 1.3 Properties of Materials as a Function of the Bond Type 14 1.4 Molecules 21 1.5 Chemical Reactions 30 1.6 Physical Transformations—Solutions and Emulsions 36 1.7 Hydrophilic and Hydrophobic Materials Part 2 38 Mortars, Bricks, and Concretes: Earth, Gypsum, Lime, and Cements 38 2.1 Earth as a Building Material 43 2.2 Ceramic Materials 47 2.3 Gypsum 50 2.4 Lime and Lime Mortars 54 2.5 Pozzolanic Mortars 58 2.6 Hydraulic Lime 61 2.7 Cement 65 2.8 Modern Concrete 67 2.9 Reinforced Concrete 69 2.10 Compatibility Problems Related to the Use of Cement in Architectural Conservation Part 3 72 Deterioration of Porous Building Materials 72 3.1 Mechanical Deterioration Processes 81 3.2 Physical Processes of Deterioration of Porous Materials 87 3.3 Chemical Deterioration Part 4 96 Conservation of Architectural Surfaces 96 4.1 Basic Principles 97 4.2 Cleaning of Architectural Surfaces 102 4.3 Consolidation of Architectural Surfaces 107 4.4 Protection Part 5 110 Metals 110 5.1 Ferrous Metals 138 5.2 Notes on Non-ferrous Metals Relevant to Architectural Conservation Part 6 147 Natural and Synthetic Polymers 147 6.1 Polymers 149 6.2 Natural Polymers 152 6.3 Wood: A Short Note on Properties 154 6.4 Linear Synthetic Polymers—Thermoplastics 164 6.5 Cross-linked Synthetic Polymers—Thermosetting Resins 173 6.6 Aging—Oxidation of Organic Molecules Part 7 175 Silicates, Silanes, and Silicones 175 7.1 Silicates and Fluosilicates 180 7.2 Silanes 182 7.3 Silicones 186 Bibliography 194 About the Author Foreword “Everything happens at the atomic level,” I used to tell my students. This means that if a bridge collapses, ultimately it is because a few atoms have let go of their bonds and started a small crack that continued to expand, resulting in disaster. If this paradox is true, an in-depth understanding of the mechanisms at work on a microscopic level is fundamental to the successful work of engineers and architects. The difficult part is bridging the gap between the microscopic and macroscopic lev- els from the atom to the building. Giorgio Torraca does this superbly. For many years the Getty Conservation Institute has applied the expertise of scientists and conservators in bridging that same gap. The study of the mechanisms of salt crystallization and salt extraction in order to save thousands of square feet of mural paintings in the Mogao grottoes is a typical example. Other examples include the GCI’s research into the influence of clay expansion with water and its effects on limestone in projects involving the conservation of churches and cloisters in Yorkshire and the great Maya pyramids at Copán. All of these conservation endeavors require the merging of knowledge from various branches of science. Torraca’s ability to synthesize concepts and knowledge from various fields and present them in plain, comprehensible fashion to the reader is remarkable. His previous books, Porous Building Materials and Solubility and Solvents for Conservation Problems, are the fundamental texts on which several generations of cultural heritage professionals have been educated. A characteristic that these books share with the present volume is the apparent unrefined quality of the figures and drawings. In an era of computer imaging, Torraca still draws his pictures by hand—a brilliant move that allows each illustration to convey the required concept with precision, clarity, and simplicity. Nothing is redundant. Giorgio Torraca has been my mentor, colleague, and friend for more than forty years. During this time I have had the opportunity and good fortune to appre- ciate and benefit from his ability to tackle complex problems and immediately get to the core of them. This is what the reader will find in his Lectures on Materials Science for Architectural Conservation, which the GCI presents in the same spirit of bridging the fields of science and conservation. I am sure that architectural con- servators, engineers, and conservation scientists not only will enjoy this work but will be enriched by the formative ideas presented within it. Giacomo Chiari, Chief Scientist The Getty Conservation Institute March 2009 Preface This text is based on notes and sketches I prepared for an undergraduate course titled “Chemistry of the Environment and of Cultural Property,” which I taught at the “Valle Giulia” Faculty of Architecture, University of Rome “La Sapienza,” from 2001 to 2004. The lecture notes were published in 2002 by the Scuola di Specializzazione in Restauro dei Monumenti, which kindly allowed the use of the material for an English version. The English text is not truly a translation because my intent was to find equivalent ways to express the concepts in a new language and not to translate the words; furthermore, several parts have been revised and some completely rewritten. This work was produced with the support of the Getty Conservation Institute, and I am deeply grateful to Leslie Rainer for her accurate review of the text, pin- pointing errors and suggesting improvements in the language, and to Giacomo Chiari for his enthusiastic support and suggestions (which would have increased the size of this text considerably had I the strength to carry out all of them). In the Rome lectures, the chapters were organized according to the system used in the textbooks on materials science, starting with a summary of the scientific theory of the structure of materials, with some basic chemistry added as required by our field of interest. This order is maintained in the present version, but with some reservations on my part as, having taught technology to engineers and post- graduate architects for a long time, I know how allergic to chemistry they are; so, starting a book with a chapter that is essentially chemistry did not appear to be the best way to encourage a reader to advance further. At some point I came to the conclusion that it would have been wise to rele- gate the chemistry to an appendix, but it was late in the project and I lacked the courage to do so mainly because it would have required renumbering all chapters and sections and correcting all cross-references (I use a lot of them), and most likely would have resulted in several errors. As an alternative, I have a suggestion for the chemistry-wary reader: Start reading at part 2, using part 1 mainly for reference when encountering words or concepts with which one is not familiar. I have tried to support this method of reading by providing cross-references to relevant sections in part 1 whenever I thought that such a problem might arise. In the Rome lectures, I tried to downplay the role of chemistry in the course by reducing its importance in the final exam; the students were told that the (oral) exam would start with a question on building materials and their properties, dete- rioration, and conservation (parts 2, 3, and 4, respectively), followed by a question on metals, corrosion, and conservation (part 5); then, for the last of the traditional x Preface three questions on Italian university exams, they would have to choose between structure of materials plus basic chemistry (part 1) and modern plastics (part 6), silicates, and silicones (part 7). This system worked well because most students, encouraged by some success on the first two questions, managed to address the third without excessive damage. The fact that a vast majority chose structure and chemistry showed, however, that plastics was even more difficult for them, even if it is a more interesting topic to an architect. In the present version, I attempted to reorganize parts 6 and 7 to improve readability, but still they are not as smooth and clear as they should be. My problem in teaching technology is that I think the aim should be to pro- vide ideas rather than information; although information is easily available in handbooks and on the Internet, what is missing for a student or a professional are the general concepts that allow him to organize the material in his mind so that he is able to pass an exam or use the information when evaluating problems on a drawing table or at a worksite. In the case of modern plastics, the amount of information available is enor- mous, but it is not easy to extract from it guidelines that an architect or an engi- neer could use when evaluating their successes and their failures (e.g., simple models of molecular structures and relation between structure and properties).