Glazed and Confused: Exposing the Mysteries of Glazed Architectural Terra Cotta

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AbstractGlazed and Confused, is a compilation of two investigative studies. The first is a chronological annotated bibliography of physical property experimentations interspersed with key historical moments pertaining specifically to glazed architectural terracotta. The second is an experimental model created for architectural conservation, to be utilized prior to conservative treatments. This thesis began with wanting to test the commercially available repair and patching mortars employed during terracotta repair, hoping to discover which performed most similar to glazed terracotta. However, after speaking with several practicing conservators within the United States, it quickly became apparent there was a lack of knowledge surrounding the basic physical properties and behaviors of architectural terracotta. It is impossible to judge a repair system using unknowns as criterion, consequently the topic was shifted to discovery of previously established properties, the procedures used to procure this information, when theses experimentations were completed and why. Using the ascertained historical information a research program was developed specifically focused towards architectural conservation, with the goal of creating a program of examination with the findings aiding in the choice of conservation treatments. The model included five laboratories: visual analysis, petrographic analysis, cross-section inspection, water vapor transmission and inverted cup water vapor transmission. The historic documentation research exposed a wealth of material testing programs done and presented through The American Ceramic Society, The National Terra Cotta Society and The National Bureau of Standards, all of which dissipated by 1961, when the last document was produced. There are currently no required testing programs for manufactures of glazed architectural terracotta in the United States, and consequently no required specifications for material performance.The materials testing program was employed on six historic terracotta blocks fabricated between 1910 and 1921, revealing that any defect in the glaze fit, such as crawling and/or crazing results in an increased rate of water vapor transmission. The microscopic examinations divulged information pertaining to the varying vitrification levels of the glaze and body. The petrography and cross-section analysis proved that crazing of a glaze can continue into the body of the block and also that crawling exposes the body to exterior elements, in addition the cross sections showed that every specimen had a glaze layer thickness of 200 m, suggesting mechanical application methods. Full Title: Glazed and Confused: Exposing the Mysteries of Glazed Architectural Terra Cotta.Author: Xsusha Carlyann Flandro Graduation Date: May 2009 Thesis Advisor: Norman Weiss

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<p>FL A N 20 D R 09 OBy Xsusha Carlyann Flandro1- FLANDRO</p> <p>GLAZED AND CONFUSED Exposing the Mysteries of Glazed Architectural Terra Cotta</p> <p>Submitted in partial fulfillment of the requirement for the degree Master of Science in Historic Preservation</p> <p>Graduate School of Architecture, Planning and Preservation Columbia University May 2009</p> <p>2- FLANDRO</p> <p>FL A N 20 D R 09 O</p> <p>Permission to Copy Copyright 2009, Xsusha Carlyann Flandro. For information about this work, please contact Xsusha Flandro, c/o Jennifer Flandro 277 Washington Avenue Apt. #2K, Brooklyn, NY, 11205. Permission is hereby granted to reproduce and distribute copies of this work for nonprofit educational purposes, provided that copies are distributed at or below cost, and that the author, source and copyright notice are included on each copy. This permission is in addition to rights of reproduction granted under Sections 107, 108, and other provisions of the U.S. Copyright Act. Before making any distribution of this work, please contact the copyright owner to ascertain whether you have the current version.</p> <p>FL A N 20 D R 09 O3- FLANDRO</p> <p>4- FLANDRO</p> <p>FL A N 20 D R 09 O</p> <p>AcknowledgementsThere are many people who need to be thanked for their consistent pressure on me to do a good job and their ability to help me along by supplying me with their continual support, knowledge and expertise. Namely, Professors Norman Weiss and Dr. George Wheeler, whose enthusiasm for teaching and interest in material conservation continually amazes me. Judy Jacob, for offering her time to assist me in my research and serving as my thesis reader. Professor Richard Pieper who showed me that it is a gift to be obsessed with one material. And of course my family who have continually supported, mocked and humored me throughout my full-time ten-year college educational path.</p> <p>FL A N 20 D R 09 O5- FLANDRO</p> <p>6- FLANDRO</p> <p>FL A N 20 D R 09 O</p> <p>AbstractGlazed and Confused, is a compilation of two investigative studies. The first is a chronological annotated bibliography of physical property experimentations interspersed with key historical moments pertaining specifically to glazed architectural terracotta. The second is an experimental model created for architectural conservation, to be utilized prior to conservative treatments. This thesis began with wanting to test the commercially available repair and patching mortars employed during terracotta repair, hoping to discover which performed most similar to glazed terracotta. However, after speaking with several practicing conservators within the United States, it quickly became apparent there was a lack of knowledge surrounding the basic physical properties and behaviors of architectural terracotta. It is impossible to judge a repair system using unknowns as criterion, consequently the topic was shifted to discovery of previously established properties, the procedures used to procure this information, when theses experimentations were completed and why. Using the ascertained historical information a research program was developed specifically focused towards architectural conservation, with the goal of creating a program of examination with the findings aiding in the choice of conservation treatments. The model included five laboratories: visual analysis, petrographic analysis, cross-section inspection, water vapor transmission and inverted cup water vapor transmission. The historic documentation research exposed a wealth of material testing programs done and presented through The American Ceramic Society, The National Terra Cotta Society and The National Bureau of Standards, all of which dissipated by 1961, when the last document was produced. There are currently no required testing programs for manufactures of glazed architectural terracotta in the United States, and consequently no required specifications for material performance. The materials testing program was employed on six historic terracotta blocks fabricated between 1910 and 1921, revealing that any defect in the glaze fit, such as crawling and/ or crazing results in an increased rate of water vapor transmission. The microscopic examinations divulged information pertaining to the varying vitrification levels of the glaze and body. The petrography and cross-section analysis proved that crazing of a glaze can continue into the body of the block and also that crawling exposes the body to exterior elements, in addition the cross sections showed that every specimen had a glaze layer thickness of 200 m, suggesting mechanical application methods. Full Title: Glazed and Confused: Exposing the Mysteries of Glazed Architectural Terra Cotta. Author: Xsusha Carlyann Flandro, May 2009 Advisor: Professor Norman Weiss</p> <p>FL A N 20 D R 09 O7- FLANDRO</p> <p>8- FLANDRO</p> <p>FL A N 20 D R 09 O</p> <p>PrefaceThere are literally hundreds of books and articles written about architectural ceramic uses in the United States. My goal here is not to rehash what has already been stated but rather to survey the descriptive written works of testing procedures used throughout history to study the physical properties of glazed architectural terra cotta. A brief historical introduction outlining the use of glazed terracotta in the United States has been included as a necessary means to the evaluation of terra cotta. My research had two main goals, first, to compile a history of glazed terracotta manufacturing and testing procedures completed in the United States and secondly, armed with the knowledge that there is currently no published data with regards to the rate of water vapor transmission of glazed terracotta experiencing crazing and crawling, to perform this relatively simple test and publish the findings. The hope is that through this test and very basic microscopic examination of the material the value in performing a few basic tests prior to restoration or replacement of terra cotta pieces will be exposed and become a best practice in the conservation field. In addition, credit has to be given to Harry G. Schurecht (1893-1968), a ceramic chemist who tested terracotta with such fervor and persistence, that I strongly believe without him architectural terracotta would have never have reached its peak.</p> <p>FL A N 20 D R 09 OMr. Harry G. Schurecht11</p> <p>As those have done before me the term terracotta has been hyphenated when used as an adjective, but not when used as a noun.</p> <p>Image from. Terra-Cotta Skyline by Susan Tunick, 1997, page 135.</p> <p>9- FLANDRO</p> <p>10- FLANDRO</p> <p>FL A N 20 D R 09 O</p> <p>TABLE OF CONTENTS</p> <p>INTRODUCTION.....13 A Short History of Architectural Terra Cotta in the United States.....18 SECTION I: Previous Research and Key Historical Moments.24 SECTION II: Material Testing...............92 Testing Methodologies............95 Extracting Samples........95 Specimen labeling..........96 Visual Analysis.......................96 Petrographic Analysis............96 Cross Section Analysis...........97 Water Vapor Transmission Test..............98 Inverted Cup Water Vapor Transmission Test............99 Normalized Data......................................................................................100 Water Vapor Transmission Comparative Analysis...................................101 Terra-cotta Block #1 Test Results........102 Terra-cotta Block #2 Test Results....109 Terra-cotta Block #3 Test Results....118 Terra-cotta Block #4 Test Results........125 Terra-cotta Block #5 &amp; #6 Test Results.......130 Water Vapor Transmissions Comparative Analysis.....137 Material Testing Conclusion............................141 RECOMMENDATIONS.145 CONCLUSION149 BIBLIOGRAPHY152 LIST OF FIGURES.170 CERAMIC NOMENCLATURE......................................................................................176 APPENDIX..185 Water Vapor Transmission Raw Data......186 Inverted Cup Water Vapor Transmission Raw Data.....189 Rate of Water Vapor Transmissions.........192 Comparative Analysis Water Vapor Transmission Rates.....194 Dixons Q-Test Mathematics............202</p> <p>FL A N 20 D R 09 O11- FLANDRO</p> <p>12- FLANDRO</p> <p>FL A N 20 D R 09 O</p> <p>FL A N 20 D R 09 OINTRODUCTION13- FLANDRO</p> <p>FL A N 20 D R 09 OGlazed architectural terracotta on 154-160 West 14th Street, New York City (ca 1912).14- FLANDRO</p> <p>The initial plan for this thesis was to evaluate patching materials used in the United States for the repair of damaged glazed architectural terracotta. With two major brands being marketed in the United States, the goal was to determine which one was the most effective and the pros and cons of each product, hoping that this information would lead to better repairs. Through a series of conversations and a search of historical references, it was found that little had been recently completed to determine the physical properties of terra cotta. It is hard to judge the quality of a repair when the properties of the original fabric are unknown. An exhaustive research project was undertaken to determine what experiments had been previously completed on glazed architectural terracotta to establish its physical and mechanical properties. From this research an experimental design was modeled specifically for glazed terracotta materials conservation and tried out on pieces of glazed terracotta taken from four historic buildings, in hopes that the information revealed from this experiment would be sufficient to make more informed decisions prior conservation treatments. In the United States there are two main companies that produce repair mortars for terra cotta, Cathedral Stone Products (also known by their German name, Jahn) and Edison</p> <p>Coatings Incorporated. Each company has excellent and readily available representatives, as well as external and on-site training sessions (a three day training course is required for the purchase and use of Cathedral Stone Products). Cathedral Stone Products terracotta repair mortar is Jahn M100 to be used in conjunction with Benjamin Moores AURA line of paints to replicate glaze sheen (color is achieved by the matching of the mortar through the addition of synthetic pigments). For varying sheens AURA Low Lustre 634, Semi Gloss 632 and Flat 629 are used. During the time of this research and shortly before its publication, Cathedral Stones repair mortar, Thin-Set 125, used for the repair of shallow glaze spalls and their own system of glaze replication, TerraCoat Glaze Repair, was removed from the market due to reoccurring improper installation and subsequent failures. The claim the company makes about M100 is that it is completely water permeable and contains no latex or acrylic bonding agents or additives.1 Their goal is to match the physi-</p> <p>FL A N 20 D R 09 O1</p> <p>cal properties of the substrate to allow salts, water vapor and liquid water to penetrate the repair. While the patching mortar may be compatible with the substrate once the addition of AURA is applied this no longer maters. AURA paints are 100% Acrylic latex; thereby eliminating any water vapor transmission from the rear of the terracotta through the front. This fact was discussed with the laboratory personal at Cathedral Stone and it was revealed that they are aware of this and are unconcerned, as terracotta glazes are believed to be impermeable, so the paint mimics the impermeable characteristic of ceramic glaze. Edison Coatings has a variety of products available for glazed terracotta repairs, but the most common ones used are: Thin-Fill 55 for filling shallow losses (1/32 to 1/8 deep), Custom System 45 TC or Custom System 45 for deeper repairs, Elastowall 351 and Aquathane UA210 type NCL used for glaze replications and Aquathane UA210-E and AquaSpex 220 for glaze detail replication (such as speckles, smears and mottling inherent in some terra-cotta glazes). Edisons claims about their products are that they are latex-modified mortars, which allows them to be tough, durable, have excellent water vapor transmission rates, high tensile bond strengths, and low modulus of elasticity ensuring good adhesion. In addition the latex allows for achievement of incredibly smooth finishes (the repairs can be polished with 400 grit sand paper) and they are available in over 900 standard colors. Their goal is to allowCathedral Stone website, product literature for Jahn Repair Mortar M100. www.cathedralstone.com, 2009.</p> <p>15- FLANDRO</p> <p>the repair to move with the substrate, eliminating stresses in either material. The glaze replication products are waterborne polyurethane based coatings with varying degrees of hardness and permeability. They can be applied once the repair mortar has set. Once the differences between the two different repair mortars were determined, several architectural conservators throughout the United States were interviewed. The goal of these interviews was to reveal which product was used more and how the decision to go with a specific product was made, was it workability? Durability? Water vapor transmission rate? Color? Inquiries were made specifically about testing procedures used prior to deciding which product to purchase. The interviews revealed a national phenomenon among conservators as well as a lack of knowledge about the basic properties and mechanics of polychrome terracotta. Choices between the two products were being made based on best business practices, with little or no materials testing being done at all. The company that the conservators choose to supply the treatment materials was synonymous with the company that could provide the best warranty on their product (this also allowed for one supplier to be held libel in the event of premature failure). The thought arose that perhaps the physical properties of glazed terracotta had already been determined and therefore the material testing of historic fabric was not required prior to repair. Research was done for any publications stating the physical properties of glazed architectural terra cotta and any testing procedures completed to determine said properties, a particular focus was given to glaze fit tests. This research was compiled chronologically to display the evolution of glazed terracotta manufacturing and testing programs (who knew what and when). The most recent publication retreived was printed in 1961, by the American Institute of Architects and had a specific focus towards ceramic veneer rather than terracotta. It was also revealed that there are currently no required ASTM (American Society for Testing and Materials International) standard tests pertaining specifically to the physical properties and glaze fit on architectural terracotta and consequently no minimum material standards that have to be met in the current manufacturing of glazed architectural terracotta. In addition while the ASTM has standardized some nomenclature pertaining to ceramic materials, the document is not thorough, to help resolve this issue a new glossary of ceramic terms was compiled and can be found in the appendix. Using the information discovered in t...</p>