Reglazing Modernism


Angel Ayón, Uta Pottgiesser and Nathaniel Richards


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The following is an excerpt from the recently published book, Reglazing Modernism - Intervention Strategies for 20th Century Icons, by Angel Ayón, Uta Pottgiesser and Nathaniel Richards, which was awarded the 2021 Lee Nelson Book Award from the Association for Preservation Technology International (APT). The publication was recognized for being the most outstanding and influential book-length work on preservation technology. 

Reglazing Modernism provides 20 in-depth case studies of Modern architectural icons in both Europe and the Americas. Focusing on interventions to their steel-framed glazing assemblies, the book offers a critical assessment of these, while also exploring emerging technologies that may offer higher performance in the future. Visit to learn more or to order a copy.


Modernism is the most defining architectural expression of the 20th century. The sheer quantity of Modern-era buildings world-wide is in itself impressive, especially considering that more build-ings were built in the 20th century than in all preceding ages com-bined.1 From the first late-19th-century examples whose stylistic vocabulary started to depart from Classical architecture to the most recognizable Modern works from the post-WWII period, Modernism transformed the built environment across the globe unlike any other previous period in civilization. The issues affecting these buildings today are increasingly at the forefront of discussions about sustainability, heritage conservation and building sciences, particularly in light of current global challenges like climate change and the need to increase energy efficiency of the built en-vironment.

Suffice it to say that, along with notable quantity and cultural significance, poor performance and limited longevity are also regular hallmarks of Modern buildings. More often than not, these deficiencies involve what is unquestionably one of the most character-defining features of Modern architecture—the exterior glazed enclosures. As a result, the need for interventions to address un-desirable conditions at the frames, glass panes and other components of Modern glazed assemblies—referred to in this book ge-nerically as reglazing—is ever growing. However, unlike historic materials and assemblies from earlier periods and other architec-tural expressions, whose intervention criteria has been well established for decades in both the academic and professional fields, there are no proven guidelines validated by time-tested practices that can help to evaluate the appropriateness of past interventions or guide practitioners on current or future projects directed at reglazing Modernism. This book is the first to compile and present critical assessments of a range of intervention approaches to reglazing Modernism. Its goal is to help to fill an informational and analytical void within the profession, and ultimately to help define best practices for intervening on Modern glazed enclosures.

The research presented in this publication is based on the analysis of 20 case studies—nine in the US and eleven in Europe—that together exhibit a wide range of construction typologies and interventions on single-glazed steel frame curtain wall and window wall assemblies. Most of the case study buildings have been retrofitted within the last ten to 15 years in response to one or more driving forces, leading to different intervention approaches—broadly categorized in this book as restoration, rehabilitation or replacement. These categories, the interventions and their moti-vating factors—such as increasing energy performance or comfort requirements, safety and security concerns, changing functions, and reuse, or material decay, as well as stipulations of historic preservation and heritage conservation—are part of a general discussion in the Intervention Categories section that leads to the presentation of the case studies themselves. In each case study, the book highlights through research and illustrations how the glazed enclosures contribute to the cultural significance of each building, what the conditions were before and after the interven-tions and offers the authors’ opinions on the outcome.

With regard to the selection of the case studies, it was clear early on in the research that reglazing Modernism is far too wide a topic to address in one book. Several strategic choices were made in order to select representative case studies. While these are pri-marily explained in their own section preceding the case studies, the critical decision to focus mainly on steel frame Modern glazed enclosures is worth noting. There are a wide range of Modern glazed assemblies built with frames made of wood or other metals (like aluminum, bronze or even other types of steel, such as stain-less or weathering steel). However, the most commonly encoun-tered assemblies on low-rise, low-density, highly significant Mod-ern buildings are those built with mild steel. Clearly, research on the intervention approaches to other types of Modern glazed as-semblies is still needed, and we hope that this book may be only the first of its type in a series that can go on to explore the range of intervention approaches and issues unique to glazed assemblies made out of other materials.

The book concludes with sections summarizing critical findings and pointing to emerging trends derived from the analysis of the case studies. Topics where additional research and development is required are noted, as are high-performance systems that are becoming available in the marketplace as a response from the fenestration industry to the challenges posed by some of the inter-ventions presented in this book. Relevant bibliographic information is also provided so that readers can further explore each case study on their own. While we expect the analysis and criteria included in the book to be of use to both professionals and students, we hope that the graphic information and supporting 3D models will also be of interest to a wider audience.

Case Study: Zeche Zollverein

Essen, Germany
Fritz Schupp and Martin Kremmer, 1932/1961

The first coal mine shaft near Essen was opened in 1847 and was named Zollverein after the German Customs Union, Deutscher Zollverein, which had been founded in 1834. By 1914, the Katernberg area near Essen, in the state of North Rhine-Westphalia (NRW), had become home to four independent mines, with a total of eleven shafts. In order to ensure its economic survival after WWI, the plant systems were consolidated and mechanized for efficiency and high performance with largely automated work processes.(1) Zollverein Shaft XII was designed for this purpose in 1927 by German architects Fritz Schupp (1896–1974) and Martin Kremmer (1894–1945) and completed in 1932. Together with the corresponding above-ground structures, it was designed to improve the system of coal extraction and processing. Undoubtedly inspired by the Bauhaus, which had moved in 1926 to Gropius’ building in Dessau-Roßlau, Schupp and Kremmer’s design for the industrial buildings embodied the school’s shift towards Cubism and Functionalism. Their double-truss pit frame for Zeche Zollverein has become an icon of the region’s industrial culture. The architects created a symmetrical and geometrical complex arranged along two axes, which resulted in a unique industrial plant model, often considered to be the “most beautiful mine in the world.”(3) Fritz Schupp used the same architectural vocabulary for the coking plant which was completed in 1961. Zeche Zollverein was the largest and most powerful coal processing plant in the world after 1932 and right up until the mine closure in 1986. The coking plant remained open until 1993.

In 2001, the complex was included in the UNESCO list of World Heritage Sites, cited as being “a monument of industrial history reflecting an era in which, for the first time, globalization and the worldwide interdependence of economic factors played a vital part.”(4) The World Heritage designation also states that Zeche Zollverein’s buildings are “outstanding examples of the application of the design concepts of the Modern Movement.”(5) Exceptional value was attributed to the Bauhaus-influenced architecture of the industrial complex, which for decades provided the model for modern industrial construction.(6) Today it is one of the anchor points of the European Route of Industrial Heritage and the whole industrial complex is an ideal place to learn about the mining history and development of industrial architecture in one of Europe’s most significant industrial regions.(7)

Condition Prior to Intervention

The exterior architecture of Zeche Zollverein is characterized by large expanses of red brick infill laid within the orthogonal grid of its exposed post-and-lintel steel framework (which is painted red). Set within the masonry infill are wide bands of fixed and operable steel-framed ribbon windows that are single-glazed with 1/4″ (6mm) thick cast glass to provide diffused daylight to the interior. A structural skeleton system is used throughout most of the complex, consisting of full building-height steel portal frames (that remain exposed) and a concrete-encased steel frame construction forming the floor and roof decks.

The thin exterior single-wythe brick wall construction and the non-thermally broken single-glazed steel frame windows have poor thermal performance, making it more difficult to adapt the facades to other uses. Fortunately, the buildings were well-maintained during the years between 1986 and 1993, and there was no significant damage before the interventions began. It also helped that some of the buildings in the complex were used for temporary cultural events starting as early as 1987, and so had been kept in good condition.


Most of the buildings in the complex were refurbished (primarily by architects Heinrich Böll and Hans Krabel) and reused for permanent cultural and historical purposes in 1989.8 Each building was treated differently, which has made the complex a showcase of facade refurbishment at its best. Interventions have included: simply restoring the steel profiles with the existing single-glazed panes or with new wired glass panes (Halls 2 and 5); adding new slim IGUs partly with wired glass panes (Hall 7); installing a thermally broken frame with IGUs with wired glass (Hall 9); and installing insulated partitions with secondary glazing consisting of single-pane steel frame casement windows set within an interior insulated partition built parallel to the existing exterior walls, thereby creating a large insulating air cavity (Halls 6 and 10).

Since 1992, the former central workshop (Hall 5) has been used as an exhibition space that required only moderate heating up to 14°C maximum. This circumstance maintains the building’s austere industrial character internally and allowed the steel frame windows to be restored with their original single glazing. In nonglazed areas an additional interior layer of lightweight perforated bricks with a lime cement mortar finish was added to the inboard side of the exterior walls to improve thermal performance. The hall famously hosted the work of Ulrich Rückriem as part of the Documenta 9 art exhibition in 1992. Between 1996 and 1997, the Boiler House (Hall 7) and the former Low Pressure Hall (Hall 9) were transformed into the Red Dot Design Museum and the restaurant Casino Zollverein. Both adaptations demanded higher comfort requirements. At Hall 7 (museum), the approach to the exterior was to restore the building facade and remove several later additions in order to reveal its original form, while in the interior “the heavy industrial feel of the building was maintained.” The original single glazing was replaced with new IGUs within the existing steel frames (installed from the interior to prevent the costly use of scaffolding). For economic reasons, the interior is only moderately heated to a maximum of 16°C. Altogether, the interventions maintain the calm and homogeneous industrial appearance both inside and outside.

There were higher requirements for indoor climate and thermal comfort for the restaurant in Hall 9. To address this, a replacement thermally broken cold-formed steel frame window system with 1″ (25mm) thick IGUs was developed using the same sightlines as the existing windows. The new windows were installed in a new interior stud wall that was built parallel to the existing exterior masonry wall. The new windows align with the original single-glazed steel frame units and provide them with secondary glazing to deliver a high level of thermal protection while maintaining the industrial interior appearance. Some of the original single-glazed wired glass in Hall 9 was replaced with transparent IGUs in an effort to enhance a visual connection to the exterior.

The same intervention was implemented at the former electrical workshop (Hall 6) when it was converted into an exhibition hall and store. To allow for the additional interior stud wall and secondary glazing, the inner pumice-concrete block facing was removed and supplemental rails were installed to anchor and secure the historic facade. Overall, the steel post-and-lintel framing retained its structural function and refurbishment was not necessary for the most part. The exception was at the perimeters of the ribbon windows, gates and door openings of the existing postand-lintel structure which were refurbished “by applying a complete rustproof coating or by replacing individual members.” All other facade steel profiles were sandblasted and coated on their exposed surfaces only.

A different approach of additional interior layers was chosen for the former warehouse (Hall 10). There, a box-within-a-box con-cept was utilized to accommodate permanent spaces such as workshops, stores, classrooms, offices, changing rooms and sani-tary facilities. By constructing a new thermally insulated structure within the existing hall, it was possible to leave the historic facade intact without any modifications. The corridors between both structures served as a thermal buffer and are used seasonally for teaching and administrative purposes.

The coal mine complex and the coking plant have been owned by the Zollverein Foundation since 2009 and 2010 respectively. New buildings have been added to the adapted historical buildings at the complex, extending its new cultural functions. De-signed by the Japanese architectural office Sanaa, the ‘Zollverein Cube’ was built in 2006, the first new building at Zollverein in 50 years. Its thin rectangular facades, cubic shape and the oversized room heights are reminiscent of the iconic designs of Schupp and Kremmer. The Zollverein Cube was used by different design schools until 2017, and today is part of the Folkwang Zollverein World Heritage Campus, serving as a starting point for a future “design city” aimed at building up a larger design campus. In 2017, MGF Architects designed a new campus in the Northern Quarter to host the design department of the Folkwang University of the Arts. The new building’s orthogonal facades made of flush grey galvanized steel plates interpret Zeche Zollverein’s industrial char-acter qualitatively rather than imitating it.


The variety of approaches to the rehabilitation of the steel frame glazed enclosures at Zeche Zollverein is unique within the field of conservation of Modern architecture. The number of different in-terventions is directly related to the diversity of uses and new functions added to the complex over time, with each program re-quirement resulting in a unique tailored approach. As the industri-al complex was still in use and not modified prior to the first inter-vention, the original steel frame glazed enclosures were mostly intact. For this reason, Zeche Zollverein’s glazed enclosures are essentially an authentic as-built documentation of its original de-sign—characterized by its large-scale uniform detailing and mate-rial usage.

The challenge for the rehabilitation and conservation of the complex was not in the identification of the original members and materials, but rather in the decision-making process around adapting the “simple” industrial envelopes to the new uses and functions, each with different requirements and standards that had to be taken into consideration. Throughout the process, it was critical that the complex’s recognizable exterior appearance, with its unique and elegant industrial materiality, be retained. To accomplish this, each building and its interventions were treated differently, particularly when it came to their specific thermal re-quirements, where a range of interventions to the glazed enclosures were applied. As part of the conservation plan devised for the complex, the interior spaces were also left as much in their original state as possible, often with remaining equipment in place, in order to retain and highlight the authentic industrial at-mosphere. Across the entire complex only limited architectural interventions were made to adapt to the new functions, adding elements such as new staircases and elevators (like the large ex-ternal escalator leading to the Ruhr Museum at the coal washing plant, for example) and sometimes creating new levels to provide access into the former industrial areas and to specific functions, such as new kitchens and offices.

The diverse solutions implemented throughout the complex range from subtle restorations that are hardly noticeable to new additions that are clearly readable (such as the new interior walls and secondary glazing which are visible from the exterior due to their silver aluminum finishes). These localized changes in the his-toric appearance do not distract from experiencing the complex’s unity as an architectural ensemble though. The long-term effort of Heinrich Böll and Hans Krabel as the architects-in-charge is cer-tainly commendable—under their guidance, selective and well-purposed modifications and new additions have been under-taken. Many suitable approaches were created and implemented with a high degree of technical craft and skill, which has been re-warded with several architectural accolades.



First coal mine shaft was cut


Zeche Zollverein had grown to four independent pits with ten shafts


Operations are consolidated;Shaft XII is built


Coking plant is completed


Zeche Zollverein closes and site is bought by the State of North Rhine-Westphalia


Some buildings used for cultural activities


Böll and Krabel Architekten are brought to oversee work (Halls 2, 5, 6–10, 12, 15, 21)


Halls 5 and 6 are transformed into event spaces


Coking plant closes; Hall 2 and Hall 10 are adapted into workshop and office spaces; Heritage Trail ZOLLVEREIN® is established


Hall 12 converted into studios and rehearsal stage


Hall 9 transformed into Casino Zollverein restaurant; Hall 15 converted into heating plant


Hall 7 Boiler House transformed into the Red Dot Design Museum by Foster and Partners


Stiftung Zollverein established to further develop the site; Hall 21 transformed into Folkwang Media Institute Interartes


Listed as a UNESCO World Heritage Site


Master plan by OMA/Rem Koolhaas


Zollverein Cube by Sanaa is completed and coal washing plant transformed into Ruhr Museum and visitor center by OMA/Architect Böll


Hall 4 Hoisting Machine Hall converted into a restaurant by architect Böll


Design department of Folkwang University of the Arts by MGF Architects is completed



1. Zollverein, “What happened until now,” accessed 30 December 2018,

2. Frieder Bluhm, “Symbol für Zuversicht und Wandel. Unesco-Welterbe Zollverein in Essen,” Industriekultur (March 2013): p.31.

3. World Heritage Germany, “The ‘most beautiful coal mine in the world’: the Zollverein UNESCO World Heritage Site Tourist development, products and highlights,” last modified 21 January 2018, accessed 3 February 2019,

4. UNESCO, “World Heritage List - Zollverein Coal Mine Industrial Complex in Essen,” accessed 4 November 2018,

5. Ibid.

6. UNESCO. “UNESCO-Welterbe Industriekom-plex Zeche Zollverein in Essen: Industriedenk-mal im Stil des Bauhauses,” accessed 31 December 2018,

7. For further information, see (home page of the Zollverein complex) and (home page of the European Route of Industrial Heritage).

8. Heinrich Böll and Hans Krabel, Arbeiten an Zollverein. Projekte auf der Zeche Zollverein Schacht XII seit 1989 (Essen: Klartext-Verlag, 2010), p.147.

9. Foster and Partners, “Red Dot Design Museum,” accessed 7 December 2018,

10. Heinrich Böll and Hans Krabel, “Zollverein Coal Mine, Pit XII in Essen – Conversion and Extension of an Industrial Monument Dating from 1928–1932,” Detail 6 (1997): p.877.


Zeche Zollverein (1932/1961) Essen, Germany

ArchitectsFritz Schupp and Martin Kremmer

ClientBauhütte Zeche Zollverein
Schacht XII
GmbH/Stiftung Zollverein

Coordinating/Contact Architects
Böll and Krabel Architekten
Heinrich Böll Architekt, Essen

Other Architects
Foster and Partners, London
OMA/Rem Koolhaas
Kazuyo Sejima + Ryue
Nishizawa, Tokyo
MGF Architekten GmbH, Stuttgart

About the Authors

Angel Ayón, AIA, NCARB, NOMA, LEED AP, has more than twenty five years of experience working with historic buildings. Trained in his native Havana, Cuba, Washington, D.C., and New York City, his expertise ranges from building-envelope evaluation and repair to full-scale rehabilitation and adaptive reuse of commercial and residential properties, as well as cultural and educational institutions. Angel believes it is the responsibility of the current generation to save and secure our built heritage as a cultural asset. He currently serves as Vice-President of Save Harlem Now!, member of the Advisory Board of the Historic Districts Council, the Historic Preservation Committee of The Municipal Art Society of New York (MAS), Vice Chair of the Board of Trustees of the Preservation League of the NY State, and member of the Board of Directors of the James Marston Fitch Charitable Foundation. He is also an active member of Columbia University’s Preservation Alumni, as well as the AIA, NCARB, nycoba/NOMA, USGBC, APTi, APT NE, US/ICOMOS, Docomomo US and Docomomo US/New York Tri-State. Angel holds a professional degree in Architecture and a Master of Science in Conservation and Rehabilitation of the Built Heritage from Havana’s Higher Polytechnic Institute, as well as a Post-Graduate Certificate in Conservation of Historic Buildings and Archaeological Sites from Columbia University in New York.

Uta Pottgiesser, Architect, PhD is Chair of Heritage & Technology at TU Delft (Faculty of Architecture and the Built Environment), The Netherlands and also Professor of Building Construction and Materials at OWL UAS (Detmold School of Architecture and Interior Architecture), Germany, where she also served as vice-president (2006-11) and dean (2012-16). From 2016-2019 she was appointed Professor of Interior Architecture at the University of Antwerp (Faculty of Design Sciences), Belgium. Dr. Pottgiesser has more than twenty-seven years experience as a practising architect and researching scientist concerned with the protection, reuse and improvement of the built heritage and environment. Among other organizations, she is a member and vice-chair of Docomomo Germany and a member and chair of the Docomomo International Specialist Committee of Technology (ISC/T). As a member of the European Facade Network (efn) and a co-founder of the international "Master of Integrated Design (MID)” at OWL UAS, she has been researching, teaching and lecturing internationally. She is a reviewer and (co-) author of several international journals and publications with a focus on construction and heritage topics and acts a jury member in architectural competitions and PhD commissions. Dr. Pottgiesser holds a professional degree as an Architect (1993), graduated with a Diploma in Architecture from TU Berlin (1991) and obtained her PhD (Dr.-Ing.) from TU Dresden (2002), both in Germany, on the topic of “Multi-layered Glass Constructions. Energy and Construction.”

Nathaniel Richards, LEED AP, is a Senior Design Manager with Elicc Americas Corporation. He is a seasoned professional with 15 years experience in the construction industry. He has a broad breadth and depth of knowledge coupled with years of hands-on experience successfully managing large scale complex construction projects, from designing entire apartment communities beginning at conceptual stage, drafting contract documents and performing submittal reviews, to engineering fabrication sets of high performing building enclosure systems while managing production and overseeing installation, to concurrent execution of a dozen trade contracts across multiple challenging projects from Staten Island to the Bronx.

Angel Ayón, Uta Pottgiesser, Nathaniel Richards, Reglazing Modernism. Intervention Strategies for 20th-Century Icons, Birkhäuser 2019 (ISBN: 978-3-0356-1845-7).