La materia e il tempo. I possibili utilizzi dell'acciaio corten

Weathering steel, also known as corten, is a modern steel alloy created for the industrial sector that has become a privileged tool of recent contemporary artistic and architectural research. Over the past few decades, and not without controversy, this material has come to be widely used in projects situated in historical-monumental contexts. Consensus around its use tends to be founded more on its expressive qualities than its interesting technical properties. The notable success of this material, and its frequent presence in highly fragile contexts, demand a pressing and twofold reflection on its effective durability and compatibility with historical materials; aspects that have been scarcely considered to date, as corten is largely presented as self-protecting steel. In reality, the first verifications of its ability to stand the ‘tests of time’ present a more complex and largely unexplored situation. What emerges is a steel alloy with a complex and variable chemical composition, conceived for other purposes, and whose use requires a great deal of attention. This situation suggested the need for a study that expands on what we know about weathering steel, the techniques related to its use and the treatments often used to protect its integrity. In parallel, the study also examines its multiple applications in design with respect to pre-existing structures. The objective is to understand proper methods and correct uses, effective conditions of conservation and, where necessary, evaluate eventual and suitable conservation strategies. Presenting the results of this research, this publication begins with a selection of projects and discussions, where possible, with their designers. Further support is provided by a comprehensive review of related scientific literature in order to understand choices made, existing knowledge and the critical awareness of those who use this material, eventual difficulties encountered and, finally, the validity of results obtained. Looking to contemporary works, the selection focused on a number of projects considered emblematic of the many ways of using this material and its preferential use in texts or contexts in a state of ruin or subject to a significant interruption in the continuity of their use. The study privileged South-Western Europe for the elevated number of examples it offers, though the Northern or Eastern regions of the continent present no shortage of interesting examples; this choice was corroborated by an evident interest in this particular steel, and the greater availability demonstrated by architects. The latter were invited to collaborate by explaining the technical and conceptual reasons behind the choice to use weathering steel and sharing useful information about the entire design process and the evaluation of expected results. The book is structured in three, intrinsically connected chapters. The final one, edited by Chiara Mariotti and Alessia Zampini, concentrates on twelve case studies, selected from the list of exemplary projects, described and analysed in more detail. The first part of the study begins with a short reflection on the aesthetic prerogatives and perceptive mechanisms that have made weathering steel one of the most esteemed materials for contemporary interventions in existing built environments; a metaphor of caducity and the passing of time, but also of resilience communicated by its unique rusted patina. This premise introduces the analytical structure of the research, intended as an articulated evaluation of the results of specific projects in relation to their historical, material and environmental context of reference. The study of the selected projects follows a tripartite structure in relation to the different environments identified. It accounts for the various strategies employed by contemporary architecture to confront pre-existing scenarios in architectural or archaeological contexts or abandoned landscapes. For each of the experiences synthetically presented an attempt was made, where possible, to consistently report the geoclimatic and material characteristics of the specific environment, the type of alloy utilised and the methods of installation; while not always known, this information is determinant to the success of a project. This data set was then used to define a complex set of forms, languages, design solutions and technical methods for the use of this material, which proved fundamental for successive studies. The next section concentrates on the material itself, with the intention of clarifying the precise technical confines and formal restrictions that regulate its correct use and to initiate a parallel reflection on the dialectic between new materials and actions of conservation. A brief historical introduction of the events that led to the metallurgic definition of the alloy is followed by an account of the principal requisites of corten steel. A description of its different chemical formulations is followed by a look at its physical-technical properties, systems of connection and interactions with the environment, often largely under-evaluated. Particular attention was focused on understanding the morphological characteristics and articulated mechanism that generates the process of surface oxidation responsible for the protective layer of rust that comprises the primary quality of this steel. Finally, gathering together the results of past experiences, the opinions of designers and indications provided by manufacturers, the study attempts to better understand the controversial question linked to the nature of the numerous treatments (oxidants, inhibitors and protective) often employed and their true usefulness. The second part of this chapter is dedicated to evaluating the durability of COR-TEN steel and the physical-chemical compatibility of its use in evidently delicate contexts. The possibility to verify a series of interventions that stand the ‘tests of time’ by comparing data from different projects made it possible to identify the recurring inherent criticalities of weathering steel and its effects on historical materials, and to hypothesise possible causes. Problems include discoloration, extensive or localised corrosive attacks, acts of vandalism and, to conclude, staining the notorious percolations of iron oxides that represent one of the most frequent and feared phenomena of decay. The coherent organisation of this information outlined a summary framework of the diverse typologies of problems encountered, often linked to an incomplete understanding of the characteristics of this material, as well as a lack of maintenance. This mosaic is used to suggest general strategies for contrasting the onset of these mechanisms, resolving eventual difficulties and, above all, increasing awareness about the appropriate use of weathering steel. The book is completed by a chapter dedicated to an exploration of twelve case studies, selected from among the projects presented in the first part of the research. This section presents a detailed reading of the technical, aesthetic and functional values of each intervention. A dedicated text analyses these projects, beginning with its condition ante operam, followed by a more detailed focus on the phase of construction with particular reference to the role of weathering steel and a conclusive evaluation of its current state of conservation. What clearly emerges in the conclusion is the fundamental value of a more in-depth design phase, guided by an informed ‘director’ whose specific understanding of the performance characteristics of this material is capable of defining and controlling not only design and construction, but also the necessary future actions and successive periodic maintenance required over time. It is thus left to the designer’s technical and critical capacities and knowledge, beyond preference and criticisms, to responsibly ‘direct’ the entire process and ensure, through a judicious use of weathering steel, the introduction of a new spatial and temporal dimension within a historical context that welcomes its introduction thanks to the respect for its character sedimented over time.