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Abstract
The coal tower, also called steel coal tower due to its steel construction, is an elevated steel structure 19m tall and approximately 10m in diameter. With a holding capacity of 1,187m3 it rests on interconnected steel supports, is held together by horizontal steel bands, and was used to store fine-grained coal. By 1942 it had been retired and subsequently served only as structural support for redirecting a conveyor belt. The tower, built in 1897, has a high-value as a monument because it is one of the oldest components of the coking plant. Its most prominent features are its style of construction and its former role in the production process. The coal tower has been in poor general repair. In 2008, securing the structure became urgently necessary. The goals of the renovation included the preservation of its corroded appearance through corrosion protection pursuant to DIN EN ISO 12944 and the conservation of the ruins through restorative retouching. A so-called "Window into History" serves as primary documentation. The stability of the structure was assured through scaffolding on the inside.
Framework for Monument Preservation
In the context of regular monitoring of protected historical monuments of the World Cultural Heritage Site Voelklinger Huette, the old Raw-Coal Tower of the Coking Works (built in 1897) was, as of calendar year 2007, classified as urgently in need of renovation. Because of its steel construction, it is also called the Steel Coal Tower (SCT). The structure had been decommissioned in 1942, but was preserved basically unchanged. Its function was taken over by a bigger concrete silo dubbed the Concrete Coal Tower (CCT). The old steel tower was simply left standing. Filling of the CCT was accomplished through inclined conveyor belts, the Coke Belts. After the SCT had been retired, a station to divert the conveyor belt was erected on its roof. Thus the SCT acquired a new role, without which it most likely would not have been kept around. Maintenance of its sheet metal exterior was discontinued for obvious reasons, only its supports had to be kept intact since they were needed to bear the load of the conveyor belt redirection hardware. The cylinder-shaped container, constructed from riveted sheet metal and tapering towards the bottom, is one of the oldest unchanged components not only of the Coking Works, but of the entire iron production works in Voelklingen in general, which have been designated a World Cultural Heritage Site. The SCT has a very high value as a monument because it is one of the oldest parts of the coking plant. Most prominent are construction style and its former role in the production process as a silo for the storage of fine-grained coal for the coking ovens (figure 1).
As the primary structure and the diversion apparatus of the conveyer bridge had been restored between 2002 and 2003, in 2008 the latest renovation started with the gradual removal of the coal dust. It had remained inside the tower since its decommissioning and was carried off by make-work crews. During this process, the full extent of the damage a restoration plan for the sheet metal construction would have to address became visible. Rust had destroyed crucial parts of the contact surfaces between sheet metal and the horizontal steel bands affixed to the outside of the container for rigidity, so that no residual static load-bearing capacity remained.
The following additional adverse factors had to be considered in planning the restoration: 1. Development of a permanent workable solution that does not require additional expenditures for a period of approximately 20 years. 2. Conservation of original components to the greatest extent possible while simultaneously preserving the historical appearance (especially the rusty container surface). From the standpoint of the building engineers involved in the process, the preservation of the sheet metal hull seemed to make little financial sense, particularly in light of the specifications for monument preservation. Instead, dismantling and extensive reconstruction of the entire structure was considered as a workable alternative. The SCT seemed to get lost between the conflicting requirements of conservation of existing building materials, of passing on to posterity the original ('ruinous') appearance, and of making the reconstruction last (i.e., protecting against corrosion). In addition, the cost estimates that had been performed assumed that replacement of the building would be the most cost-effective alternative. The solution to these complex problems required the services of a planning expert because basic principles of the field of object restoration had to be applied to a large-scale structure. Conservation Goal The "preservation of the state of decommissioning in 1986" was the designated goal of the restoration. Therefore, the corroded appearance of the SCT had to be preserved and restored. To understand this unusual conservation goal, it is important to consider the time slice that applies to all components of the World Cultural Heritage Site Voelklinger Huette. By time slice we mean the point in history that is intended to be preserved through conservation measures. For Voelklinger Huette this is the time when the plant was closed. The time slice applies to all components of the site. As a consequence, all structures that were in ruinous condition at the time of closing are conserved as ruins and are not brought up to par with structures that were maintained better. Steps in the Decision Process 1. The starting point was abandoning the concept of dismantling the entire structure. The requisite static stability of the object was to be achieved through the addition of scaffolding on the inside of the building. Thus, load was taken off the historic building fabric so that a substantial rebuilding of the existing structure was unnecessary. 2. Next, a detailed map of the points of damage was compiled. All existing pieces of sheet metal were to be preserved as originals with the parts to be replaced kept to a minimum. 3. The question of appearance left several options of how to deal with the object. Originally, all building components had been covered with an anthracite-black coat of paint. This was partly preserved in the funnel area and the lower parts of the cylinder. A complete renewal of the corrosion protection in this hue, however, would have completely altered the appearance, and the character of the SCT, and with it the prominent position within the coking plant as a 'rusty ruin', would have been lost. Therefore, planners, restorers, and client decided to colour the new and newly corrosion-protected surfaces of the SCT such that the impression of a 'rusty ruin' remained. In this way it was also possible to ensure the specified period of 20 years that the renovation was expected to last. 4. Parts on the side of the building protected from the weather were to be preserved unchanged in appearance to form a 'Window Into History'. This way, for the trained eye a comparison between real and painted ruin is possible as primary documentation. Condition The SCT is an elevated steel structure 19m tall and approximately 10m in diameter resting on interconnected steel supports, with horizontal steel bands around the silo added for rigidity (figure 2). It has a holding capacity of 1,187m3. The sheet metal casing had rusted through in several places along stretches several meters in length, especially on the windward side. The entire hull structure was therefore in serious danger of collapse. Apparently, this was not a new problem either: rusted-through metal sheets had previously been patched up in a makeshift manner with coarsely welded-on hull plating (figures 3 and 4). Figure 1. Old picture of the Steel Coal Tower from “Richard Nutzinger, Hans Boehmer, Otto Johannsen: 50 Jahre Röchling. Saarbrücken 1931”. Figure 2. The Steel Coal Tower and its integration into the surrounding components of the coking plant: Steel Coal Tower (1), Concrete Coal Tower (2), Redirection Station (3), Coke Conveyor Belt (4). The picture was taken in 2005, before the work started. Figure 3. Rusted-through metal sheets patched up in a makeshift manner. Figure 4. The sheet metal casing had rusted through in several places along stretches several meters in length. The condition of the exterior of the SCT fully exposed to the weather can be divided into three categories (figure 5): 1. Hull plating and horizontal steel bands: corroded; load-bearing capacity partially reduced; original black coating remains only in small remnants. 2. Repair panels on top of old hull plates: corroded; able to bear loads; no coating. 3. Funnel surface: Condition better than that of hull plating; remnants of original black coating present. Stability Statically securing the sheet metal hull panels was a prerequisite for the preservation of the SCT as a ruin. It was achieved through a new integrated steel structure inside the container which served as scaffolding and to which the disintegrating sheet metal plating was attached. The support structure was connected to the existing steel structure of the redirection station and thereby secured horizontally in two directions (figure 2): through the sloping coke conveyor belt (4) of the redirection station (3) and through the CCT (2) with which the redirection station (and thus the SCT) was connected, again through the coke conveyor belt. At the bottom, the structure is supported horizontally and vertically by the existing steel beams of the SCT. There, stability is assured because the supports were originally designed for a fully loaded tower and hence for a much heavier load. The sheet metal hull of the tower, damaged heavily in parts, and the horizontal bands were attached to the new support structure with special screws ['Fuchsschrauben'] protecting them from falling (figures 6-8).
Figure 5. Excerpt of the damage map for the steel hull (drawing by WPM Ingenieure GmbH, Neunkirchen).
Figure 6. Anchoring the support structure of the Steel Coal Tower at the top through the coke conveyor belt (4) and the Concrete Coal Tower (c.f. red markings; drawing by WPM Ingenieure). Figure 7. Inside support scaffolding built from raw steel.
Figure 8. Detail of bottom connection of the scaffolding to the existing steel supports.
Corrosion Protection and Retouching Because of the scaffolding on the inside, neither the rust-damaged horizontal steel bands nor the metal sheets of the hull needed replacement or repair. Because they were weathering openly, they were protected against corrosion on the outside, including the edges, according to DIN EN ISO 12944 Korrosionsschutz von Stahlbauten. The corrosion protection was applied by sandblasting with granularity SA2 for surface preparation and by applying three coatings with a combined thickness of at least 240 micrometers. Areas split apart by corrosion were carefully cleaned, sealed, and also painted three times. The constructional corrosion protection of the horizontal steel bands was improved by drilling holes into the horizontal joint profiles. This allows rain water to drain away more easily (figure 9). Finally, restorative retouching was applied to the top layer through a varnish. This retouching simulates the corroded appearance of the SCT by employing different shades of rust colouring and - most importantly - varying the texture upon application through the use of different brushes. Only in this manner was it possible to create the impression of a corroded steel surface when looked at from within normal viewing distance (figures 10 and 11). The steel surface on the inside of the SCT remained untreated. This was done so that in the long run, the former usage context would remain visible on the object itself through preservation of the very tough and highly adhesive coal dust deposits (figure 7). Window Into History (WIH) On the leeward side, the well-preserved sheet metal plates were preserved without corrosion protection or retouching over a continuous area of 30m2 together with the entire exterior of the funnel as a "WIH". Only temporary protection in the form of an anti-corrosion wax was applied. Preparing the surface demanded purity grade P St 2 in accordance with DIN EN ISO 12944. The "WIH" is intended to show the existing ruinous state directly at the object as primary documentation (figure 12).
Figure 9. Corrosion Protection: Corrosion Protection through holes drilled into the steel bands (1), sealing of gaps (2), surface coating (3) on top of two primer coats.
Figure 10. After application the retouching must be textured with different brushes. Figure 11. For the final decision on retouching, test surfaces were created on a sample. Visible below the paint line is the original state (1), above is the sample surface for retouching (2). Figure 12. The Window into History. Documentation To record the corroded hull surface, a precise photo documentation was created before sandblasting commenced. In addition, an unrolled projection of the hull surface was created as a CAD drawing, in which the damage to the SCT was cartographically recorded. All methods, materials used, and procedural explanations were delivered to the client for documentation (figure 13).
Acknowledgments
The comments concerning structural stability and the damage map are based on documentation by WPM Ingenieure GmbH, Neunkirchen.
About the authors
Kornelius Götz
Industrial Heritage | Senior Consultant Conservator
Bureau for Conservation Advice Contact: goetz@restaurierungsberatung.de Kornelius Götz, M.A., is a conservator-restorer specialised in the conservation of industrial monuments. He worked as a conservator of industrial objects at the Landesmuseum für Technik und Arbeit (Technical State Museum for Technology and Labor) in Mannheim from 1984 to 1996. Since 1996 he works as freelancer conservator. He also lectures at the Fachhoch-schule für Technik und Wirtschaft Studiengang Restaurierung in Berlin and the Haute École d'Arts Appliqués du Canton de Neuchâtel, La Chaux-de-Fonds, Switzerland. Axel Böcker Dipl.-Ing., Landesdenkmalamt Saarland, Ministerium für Umwelt Keplerstr. 18, 66117 Saarbrücken Contact: a.boecker@denkmal.saarland.de
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