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Galvanic removal of metallic wrought iron from marine Encrustations

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  • Ships of Discovery

Abstract

Historic shipwrecks frequently contain a large proportion of wrought-iron tools, fasteners, and other types of artefacts. Encrustations readily form around such objects in many marine environments. Depending on many factors, the iron objects inside these encrustations may be well preserved, completely disintegrated, or poorly preserved but still present. The latter type of encrustation is every conservator's nightmare because removing the encrustation yields merely a poorly preserved artefact still in need of extensive additional conservation. Neatly separating the artefact from its encrustation in order to obtain a natural mould for casting is generally impossible. Having encountered this problem many times, the authors began to experiment with the extraction of metallic iron from poorly preserved encrusted artefacts using galvanic dissolution. Data on rates of dissolution were gathered for three experimental configurations. The results of a test conducted on an encrusted artefact were promising, but inconclusive.
... The rate of the cathodic (protective) or anodic (corrosive) reactions that can take place will depend on the relative surface areas of the two metals, reaction kinetics and the difference in E corr values (North and MacLeod, 1987: 72). Carlin et al., (2002) experimented with the rate of galvanic corrosion of archaeological wrought iron in a 2% NaCl solution (sodium chloride). ...
... Concretions may encapsulate several iron artefacts, which an X-ray may reveal, and if the residual metal has totally corroded and disappeared, once removed the concretions can be kept as a mould for replicating artefacts (Carpenter, 1990: 25). While the previously mentioned study by Carlin et al. (2002) was highly effective in testing corrosion with variables of galvanic proximity and air flow to iron, it should be noted that the overall aim was to completely dissolve the metallic wrought iron so that conservators could create an artefact mould from the encrusting concretion. Encouraging further loss of artefact iron to produce a replica is fraught with risks, may not work with larger artefacts and does not comply with the ethical responsibility that conservators have to prevent further deterioration. ...
Thesis
Marine iron artefacts are particularly vulnerable to rapid corrosion due to factors pertaining to their metallurgy and manufacture, surrounding environmental conditions, and when in the contact with other metals exhibiting higher electrochemical potentials (more noble). In order to preserve them for future generations, active strategies aimed at prolonging their lifespan involving stabilisation and conservation, must be implemented. Three historic 19th-century iron Rifled Muzzle Loading cannon may presently be found in an underwater setting beneath a natural rock formation off Malta’s Fort Ricasoli. This thesis explores their history, chronology, deposition, and current status. Three feasible solutions are hereby proposed for their conservation including: recovery and conservation, temporary relocation and future recovery, or permanent relocation and the creation of a recreational dive trail site. These options take into consideration the environmental factors affecting their stability, their assessed level of significance, and the relative expense entailed for each option. Due to a number of reasons that will be explained, the most advantageous option in the near future is for these cannon to be relocated, and then revisited with funding for a long-term plan.
Article
Les fouilles sous l'eau effectuées par le Musée Universitaire de l'Université de Pennsylvanie près de Bodrum en Turquie, ont ramené plus de 150 objets de fer d'un navire du septième siècle. Cet article décrit le procédé de ré-création qui fut accompli sur plusieurs outils de charpentiers, ustensiles agricoles, accastillages de bateaux et ancres. Des moules durs comme le béton s'étaient formés autour des objets de fer en décomposition. Chaque concrétion fut d'abord sciée avec une scie lapidaire. On prit ensuite des empreintes des moules. Au cours du procédé de moulage on expérimenta avec deux composés de caoutchouc de polysulfure et un composé de caoutchouc au silicone qui, sous forme liquide, lorsqu'ils sont mélangés à un catalyseur, durcissent pour former des caoutchoucs solides. Après la polymérisation des composés, les concrétions furent soigneusement brisées et les morceaux furent lentement enlevés pour révéler des facsimilés parfaitement identiques aux objets originaux. On étudia ensuite les empreintes prises avec ces caoutchoucs artificiels et on les exposa au Musée de Bodrum. Le projet a été complété voici déjà un an, et les empreintes ne se sont pas détériorées depuis. Nous pensons que cette méthode de recréer des objets en fer décomposés, recouvrés de la mer, est efficace, précise et satisfaisante du point de vue esthétique. /// Während der Unterwassergrabungen, die vom Universitätsmuseum der Universität von Pennsylvania in der Nähe von Bodrum (Türkei) durchgeführt wurden, konnten 150 eiserne Gegenstände aus einem Schiff aus dem 7.Jahrhundert an die Oberfläche gebracht werden. In dem Artikel wird der Prozeß der Nachbildung beschrieben, der auf eine Reihe von eisernen Tischlerwerkzeugen, landwirtschaftlichen Geräten, Schiffsgeräten und Ankern angewendet wurde. Um die zersetzten Eisengegenstände hatten sich betonartige Formen gebildet. Jede dieser Versteinerungen wurde zunächst mit einem Edelsteinschneider in zwei Hälften zersägt. Später wurden die Hälften gründlich von allen zurückgebliebenen Eisenoxyden gereinigt. Dann wurden mit Hilfe der Formen Gußstücke hergestellt. Beim Gießen experimentierte man mit einer Silicon-Kautschuk-Verbindung und zwei Polysulfid-Kautschuk-Verbindungen. Diese flüssigen Verbindungen verhärteten sich zu gummiartigen festen Körpern, wenn sie mit einem Katalysator vermischt wurden. Nachdem die Verbindungen hart geworden waren, wurden die Versteinerungen sorgfältig abgebröckelt. Die zerschlagenen Stücke wurden langsam abgeschält und legten perfekte Nachbildungen der ursprünglichen Gegenstände frei. Die mit diesen synthetischen Gummiverbindungen hergestellten Gußstücke wurden dann untersucht und im Museum von Bodrum ausgestellt. Seit der Durchführung dieses Projekts haben die Gußstücke nichts von ihrer guten Qualität eingebüßt. Wir halten diese Methode für die Nachschaffung vom Meeresboden geborgener zersetzter Metallgegenstände für zweckmäßig, genau und ästhetisch befriedigend. /// Gli scavi subacquei compiuti dal Museo dell'Università di Pennsylvania nei pressi di Bodrum, in Turchia, hanno fruttato 150 oggetti di ferro provenienti da una nave del settimo secolo. In questo articolo si descrive il processo di ricreazione condotto su un certo numero di utensili da falegname, attrezzi agricoli, accessori e ancore per nave, tutti in ferro. Intorno agli oggetti di ferro in decomposizione si erano formati delle forme aventi la consistenza del calcestruzzo, ognuno dei quali venne per primo segato in due mediante una sega da marmista, dopo di che si pulirono a fondo di tutti i residui di ossido di ferro. Nel corso della colata si sperimentarono un composto di gomma siliconica e due composti di gomma al polisolfuro. I composti liquidi, dopo essere stati miscelati con un catalizzatore, si sarabbero induriti formando dei solidi gommosi. Dopo la polimerizzazione dei composti, i solidi furono attentamente spezzettati e i pezzi lentamente pelati, rivelando cosí repliche perfette degli oggetti originali. Le forme ricavate con questi composti di gomma sintetica furono quindi studiate ed esposte nel Museo di Bodrum. Nell'anno che seguí al completamento delle operazioni non si sono verificati deterioramenti nelle forme. Si ritiene che questo metodo di ricreare oggetti di ferro decomposti recuperati dal mare sia efficace, preciso ed esteticamente soddisfacente.
Article
This article describes the techniques developed at the request of archaeologists for the total reconstruction and long-term conservation of an extremely rare French pistol dating from the Revolutionary period. The pistol was retrieved from the wreck of ‘Le Cygne’, a brig which sank in 1808 off the coast of Martinique (West Indies). The methodology and techniques used illustrate the complexity of the cleaning and conservation operations applied to this composite object whose wrought iron had disappeared leaving only its imprint in the concretion. A new X-ray photographic technique using a contrast gel is described, as well as the procedures which were used in the reconstruction of the wrought iron and the stabilization of the wood and brass mountings of the object.
Conservation of metals
  • C Pearson
Pearson, C., 1987, Conservation of metals. In C. Pearson (Ed.), Conservation of Marine Archaeological Objects. 207–252.