Studies in Conservation, 17 (1972), 39-43 39

SOME ASPECTS OF CONSERVATION OF IRON OBJECTSFROM THE SEA

S. G. REES-JONES

Abstracts-A description is given of the methods used to conserve iron objects from thewreck of the Spanish galleass Girona which sank in 1588 off the North coast of Ireland.Reference is made to the instability of cast iron after prolonged immersion in sea water.

1. INTRODUCTION

In October 1588 the Spanish galleass Girona was wrecked on the northern coast of Ireland atthe Giant's Causeway. Material from the wreck has recently been recovered [1] and for thepast three years conservation work has been in progress on it. Iron objects were found topresent the most problems, and, there being little published material on the conservationof iron from the sea, an account of some of the work undertaken and observations relatingto it is presented below.

2. CONSERVATION OF IRON

2.1. Field treatmentsObjects recovered from the sea require some form of immediate treatment to preventdeterioration while awaiting permanent conservation. This should involve only superficalcleaning to remove loosely adhering material, followed by washing in fresh water to removeas much of the soluble salts (mainly chlorides) as possible.Objects should then be examined to determine whether they are cast or wrought. Cast ironshould not be allowed to dry out before or during treatment (although it may be exposedto the air for short periods when only surface water is evaporated). This in principle onlyapplies to objects that have been chemically and/or physically eroded sufficiently to allowcorrosion to extend to the inner layers; but as it is impossible to be certain of the con-dition prior to cleaning, all cast iron must be assumed to be unstable (Fig. 1).The most suitable method for its temporary preservation is immersion in a solution of 5per cent sodium sesquicarbonate in water, which will inhibit further corrosion and commencethe process of chloride removal.Wrought iron may be carefully dried, although any surrounding encrustations may tendto crack with possible damage to the underlying metal. In this case the sodium sesqui-carbonate treatment may be applied.

2.2. Removal of encrustationsThe first stage in the cleaning involves the removal of encrustations and is carried outmechanically, except in the case of fragile objects or objects with a loose corrosion layer,where chemical methods are used.

40 S. G. Rees-Jones

Encnl~tations on large substantial objects are removed by careful use of a small hammerand fine chisel, or on smaller objects with an electrical or compressed-air vibrator [2].There is a wide range of chemicals which will dissolve calcareous material without damagingthe underlying metal. As these encrustations often contain iron salts from the corrodingiron, a 20 % aqueous solution of orthophosphoric acid has in practice been found verysuitable by the author, since it will dissolve the iron salts as well; but for this reason ironobjects should not be subject to prolonged immersion.

Fig. 1 Fig. 2

FIG. 1 Cannon ball showing the result of drying without stabilization.FIG. 2 The result of electrolytic cleaning on a very porous cannon ball.

2.3. Removal of chlorides and corrosion productsOne of the methods of removing chloride contamination and corrosion products on ironobjects is electrolytic reduction as described by Plenderleith [3]. This is followed by boilingin successive baths of distilled water until all the chloride has been removed. Some groupsof objects, however, cannot be cleaned in this manner; these include cast iron that is porous,very large objects where the method is not practical and very heavily corroded objects.In the case of corroded cast iron that is porous, electrolytic reduction will cause the evolutionof hydrogen in the porous inner region of the object, resulting in expansion and crackingof the object (Fig. 2).For these types of objects corrosion products are removed mechanically as described aboveand the chlorides are removed by repeated washing in distilled water. This is a very slowprocess, but for smaller objects it can be accelerated by boiling the object repeatedly in thedistilled water. If the water is changed six times per day the object is free of chlorides in2-3 weeks.For large objects a suitable container is made, to suit the shape of the object. The simplestand cheapest container is a wooden box lined with polythene sheet (Fig. 3), which, if suitablelifting tackle is not available, can be made without a base, allowing the object to be man-handled on to the polythene and the sides to be placed over it. The object is then soaked infresh water which is changed daily until the chloride concentration remains the same as forfresh water. Distilled water is then substituted for the fresh water, and changed weeklyuntil all the chloride contamination is removed. As this is a lengthy procedure, 5 per centsodium sesquicarbonate is added as a rust-inhibitor.The silver nitrate test is used for the measurement of chloride concentration. Two samples

Studies in Conservation, 17 (1972), 39-43

Some aspects of the conservation of iron objects from the sea 41

of the water to be tested are put in test-tubes, and two or three drops of dilute nitric acidare added to each; if effervescence results due to the presence of sodium sesquicarbonatein the test water, further drops are added until there is no more effervescence. Then fivedrops of 0·1 normal silver nitrate solution are added to one test-tube. Both test-tubes are

FIG. 3 Cannons from the Girona under treat-ment inwooden tanks lined with polythene.

shaken and then compared; if any chloride is present, this will combine with the silvernitrate to give a white precipitate of silver chloride.Other methods that have been used to remove corrosion and chlorides from iron objectsfrom the sea are: electro-osmosis in 2 per cent sodium hydroxide followed by washing inchromate-inhibited distilled water [4]; electro-chemical reduction followed by intensivewashing [5] and heating in a closed furl1;a'geto 850°C [6].With the exception of porous cast iron ~ll 'objects are now ready for the final stage of treat-ment: protection from further contamination.

2.4. Stabilizing porous cast ironPorous cast iron, although cleaned and :chloride-free, is still potentially highly corrosible,and it is necessary to inhibit the iron against the rapid oxidation which will take place ondrying. Corrosion inhibition has been achieved on the most unstable cannon balls by im-pregnation with 20 per cent solution of orthophosphoric acid. To achieve complete pene-tration a vacuum tank is used [7],which promotes penetration of the phosphoric acid intothe porous areas. After this treatment an object can be dried out with safety, as the acidwill complex with the iron and iron corrosion products to form stable iron phosphates.

2.5. Protection from further corrosionTo prevent further corrosion of iron objects, it is essential to seal the surface after cleaning.

Studies in Conservation, 17 (1972)) 39-43

42 S. G. Rees-Jones

The object is first brushed clean of any loose material which may prevent complete adhesionof the protective coating, and then thoroughly dried either in an oven at 110°C or, for largeobjects, under infrared lamps. The length of time is dependent on the size and porosity ofthe object.The object is then coated with two applications of a suitable protective coating, preferablyone that can be removed if necessary. The coating used in this instance was 'Ebonide'[8] which gives a dead black finish very suitable for iron, and good resistance to moisture.This will protect the object under normal conditions, but no protective coating is completelyimpervious to the slow diffusion of salts and moisture which will result in further corrosion.Therefore objects should be kept in the correct conditions of humidity to minimize this.

ACKNOWLEDGEMENTS

I should especially like to thank my colleague, Mr C. G. Slack, for his ideas and assistancein solving the problenl of the disintegrating cannon ball, and also for the photographs usedto illustrate this article.I am very grateful to Dr A. E. Werner and his staff at the British Museum Research Labo-ratory and Mr McCandless and his staff at Harland & Wolff's Testing Laboratory, for theiradvice on the structure of cast iron cannon balls.

NOTES AND REFERENCES

STENUIT,R., 'Ireland's Rugged Coast Yields Priceless Relics of the Spanish Armada', NatiOl:alGeographic, 135 (1969) no. 6, 745-777.

2 1. 'Burgess Vibrotool' obtainable from Burgess Products Co. Ltd., Small Tool DivisionSapoote, Leicestershire.

2. 'Desoutter Pneumatic Power Pen' obtainable from Desoutter Bros. Ltd., The Hyde, Hendon,London N.W.9.

3 PLENDERLEITH,H. J., The Conservation of Antiquities and Works of Art, Oxford University Press,London 1956.

4 'Cook's Cannon Preserved', Nature, 223 (1969) no. 5210, 1000-1001.5 PETERSON,M., History under the Sea, Smithsonian Institute, Washington 1965.6 ERIKSEN,E., and THEGAL,S., Conservation of Iron Recovered from the Sea, T0jhusmusects

Skrifter 8, Copenhagen 1966.7 REES-JONES,S. G., 'A Simple Vacuum Impregnation Tank for Pottery and Other Objects',

Studies in Conservation, 8 (1963), 67-71.8 'Ebonide' (nitro cellulose lacquer), W. Canning Ltd., Great Hampton St., Birmingham 18.

STEPHEN G. REES-JONES, born 1941; Diploma in Conservation, Institute of Archaeology 1960; Demon-strator, Department of Conservation, Institute of Archaeology 1960-61; Conservation Officer in charge,Ulster Museum/Queen's University Conservation Laboratory, Belfast, 1961-71; Field Conservation Officer,University of Pennsylvania, Mohenjo D~ro Expedition, West Pakistan, 1964-65. Present position: Keeperof Conservation, City Museum and Art Gallery, Birmingham. Fellow of IIC.

Author's address: Department of Conservation, City Museum and Art Gallery, Birmingham B3 3 D H, GreatBritain.

Abstrait-Sont expliquees les methodes employees a conserver des objets en fer, provenantde l'epave du galion espagnol Girona qui a sombre en 1588 au large de la cote nord d'Irlande.II a ete refere a l'instabilite du fer de fonte apres immersion prolongee dans l'eau de mer.

Kurzfassung-Methoden zur Konservierung eiserner GegensHinde aus dem Wrack derspanischen Galeone Girona, welche im Jahre 1588 unweit der nordirischen Kiiste sank,werden beschrieben. Es wird auf die Unbestandigkeit von Gusseisen hingewiesen, das sichlange Zeit im Meereswasser befunden hat.

Studies in Conservation, 17 (1972), 39-43

Some aspects of the conservation of iron objects from the sea

Riassunto-- Si descrivono metodi applicati nel conservare oggetti in ferro provenienti dalrelitto del galeone spagnolo Girona, affondatosi nel 1588 al largo della costa settentrionaleirlandese. Viene riferita la instabilita della ghisa in seguito a prolungata immersione neU'acqua.

Extracto-Se describen metodos empleados para conservar objetos de hierro del derrelictodel gale6n espanol Girona, que se hundi6 en el ano 1588 sobre la costa norte de Irlanda. Sehace referencia a la instabilidad de hierro fundido despues de sumersi6n prolongada en aguadel mar.

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Studies in Conservation, 17 (1972), 39-43