Conserving Iron Objects from Shipwrecks: A New Approach
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CONSERVING
IRON OBJECTS FROM SHIPWRECKS: A NEW APPROACH
Dr. HERMAN A. SMITH Corpus Christi Museum of Science and History, 1900 N. Chaparral, Corpus Christi, Texas 78401
ABSTRACT Recent experiments in the conservation of iron objects from several shipwrecks have resulted in a new method to arrest corrosion and stabilize the materials for further study or exhibition. The process involves the use of electrolysis and the application of a tannin preparation to neutralize the generation of iron-destructive compounds. THE CONSERVATION PROBLEM In April of the year 1554, four Spanish merchant ships heavily laden with passengers and cargo left Veracruz bound for Havana on a return trip to Spain. A severe spring storm drove three of the vessels ashore along the coast of southern Texas, resulting in the loss of all three ships and over three hundred lives. A few survivors were able to make their way back to Veracruz and the viceroy dispatched a salvage expedition to recover the cannons, anchors, and some 80,000 pounds of silver that had gone down with the wrecks. In spite of almost heroic efforts, the Spanish were able to recover only about a third of the silver and likewise abandoned several iron cannons and anchors (1). In the late 1960s, a salvage company relocated the wrecks and began recovering artifacts from two of the vessels. The State of Texas stopped the operation before the wrecks were entirely destroyed and undertook a scientific investigation of the ship's remains. Although very little remained of the ship itself, many metal artifacts were preserved in concretions which encrusted clusters of artifacts soon after their immersion in sea water. The concrete-like coating, formed by the action of bacteria and other small organisms, frequently preserves materials that otherwise would be destroyed by the action of the salt water. Artifacts recovered from the wrecks were transported to the Texas Archeological Research Laboratory in Austin, where they were placed in holding tanks until a process could be worked out to preserve the materials that eventually would be removed from the concretions. Archeologists and conservationists from the University of Texas at Austin and the newly-formed Texas Antiquities Committee worked out what has proven to be a permanent solution to the problem of conserving iron objects that; have been immersed in sea water for protracted periods. During the time the iron is submerged chlorine ions are absorbed by
Mat. Res. Soc. Symp. Proc. Vol. 185. c 1991 Materials Research Society
762
metal. As long as the iron object remains in the salt water solution the iron will remain relatively stable. When the object is removed from the electrolytic sea water and exposed to the atmosphere, chlorides tend to seep out and combine with atmospheric oxygen and water to form hydrochloric acid and other compounds that in turn attack the uncorroded metal until no metal remains. The net result of long-term exposure of the iron artifacts to the atmosphere is total destruction. The process by which the artifact
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