The techniques of casting and molding are often used to restore and replicate artifacts. Casting replicas for exhibition, distribution, and study is only an adjunct to conservation. This aspect of casting, although of considerable importance, is not considered here, and the reader is referred to publications and brochures that can be obtained from manufacturers of casting materials, and to articles by Rohner (1964, 1970), Rigby and Clark (1965), Hamilton (1976), and Frazier (1974).
In the conservation of marine archaeological specimens, casting is used when the artifact itself cannot be treated. In some cases, only through casting can the object be saved or its form determined. As has been already noted in previous files in this on-line manual, metal objects within an encrustation can continue to corrode until little or no metal remains. In such cases, the original surfaces with identification marks, stamps, letters, or numbers are lost. Fortunately, the encasing encrustation begins to form immediately at the onset of the corrosion process. It forms a mold around the original forms, preserving any surface details. Quite often, the encrustation is more informative than the deteriorated or badly oxidized object.
There are many ways of using casting techniques during the conservation of shipwreck material. Some knowledge of the procedures used is important. A laboratory should also keep a stock of the necessary supplies and casting compounds. A number of different casting materials from many different manufacturers can be used. Products that are particularly recommended include Dow silver chloride (AgCl) silicone rubber, Smooth-On polysulfide rubber, Surgident Neo-Plex Rubber, Permamold Latex, Hysol Epoxy, plaster of Paris, and Coecal plaster. Many similar products could be substituted for those recommended here.
CASTING TECHNIQUES IN MARINE ARTIFACT CONSERVATION
The first published account of casting in marine conservation as a means of retrieving completely oxidized artifacts is that reported by Katsev and van Doorninck (1966:133-141). Using a lapidary saw, they sectioned small encrustation containing natural molds left by oxidized Byzantine iron tools. Some specimens required only one cut, while more complicated objects required several cuts. The corrosion residue was removed from the natural molds, and a piece of cardboard or plastic was made to fit between the sawn halves to compensate for the material removed by the saw. The mold then was filled with a flexible compound and the halves fitted together. The rubber cast was removed once the compound had cured. When the rubber flashing that formed along the seams of the mold was cut away, a replica of the disintegrated artifact was obtained.
Although rubber casts such as these are not permanent nor long lasting, they will last for a number of years. Their life and usefulness can be extended by storing them in plaster mother molds to provide support and to keep them from stretching and losing their form. If a permanent epoxy cast is needed, a mold must be first made of the polysulfide rubber cast; this second mold is then cast in epoxy.
After casting several molds sectioned with a lapidary saw, several disadvantages were noticed by Katsev and van Doorninck (1966). The technique is limited to small encrustation and to uncomplicated shapes, which require only a few cuts. A problem also arises in correctly aligning the two halves and the cardboard gasket required to replace the thickness sawn away by the blade. This problem is compounded when more than one cut is made. When the mold is cut with a saw, the seam flashing is very noticeable.
If X-ray facilities are available, some of the problems of casting natural molds can be overcome. Radiographs reveal the shape of the object and the extent of the corrosion. In certain encrustation, it is possible to use a pneumatic air chisel to cut openings into the distal ends or key points of an object. Through these holes, the corrosion residue can be washed out and the casting compound poured. Alternatively, the air chisel can be used to inscribe a line along or around an encrustation. By hitting along this line with a chisel and a hammer, the encrustation can be broken in a predetermined manner. Simple encrustation are easily opened and cast in this way. Because it is much more effective to break open natural molds in encrustation, the use of lapidary saws is not recommended.
The only way to recover many of the smaller, thin iron artifacts recovered from marine sites is to cast the natural mold left inside the encrustation after the artifact has corroded to a slush. The corrosion residue can sometimes be removed simply by rinsing the mold out with water; in other cases, a considerable amount of mechanical corrosion removal is required. After any residual corrosion product is removed, the void is filled with casting material. Epoxy is recommended as a general casting material, since it does not present the same long-term storage problems to the conservator as does polysulfide rubber. After the casting material has set, the surrounding encrustation can be removed with a pneumatic chisel, revealing a perfect replica of the original artifact.
Using the above technique, conservators have been able to make epoxy casts of corroded hammer heads directly onto the original wooden handles, as well as iron cleavers hafted onto the original wooden handles, a variety of iron keys, and several door locks (see Figure 16.1). It should be emphatically stated that if casting techniques are not being used on otherwise unconservable artifacts, a significant amount of data will be lost. (See Hamilton 1976:72-85; North 1987:231-232; and Muncher 1988 for a more complete discussion of the techniques of casting.)
Natural molds of disintegrated metal objects are often encountered in very large encrustation, where, even if it were possible to x-ray the piece, they cannot be detected on radiographs. To avoid destroying possibly valuable information about the encrusted artifacts, close observation is required when using air chisels to break apart the encrustation in order to reveal any natural molds. Because of the presence of these natural molds in large encrustation, the use of acids or even electrolysis to remove encrustation (see Montlucon 1986, 1987) is not generally recommended. When natural molds are detected, it is possible to open a small area on one side of the mold, clean it out, and fill it with epoxy.
The casting examples discussed above involved corroded iron artifacts; similar casting procedures are often employed on silver artifacts, which often corrode extensively in anaerobic marine environments. For example, a number of silver discs that are plano-convex in cross section were recovered from two 16th-century Spanish shipwrecks. On the flat surface of the silver discs are usually one or more stamps indicating ownership, mines, and tax marks. Many of the stamps were obliterated in the corrosion process. The encrustation, however, formed a perfect mold of the original surface of the silver, and a reverse impression of the stamps remained in the encrustation. In one example, the encrustation on a singularly encrusted silver disc was removed with an air scribe by chipping along the circumference of the disc and separating the two halves of the encrustation from the silver. The reversed silver stamps were revealed by carefully removing the corrosion products from the interior surface of the encrustation with fine bristle brushes and pointed wooden sticks. A latex peel was made of the interior surface of the encrustation which contained the reverse of the stamp. Plaster casts were made from the latex peel of the stamp impression, and the stamps were highlighted with a soft-lead graphite pencil. It is possible to recover many otherwise lost stamps through this procedure, and this technique is routinely incorporated into the conservation of encrusted silver discs in order to preserve these valuable data. Few historians or archaeologists would deny that the salvage of the stamps is historically more significant than silver discs with obliterated stamps.
The casting examples discussed above present an unquestionably strong case for the value and significance of casting in the conservation of marine shipwreck material. The recovered data are of the type that is lost daily by improper care and conservation of archaeological material. These examples emphasize the reasons why marine shipwreck material should be processed by conservation personnel, who are familiar with the material culture, as well as with the alternative techniques available for salvaging and preserving the maximum amount of data from an artifact.