CASE STUDY

Osiris Cult Statuette

An in-depth study of materials and techniques, plus spot treatment of bronze disease.

 

La Sierra University, Center for Near Eastern Archaeology, LS 02.3030

THE OBJECT

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The object is a cast metal figure of Osiris, king of the netherworld/underworld, in mummiform wearing a tight, fitted shroud and a false beard. He wears the crown of Upper Egypt (atef) composed of a central miter flanked by ostrich feathers/incised plumes. The miter has an uraes serpent on the front, extending up the crown. A pair of twisted and incised rams horns project horizontally along the bottom of the plumes, and two small uraei stand upright on either side of the plumes, with sun disks above their heads. Finally, the figure holds a flagellum (nekhakha) in his left hand and a heka scepter in his right.

 

There are two small loops extending from behind the lower part of his crown which look to be attachment points, perhaps for dangling adornments of precious metal and stone which were often hung from figures (1). The tang below his feet indicates the figure was mounted into a base, likely a portable wooden shrine that was then fitted into a stone pedestal located within the sanctuary of a temple. The object most likely functioned as a cult statuette (2).

All technological features of manufacture and metallic composition support the figure’s authenticity, and dates the object to the 1st millennium B.C.E., specifically the Late Period (ca. 650 - 330 B.C.E.).

MATERIALS

X-ray fluorescence spectroscopy (XRF) at the front of the figure's crown indicates the object is alloyed of copper, with notable peaks for lead and minor peaks for tin and iron. Identical spectra were achieved at locations on the figure’s legs and hands. Copper alloy compositions containing large amounts of lead and negligible amounts of tin are well known to ancient Egyptian castings from the 1st millennium B.C.E. (1-3).  

Copper-lead alloys are heterogeneous in their microstructure, as the two metals have very low solid solubility in each other. This is reflected in the mottled appearance of the object’s x-ray, which shows an uneven wall composition full of more radiodense (the lead) and less radiodense scatterings. The addition of lead lowered the melting temperature of the metal and reduced its porosity (3).

FABRICATION

X-ray radiography reveals the statue to be hollow cast, most likely executed by lost-wax casting. The weight of the figure indicates the core is still present, which would further imply that it was core cast. Core casting reached its peak in the Late Period.

A tube-like feature is visible in the center, starting at the figure’s torso and extending all the way into the crown. Its purpose is unclear, however, it may have been an element used to support the clay core as it was modeled. Its shape alludes to an organic item, such as a piece of bamboo, which may have burnt away during the manufacturing process after being coated by metal somehow.

The eyes lack radiopacity, which may indicate a void in the wall of the metal. Egyptian copper alloys were generally cast to their final forms, and then finished off with the addition of details. It's likely the eyes were inlayed with a semi-precious stone or glass. Discrete patinas in certain areas also suggest decorative metal inlays, however this was unconfirmed in XRF analysis.

Core supports/chaplets in ancient bronzes tend to corrode quicker than the surrounding metal, resulting in small, rectangular voids, like those seen in the front and back walls of the figure's legs. The extent of corrosion on the surface of the object prohibits identification of the core supports on the exterior of the bronze, so x-ray radiography often becomes the only method allowing for their identification. Were this a more modern figure or a forgery, a great deal of core supports would be observed in the x-ray; authentic, ancient figures contain only a modest amount (3). 

The core supports were likely iron. A comparable Late Period Osiris figure belonging to the Rijksmuseum van Oudheden has core supports identified as iron (6), and iron core supports were only used after the Third Intermediate Period (after 664 B.C.E.) (7).

The tang, extending 3.0 cm from the bottom of the figure, is tapered and four-sided with a flat end, and would appear square/rectangular in cross-section. Tangs took this form after the New Kingdom (after 1070 B.C.E.) in the Late Period (7), and were also solid-cast and mechanically inserted (rather than cast-on) into a receiving slot at the base of the object (3).  X-ray images reveal that the tang is indeed solid-cast, and appears to be inserted into the figure as well, consistent with the object’s Late Period attribution.

 
 

CONDITION

The most notable structural damage is a large break that occurs at the figure’s ankles, likely the result of it being forcibly removed from its base. This is further supported by the overall curved or bent nature of the object. The damage and loss extends within the object, where there is dark, granular material. Powdery green corrosion is also noted above this black material.

It's possible that this area is a combination of metal and core material, since core material of ancient Egyptian objects is described as a mixture of clay, sand, and organic material, which becomes charred and blackened in the casting process (1,4). However, XRF at this area showed the presence of silver, which could indicate silver chloride (characteristically black)—the product of silver oxide bronze disease treatments. Further analysis of this material, such as using x-ray diffraction, is required to accurately understand what is present.

The surface of the object clearly indicates a previous history of treatment. The corrosion—a mixture of several forms and colors of cuprous corrosion products—has been brought down to a relatively even (though pitted) level. The pitted nature of the surface suggests deleterious chemical cleaning, and there are also signs of mechanical filing. 

FILING MARKS

Several areas over the surface contain small pits filled with bright green corrosion products, often associated with active bronze disease. A few areas contained spots that tested positive for the presence of chlorides using the silver nitrate test. The presence of bronze disease would support the evidence of a silver oxide treatment at the ankle break. Despite its presence, the bronze disease does not appear pervasive and seems to be progressing very slowly, if it is progressing at all; it is possible these small pits have been in this exact state for a very long time.

BEFORE TREATMENT

 

TREATMENT

Treatment consisted of attempting to remove residues and coatings over the figure that were observed under UV-induced visible fluorescence, and pacifying areas suspected of active bronze disease. Some residues were able to be removed or reduced, however there was little success at solubilizing the aged coating(s).

 

Since the old coatings remain, it was deemed inappropriate to apply more coatings wholesale when considering how to treat the bronze disease—such as with a corrosion inhibitor and protective layer—and more importantly, it would be unsafe to immerse or expose the object to prolonged contact with the solvent contained in a corrosion inhibitor, since it is unknown how the old coatings would react. Instead, areas of bronze disease were spot-treated using silver oxide.

BEFORE TREATMENT

AFTER TREATMENT

First, the spots were cleaned of corrosion products using a scalpel until nantokite was reached, and then the areas were swiped with acetone on cotton swabs. Silver oxide was mixed with ethanol to form a paste, and then applied to the excavated pits. After a couple hours, it was assumed the mechanism had occurred, in which the silver oxide reacts with the copper (I) chloride to form harmless copper (I) oxide. Loose particles were then blown away with a bulb blower and the pits were filled with Cosmolloid 80H wax, colored with dry Kremer pigments and applied with a slightly heated spatula. Last, the spots were coated with ~5% Incralac in toluene using a brush, to provide protection from water.

Materials & Suppliers

Silver Oxide Powder—Ag2O

CAS No. 20667-12-3

Fisher Science Education,

15 Jet View Drive,

Rochester, NY 14624 USA

1-800-955-1177

 

Cosmolloid 80H—a synthetic, microcrystalline wax

Talas (# TCD021002)

330 Morgan Ave,

Brooklyn, NY 11211 USA

http://www.talasonline.com

Dry Pigments

Kremer Pigments Inc.

247 West 29th Street

New York, NY 10001 USA

212-219-2394

 

Incralac—methyl methacrylate copolymer with benzotriazole

Conservation Resources International, LLC

5532 Port Royal Road

Springfield, Virginia 22151 USA

800-634-6932

 
 

References

  1. Schorsch, Deborah and James Frantz. (1998). A Tale of Two Kitties. Appearance and Reality: Recent Studies in Conservation, The Metropolitan Museum of Art Bulletin, Winter 1997-1998, 55(3), 16-29.
     

  2. Ghoniem, Mohamed A. (2014). A Bronze Osiris Statuette from the Egyptian Museum in Cairo: Microstructural Characterization and Conservation. Mediterranean Archaeology and Archaeometry,14(1), 37-49.
     

  3. Schorsch, Deborah. (1988). Technical Examinations of Ancient Egyptian Theriomorphic Hollow Cast Bronzes-Some Case Studies, in Conservation of Ancient Egyptian Materials, eds. Sarah C. Watkins and Carol E. Brown, 41-50. Preprints of the conference organized by the United Kingdom Institute for Conservation, Archaeology Section, held in Brisol, December 15-16, 1988.
     

  4. Nichols, Paul T. and Ian Shaw, eds. (2000). Ancient Egyptian Materials and Technology. Cambridge, UK: Cambridge University Press.
     

  5. Scheel, Bernd. (1989). Egyptian Metalworking and Tools. UK: Shire Publications Ltd.
     

  6.  Hill, Marsha. (2007). Casting About: The Late Period (664-332 B.C.) and the Macedonian-Ptolemaic Period (332-30 B.C.), in Gifts for the Gods: Images from Egyptian Temples, eds. Marsha Hill and Deborah Schorsch, 128. New York: Yale University Press.
     

  7. Schorsch, Deborah. (2007). The Manufacture of Metal Statuary: Seeing the Workshops of the Temple,” in Gifts for the Gods: Images from Egyptian Temples, eds. Marsha Hill and Deborah Schorsch, 193. New York: Yale University Press.

 

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