ancient metals

This is the story of an ancient iron Chinese belt hook that was treated at the DAM conservation lab in 2006 by Julie Parker.

The iron Chinese belt hook with gold and silver foils had undergone severe corrosion due to exposure to high humidity, which triggered the conversion of ferric chloride residues from burial to the more expansive oxides and hydroxides. The DAM's treatment focused on removing a sufficient volume of corrosion products to recreate the belt hook’s original form and appearance. The museum will utilize a climate controlled storage case to maintain stability in the future.

History & Function of Belt Hooks

More than two thousand years ago, this hook was buried with its owner to convey his wealth and status into the afterlife. Much like a prizefighter’s belt or a broncobuster’s silver buckle, ancient belt hooks were signs of an individual’s rank and status in Chinese society. In different periods, regulations specified which metals or decoration could be used by members of each rank, so the type and quantity of belt hooks in a burial demonstrate the individual's status.

Belt hooks were composed of long, thin pieces of metal—usually bronze or iron—or stone. They were attached to one end of a cloth or leather belt with a rivet-like stud and hooked into the other end to fasten the belt.

Structure & Properties of Iron

Our particular belt hook was made of iron with an applied gold and silver spiral motif.  It is likely the iron core was cast, either as one piece or in sections, though the corrosion patterns might indicate some hand forging. The gold decoration on the surface was likely applied as a sheet and then incised to make the decorative shapes and to accept the silver inlay.  

Most people consider metal to be solid, but in fact all metals are made up of crystalline grains. Generally, the faster a metal cools the smaller the individual crystals will be. Crystal surfaces form grain boundaries between each other, and these are points of weakness where outside physical and chemical forces can attack the metal.

Our belt-hook was made of “cast” iron-—the name refers to the both the alloy composition and the fabrication process. It has a higher carbon content than steel and also contains slag, a glassy residue from the ore-smelting process. The molten metal is cast, or poured, into a mold and allowed to cool rapidly. The resulting iron has a structure (shown above) that contains many small crystals, spaces, and inclusions. Such metal is fairly porous and vulnerable to infiltration by moisture and soluble salts. (This is why cast iron skillets are usually sealed with oil prior to use in the kitchen.)  

In the burial environment, objects in contact with soil and groundwater can become saturated with various dissolved salts. Once the artifact leaves the stable burial environment and encounters changes in relative humidity in the air, the salts crystallize and expand. In worst cases, these crystals literally make the object explode. This phenomenon can occur with many types of porous materials including metals, ceramics, and the plaster layers of wall frescos.

With metal objects like our belt hook, the salt also reacts with metal to create an accelerated, unique type of corrosion. In addition to producing a rust-like material, a strong acidic by-product is also formed which continues to attack the metal. A cycle is set up that will continue until all metal is reduced to corrosion. Fortunately, it is possible to break the cycle and preserve the artifact in the museum through conservation.     

Treatment of a Damaged Object

At some point in the past, the belt hook experienced a drastic increase in relative humidity that caused a disfiguring growth of salt crystals and expansion of iron corrosion products. The image below shows how extensive the damage was when it entered the conservation lab in 2006. The neck was nearly detached, and corrosion products wept from the interior and covered areas of surface gilding. Acidic byproducts had also eaten away at the lining of its storage box.   

Treatment involved several steps. The corrosion layers that had broken the hook apart were manually reduced by scraping them down with a fine scalpel blade and other small tools under high magnification. The conservator applied fine Japanese tissue to the entire surface with a reversible synthetic resin to protect the decorative surfaces and hold any weakened areas together during the operation.  

After the corrosion was removed, it was possible to coax the hook fragments back into a more appropriate shape. The piece was supported throughout this process with a flexible copper armature which could be bent to match the progressing shape of the hook.

When the proper alignment was attained, the pieces were rejoined in position with a reversible synthetic adhesive. Surfaces of losses in the body were coated with an isolating layer and then filled with a mixture of adhesive, glass microballoons, and sympathetic natural pigments for added strength.

Once stabilized, it was possible to remove any remaining corrosion products from the elaborately inlaid surface and reintegrate small surfaces losses with a similar filling compound. Finally the object received a custom-made display mount created for support and to prevent any future need for direct handling.

After Treatment: Climate Control is the Key

Conservation treatment has returned the belt hook to a point where the original appearance has been largely recovered, but the piece is not really stable without good environmental control. Since complete desalinization was not possible, there are still salts within the structure of the metal, and high relative humidity could restart the corrosion growth cycle.

To protect the belt hook in the future, it will be stored in a special sealed cabinet which contains silica gel designed to keep the amount of moisture inside very low and constant. Exhibition is also possible using controlled humidity. While this solution is specifically designed for this type of sensitive archaeological metal object, it highlights several important factors that can be applied to the preservation of similar artifacts, the key words are “constant”, “controlled,” and in this particular case, “low” humidity.    

Controlling relative humidity is one of the most important things that can be done to protect art. The optimum storage or exhibition temperature and humidity level will depend on the type of material and the function that it is to serve. Metals which are subject to corrosion are best kept at low relative humidity; 30% RH is appropriate for the extended preservation of most metal objects. It is also accepted that repeated cycling between high and low humidity causes expansion and contraction in materials. Maintaining a constant level of RH is important so that the swelling and shrinking, which cause most of the physical damage experienced by an object, can be avoided. 

Other types of materials such as leather, textiles, and wood require different humidity and temperature conditions; you should consult a conservator or research specific guidelines for care of the types of objects in your own collection.

Caring for the Past, Caring for the Future

In the ancient Chinese tradition decorative ornaments like this belt hook were meant to adorn their owners in life and accompany them into eternity. Historically the family would have continued to care for and honor the deceased, but as time passed those funerary objects have come to be part of a larger family in the DAM collection. Every day in the museum, we practice diligent ongoing maintenance for the art objects in the collection, so they should remain fine examples for many future generations.

This conservation message is generously supported by the William H. Donner Foundation.