The Cleaning and Restoration of Ancient Bronzes

By: M. R. W.

Originally Published in 1933

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ALMOST without exception the bronze and copper objects received in the Museum are in the process of disintegration. This disintegration is due chiefly to the chlorides and nitrates that are to be found in the soil in which the objects have been buried, and which have converted the constituents of the bronze into an incrustation of blue or green with underlying layers of copper oxide. The degree of disintegration varies with the content of the soil and the content of the object and may result in a simple discoloration, in the formation of a thick green crust, in the complete destruction of the metal core, or, last and most serious, in various stages of the so-called ‘Bronze Disease.’ Any one of these conditions will in time reduce the object to an indistinguishable mass of powder.

An assortment of encrusted bronze objects
Plate VIII — Ancient Bronzes from Iraq, Before Cleaning

The advantages of cleaning and restoring these ancient bronzes are three-fold. First, eradicating, or at least arresting the progress of, the Bronze Disease. Second, restoring the object to its original form, which has become distorted by the presence of the incrustation, and so make it possible for the scholar to make a comparative study of the forms of the different types of bronze and copper implements. Third, bringing to light any designs which the incrustations may have obscured such as incised decorations, makers’ signatures, hieroglyphs, or cuneiform signs which would be of assistance in dating the object or more definitely establishing its use.

The cleaning of copper and bronze in this Museum has been done by the electrolytic process. This process consists of suspending the object by fine copper wires in a tank of caustic soda, through which an electric current is passed. At the end of a period lasting from six to nine months the incrustation has been broken down and the object is ready for the final steps of restoration.

With the ever-increasing number of bronze and copper specimens that come yearly into the Museum from the several expeditions in the field, it was decided recently to introduce other methods of cleaning and restoration in order that a greater number of objects could receive the necessary treatments as quickly and promptly as possible. The most effective and satisfactory of these methods is a zinc and caustic soda treatment. In this process the object is buried in zinc and covered with a solution of caustic soda. After a period of from twenty-four to forty-eight hours, the object is taken out and the incrustation is so undermined and eaten away that it is easily removed by a careful application of the mechanical treatment. This process often leaves a layer of black or red oxide which may be removed either by frequent washings in running water or by momentary emersion in an acid bath. The object is then carefully brushed and burnished and coated with a preservative shellac. It has been frequently found that this treatment not only restores the object to its original form but also restores life and elasticity so that an object which before treatment was rigid and brittle after treatment was flexible and mobile.

An assortment of cleaned bronze objects, and one encrusted bronze mirror
Plate IX — Ancient Bronzes from Iraq, After Cleaning

Plates VIII and IX, while not exact duplicates, give an idea of the condition of the bronzes before and after cleaning. The objects appearing in both plates which yielded most satisfactorily to treatment are the two ringed pins, the bracelets, and especially the so-called chatelaine, which is shown in the left foreground of both photographs.

In the center of Plate IX are two mirrors, one cleaned and the other not yet cleaned, showing the degree to which corrosion may distort and destroy an object, and the extent to which treatment may restore it. The mattock in the right center of Plate IX responded well to treatment, and cleaning revealed the trade-mark or owner’s sign in the form of five circles set in the shape of a cross. Of especial interest, however, is the lance butt (extreme left) upon which was unexpectedly discovered a well cut cuneiform sign of a distinct archaic period. (Regarding this sign, see the article by Dr. E.’ A. Speiser in the Bulletin of the American Schools of Oriental Research, April, 1933.)

M. R. W.

Cite This Article

W., M. R.. "The Cleaning and Restoration of Ancient Bronzes." Museum Bulletin IV, no. 4 (June, 1933): 110-114. Accessed July 15, 2024. https://www.penn.museum/sites/bulletin/1191/


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