Conserving the Fitzwilliam's Egyptian copper and copper alloy objects
The Fitzwilliam's Egyptian collections contain many objects made from copper or copper alloys. Copper alloys are a mixture of copper with other metals. Bronze, for example, is an alloy of copper and tin, often with the addition of a little lead to the mix.
102 of these objects are due to go into the new displays. Some of them have not been examined closely, so the period before installation in the galleries is an ideal opportunity to investigate their construction and manufacture as well as assessing their current physical condition. The first step in the process is to locate each object within the Museum's reserves and make a preliminary assessment of its condition. This makes it easier to plan for any research and investigation necessary and also to decide whether the object is in need of conservation treatment.
The primary methods of investigation for the objects are x-radiography and γ-radiography. γ-rays ('gamma' rays) have a higher energy level than x-rays, so they are able to penetrate thicker and/or denser objects more readily, producing a clearer image than x-rays. The images produced by these methods can tell us how objects have been put together and what manufacturing techniques the Ancient Egyptians used. Some of the features shown on the x-ray and γ-ray images are illustrated in the pictures on this page.
The first image is the γ-radiograph of a bronze hawk representing the god Horus wearing the double crown of Egypt. Several interesting features can be seen, the most obvious being the white mass in his crown (marked on the image as A), indicating the presence of a material much denser than the surrounding bronze. This is likely to be a lump of lead, which is sometimes found in Egyptian copper alloy sculpture, particularly in the heads of cats. The purpose is unknown.
Another feature brought out by the γ-radiography is the very fine decorative detail of the wings and feathers (B). Much of this cannot be seen with the naked eye as the surface is obscured in places by layers of thick corrosion product. This image also shows core pins (C) used in the manufacture of this object by the hollow-cast lost-wax casting process. In this method a thin layer of metal is cast around a clay core. Core pins hold the core in place in the mould during casting. More information about the lost wax casting process can be found in the Museum's Making Art web pages.
The next image is an x-ray of a statue of the goddess Neith, sitting on a throne with a miniature Amun striding before her. A lump of lead (D) can also be seen in this object, although in this case it is in the throne. This object shows how x-rays can be useful in showing past conservation treatments. There are two fragments missing from the legs of Amun and at some point in the past his legs have been repaired by soldering metal rods between the breaks and then reconstructing the rest in coloured wax, which does not show up on the x-ray (E).
This image also explains how the statue has been attached to a plaster mount (probably when the repairs were carried out). The statue and the mount have been joined together with metal rods embedded in the plaster (F). This information helps us decide how best to remove the old mount without damaging the object.
The conservation needs of the copper and copper alloy objects are varied. Many need to be removed from 19th and 20th century mounts for a variety of reasons: the mount may be broken, or it does not support the object appropriately, or more sympathetic mounting materials are now available. Other objects may have old repairs (such as the legs of the Amun in the example above), which have failed and need to be replaced.
One of the most serious problems for copper and copper alloy objects is corrosion. There are many different types of corrosion, some bad, some good. Under certain conditions a layer of protective corrosion products can form over the surface of the artefact. But more aggressive types of corrosion cause damage by creating pitting in the metal surface, thus weakening and disfiguring the object. So-called 'bronze disease' is often found on excavated objects which have been in contact with chloride salts during burial. Treatment of this 'active' corrosion is very important in order to stop further deterioration.
Conservation is also concerned with monitoring any changes in the objects. Radiographs, photographs and condition reports are used to provide a record of an artefact so that we can make comparisons at future dates to determine whether its condition has changed. Another part of the conservation process is taking steps to prevent active corrosion from occurring. Bronze disease can only form in high humidity conditions, so we store our metal objects in a special reserve area equipped with dehumidifiers and environmental monitoring sensors to ensure that they are kept in safe conditions.