bronze die 

for making box pattern backing foil in cloisonné cells

origin: Wijnaldum (province of Friesland, the Netherlands), 1996

date: 7th century AD

Microscopic cleaning of unidentified finds from the Wijnaldum excavations in the laboratory of the LCM revealed a positive die for making box pattern backing foil in cloisonné cells (figure 1). Due to the disfiguring corrosion, the object had not yet been recognized as a die. 

The cleaning was carried out with the aid of a microscope and revealed a very fine 'wafer pattern' (figure 2). Dies like these were used to make a pattern in metal foil. If such a foil is placed in a piece of jewelry under a transparent precious stone, the brilliance of the precious stone improves considerably. The gold disc-on-bow brooch of Wijnaldum (Fries Museum, Leeuwarden) shows many examples of this technique (figure 3).

Figure 4 shows a detail of the brooch where backing foil is placed behind transparent red almandine (a garnet). 

The Wijnaldum die is a positive one; the pattern of squares and squares within squares protrudes. This is comparable to the lead characters once used to print texts. Besides positive dies, negative dies are known. A negative die has incised grooves, comparable to the grooves in a copper engraving.

Dies for making patterned backing foil have so far been mainly found in Southern England, the region adjoining the current Waddenzee and Denmark. The treasure of Sutton Hoo, found in the 1930s during excavations at Woodbridge (Suffolk, England), is probably the most famous and appealing example of so-called cloisonné jewelry.

The discovery of the Wijnaldum die makes it possible to establish the technical skills of the goldsmith working at the site. Besides jewelry other tools for metal working have been found: several touchstones and a small melting pot for gold. In addition unworked almandine was found. Moreover, laboratory research on the brooch itself indicates that it was intentionally taken apart. This leads to the conclusion that the brooch was repaired or partially reused.

conservation

condition before treatment:
At first the green corrosion gave the impression that the object was just another small square plate of copper alloy.

treatment:
The hardness of the corrosion made it difficult to clean the object mechanically without damaging the artefact. Therefore an attempt was made to remove some of the corrosion by means of chemicals. The object was wrapped in zinc foil and immersed briefly in a 5% solution of sodium-hydroxide (method Krefting). After the first treatment, traces of a pattern became visible. Part of the firm corrosion was dissolved and most of the remaining corrosion of cuprite and nantokite could be removed mechanically . An X-ray photo made before the  start of the treatment revealed the presence of primal metal but gave no indication of the pattern now slowly emerging from under the corrosion. The mechanical treatment uncovered the pattern as still present (figure 1).  Unfortunately not all the nantokite could be removed. There was a risk of a new corrosion process in which nantokite is transformed into paratacamite (due to the presence of chloride ions in the metal). For this reason, the die was treated with a 3% Benzotriazole (BTA) solution in ethanol. Finally it was coated with a 5% solution of Paraloid B72 in acetone.

After the conservation treatment, tests were done to determine how the die could be used to make patterned foil. Figure 5 shows the result of an experiment in which aluminium foil was rubbed onto the die, with satisfactory results. The condition of the die did not allow punching.

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