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.
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.
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
condition before treatment:
At first the green corrosion gave the impression that the object
was just another small square plate of copper alloy.
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.
|East K.; A study of the cross-hatched gold foils from Sutton Hoo, Anglosaxon Studies in Archaeology and History
4, 1985: 129-142|
|Meeks N.D. & R. Holmes; The Sutton Hoo garnet jewellery: an
examination of some backing foils and a study of their possible manufacturing
techniques; Anglosaxon Studies in Archaeology and History 4, 1985: 143-157|
|Nijboer A.J. & J.E. van Reekum; Scientific analysis of the
gold disc-on-bow brooch, The Excavations at Wijnaldum, reports on Frisia in
Roman and Medieval times volume 1, 1999: 203-215|
|Tulp, C. & N. Meeks; The Tjitsma (Wijnaldum) die: a 7th century tool
for making a cross-hatched pattern on gold foil, or a master template?; Historical
Metallurgy 34 nr.1 (2000) |
|Meeks N.D., C. Tulp & A. Söderberg; Precision lost wax casting; Proceedings
of the 1st International workshop on Experimental and Educational aspects of
Bronze Metallurgy; Wilhelminaoord 1999|