Tag Archives: Workshops

Conference Proceedings: Gemstones in the first Millennium

Researchers from different fields like archaeology, history, philology and natural sciences present their studies on ancient gemstones. Using precious minerals as an example, trade flows and craftsmanship, but also utilisation and perception are discussed in a cross-cultural and diachronic approach. The present volume aims at three main questions concerning gemstones in archaeological and historical contexts: »Mines and Trade«, »Gemstone Working« as well as »The Value and the Symbolic Meaning(s) of Gemstones«.

 

This volume contains the proceedings of the conference »Gemstones in the first Millennium AD« held in autumn 2015 in Mainz, Germany, within the scope of the BMBF-funded project »Weltweites Zellwerk – International Framework«.

You can preview the table of content and a preface here.

Technical Experiments 1: Producing a piece of silver foil

As an alternative to high karat gold foils fire-gilt silver foils have been used to produce the patterned Early Medieval backing foils. For my experiments, I needed a silver foil of a material thickness of 0,025 to 0,03 mm. My starting material was a sheet the size of 10×10 cm and a thickness of 1 mm. It was composed of 93,5% silver, 6,2% copper and 0,3% zinc. The authentic way to reduce the metal thickness would have been to draw down the material by forging. but to save time I decided to thin the sheet by rolling using a hand-operated rolling mill. The metal is worked in a cold state. It is primarily stretched in length. Because the process creates stresses in the crystal structure, the workpiece has to be annealed after several passes though the rolls: You have to heat it to a red state and then cool it down. Rolling the silver sheet to a material thickness of 0,03 mm would mean that its length expands up to 4 m! So repeated cutting off of smaller pieces was necessary. I started to roll the sheet immediately, reducing the distance between the rollers step by step, and I could go on until a material thickness of 0,1 mm was reached. Then the gap between the rollers couldn’t be made any smaller. So I continued sandwiching the silver between two copper sheets. During the whole rolling process the workpiece had been periodically annealed. The thinner the sheet became the greater was the danger of melting. Finally, I held two silver sheets, sizes 2,7×6,6 cm and 2,7×5,2 cm, of a material thickness of 0,03 mm in my hands, with some small stress cracks along the edges and both a little bit warped, probably because of an irregular roller pressure.

Fig.1: surface area melted during the annealing process, image field: 10,5×15,5 mm (photo: RGZM/Stempel)
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Fig. 2: stress cracks along the edge of the sheet, image field: 8×12,6 mm (photo: RGZM/Stempel)

 

Lit.: E. Brepohl, Theory and Practice of Goldsmithing, Brunswick 2001.

Manufacturing and uses of backing foils – some technical experiments

Hello, I am Christiane Stempel, goldsmith, conservator and responsible for the technological examination of Early Medieval garnet objects that have been brought to the Römisch-Germanisches Zentralmuseum in Mainz for scientific analyses within the project “Weltweites Zellwerk”.

During my work, I made some observations that raised questions regarding the manufacturing and the use of the patterned foils behind the garnets.

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Fig. 1: Schematic figure of an Early Medieval garnet cloisonné (photo: RGZM/Ober)

Transparent stones were often underlaid with gold foils to reflect the light through the stone. This is particularly important when the stone is backed up with cement. In the Early Medieval period textured backing foils were used on a large scale. The three-dimensional pattern increased the reflective effect. I examined a great number of objects from different find-spots in Sweden, Anglo-Saxon England and the Rhineland and I found all the varieties of pattern commonly known from the literature such as standard waffle, boxed waffle (Varying in the number of enclosed squares [9 to 25]), ring-and-dot, lozenge, boxed lozenge and rectangle (stack bond). They only differ in fineness (number of lines/mm), depth of texture and contour sharpness.

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Fig. 2 a-f: Foil pattern (RGZM/Stempel)

 

Unfortunately, the project nears its end and I would like to use the remaining time to carry out some technical experiments:

Producing the pattern

There are indications (see figures 3-7) that at least a number of foils weren’t manufactured with dies. It looks as if lines were traced immediately on to the foils to form the grid pattern.

Fig. 3: Buckle, Ailenberg, Württembergisches Landesmuseum Stuttgart A.V. III333 (photo: RGZM/Stempel)
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Fig. 4: Disc brooch, Junkersdorf, Römisch-Germanisches Museum Köln 51,539 (photo: RGZM/Stempel)
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Fig. 5: Disc brooch, Monsheim, Römisch-Germanisches Zentralmuseum Mainz O.15370 (photo: RGZM/Stempel)
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Fig. 6: Disc brooch, Iversheim, LVR Landesmuseum Bonn 1960.667 (photo: RGZM/Stempel)
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Fig. 7: Disc brooch, St.Severin, Römisch-Germanisches Museum Köln 50.286 (photo: RGZM/Stempel)

N. D. Meeks and R. Holmes described in their article “The Sutton Hoo Garnet Jewellery” an experiment with a scriber that they used to draw the lines directly onto the foils. But in their opinion, the result was not satisfying. (N. D. Meeks/R. Holmes, The Sutton Hoo garnet jewellery: an examination of some gold backing foils and a study of their possible manufacturing techniques. Anglo-Saxon Studies in Archaeology and History 4, 1985, 143-157.) I would like to pick up the idea again, but instead of a scriber I will do the work with a profiled hand-operated tool, comparable to a creaser. It is used until today in bookmaking and leather processing to imprint decorative lines onto a leather surface. In contrast to a pointed scriber the elongated working face of a creaser can be of any cross-section.

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Fig. 8: Creaser (photo: RGZM/Steidl)

 

The theory that has to be verified in practice is as follows:  The grid pattern can be formed with a sliding forward motion of the tool, running it along a ruler with moderate pressure to form parallel lines, then rotating the workpiece by 90 degrees and repeating the action. For my experiments, I will use a small creaser-like tool that I have made by removing the cut of a square rifle file. As foil materials, I will use tin foil (0.03 mm) as well as sterling silver foils of different thicknesses (0,025 mm to 0,04 mm).

Using foil for stone securing

Not a small number of foils seem to have a further purpose: In many cases, the foils are trapped between the stone edges and the surrounding metal of the setting. Is it possible to hold a stone in place with the foil?

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Fig. 9: Disc brooch, Hürth-Kalscheuren, LVR Landesmuseum Bonn XV (photo: RGZM/Stempel)

If times permits, I would like to produce a foil in the way outlined above using a fire-gilded silver foil. In the next few weeks, I will report about my experiments and I am curious about the results and your comments.

 

Featured image: Buckle, Endre socken, Statens Historiska Museet Stockholm 484:12, detail: garnet cloisonné with foils behind the garnets. (photo: RGZM/Stempel)

Fatimid rock crystal carving techniques (10th -12th century AD)

Lecture held by Elise Morero, Oxford

The Fatimid caliphs of the 10th- to 12th-century C.E. are well known for their patronage of luxury arts, especially carved rock crystal vessels, although the manufacturing techniques employed in their production remain largely unknown. The processes of Fatimid rock crystal carving were investigated using a multidisciplinary method based largely on: the observation at various scales of the manufacturing traces on the surface of the objects; tribological analysis; the analysis of archaeological and ethnographic data; and also experimental reconstructions of ancient techniques.
These complementary methods enabled us to identify the techniques and tool-kit used by Fatimid craftsmen. The hardness of rock crystal (Mohs scale 7.0), together with its homogenous crystalline structure, makes the use of percussive tools inefficient. Instead, the crystal must be ground away using an abrasive powder and a lubricant. The mixture was carried to the surface of the crystal using different tools put in motion by a horizontal bow-lathe. It was determined that a particularly famous group of seven ewers was produced by a few specialised workshops, probably located at Fustat (Old Cairo), in about the year 1000 C.E. 11

Lecture held during the conference “Gemstones in the first Millennium AD. Mines, Trade, Workshops and Symbolism.” October 21st, 2015 at the Roemisch-Germanisches Zentralmuseum, Mainz (Germany).

Foto: Fatimid rock crystal ewer (Francis Mills ewer – Keir Collection, Dallas Museum of Art)