Tag Archives: Natural Science

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.

Gemstones and Gem Deposits of Sri Lanka – with special emphasis on Geology, Occurrence , Varieties and Mining.

Dr. Gamini Zoysa, Ceylon Gemmological Services

Sri Lanka has long been known as one of the world’s most important gem producing countries. Specially as a leading source for fine quality Blue Sapphires & its other varieties. The genesis of gemstones in Sri Lanka has been a subject for much discussion. The gems are mostly mined from the alluvial deposits underlain by Precambrian metamorphic rocks. The main gem bearing areas are confined to the geological division called highland complex which is located in the central part of the country. Over 3200 legal gem mines have been in active operation in the past years. The depth of the gem gravel in a shaft may vary from 10 to 25 meters. There are also evidences for occurrence of primary deposits of Sapphire, Moonstone , Aquamarine, Garnet within the island. In addition to the major commercially viable gem varieties such as Sapphires, Garnet, Chrysoberyl, Topaz, Zircon, Moonstone, Tourmaline, Beryl, Quartz there are over 80 varieties of nontraditional gems are recorded.

Dienstag, den 25. Oktober, 18.15 Uhr

Römisch-Germanisches Zentralmuseum – Vortragssaal

Im Rahmen der Vortragsreihe “Weltweites Zellwerk”

See the full program here!

 

Foto: (c) http://www.aigsthailand.com

Archaeogemology and ancient literary sources on gems

Lecture held by Lisbet Thoresen, Temecula, CA

Archaeology and discoveries of new gemstones and new gem sources in recent decades attest to the need for critical review and updating of literature in translation concerning gems of the ancient world. The origins and identities of gemstones used in ancient glyptic have been inferred almost exclusively from literary descriptions available in secondary or even tertiary sources after now-lost ancient original texts. To date, no epigraphical or philological study has verified the ancient gem cutters’repertoire of materials against empirical gemological examination of extant material in public or private collections. However, such objective data should improve interpretation of literary source material that is often fragmentary or contains descriptions fraught with lexical ambiguities and contradictions. A carefully qualified perspective is needed. Whether in original form or in translation, manuscripts, from antiquity to the present day, reflect some degree of current knowledge about geography and gems in the contemporary world of the author/epigrapher/translator. Contemporary knowledge attributed to earlier cultures is an unwitting bias that frequently eludes both translators and scholars. Together with critical examination of the imprint of authorial bias, a gemological review of extant material is discussed in relation to the important treatises on gemstone nomenclature, identity, and geographic origin.

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).

Examination of garnets and their provenance

Deutsche Version

Within my master thesis in the degree course geosciences at the Johannes Gutenberg-University in Mainz I examine the spectroscopic properties of garnets as well as the inclusions in them with different methods. The goal is to get better statements for the provenance of gemstones.

Ullmann_mikroskop
Working at the spectrometer Nicolete 6700 FT-IR to collect spectra in the mid-IR and near-IR spectral ranges.

The term Garnet stands for a group of various minerals, which are seen similarly in the crystal structure and formula. They may be partial mixable among themselves. In this case miscibility means that two or more different minerals from the group of garnets can occur together and form one unit, thus one crystal, because of their similar structural and chemical composition.
Essentially, there are two series, the Pyralspit- and Ugrandit series. The former is aluminum garnets with magnesium (Pyrope), iron [three] (almandine) or manganese (Spessartin), temptation are calcium-grenade with chromium (Uwarowit), aluminum (Grossular) or iron [two] (Andradit). The garnet deposits of the jewels in usual belong to the Pyralspit group.


Through to the large mass of processed garnet in the early European Middle Ages, provides the question for the originated area of the raw material. In addition, the garnets were cut into thin platelets and these were used across the board, but remains of garnet grinding services are not to be found yet. This raises further questions as to whether the garnets were imported already honed and in which direction they were exported as raw material as well as already honed exemplar.

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Examples of garnet platelets from a find in sweden. Foto: Michael Rychlicki

In order to determine possible areas of origin, the chemical compositions of as many garnet platelets are determined by X-ray fluorescence analysis. By this method, groups of garnets indicate which originated most likely in India, Sri Lanka and Bohemia. However, these allocations are based on the comparison with measurement results of garnets, which origin is not 100% guaranteed, because of the lack of exact details like the accurate provenance for example.

CaO/MgO-plot for the garnet finds in Sweden and in India. (In accordance to Greiff 2010)

Therefore it is the more gratifying that now samples are available that guaranteed are hailed from different federal states of India and were picked up by Borayin Larios himself.

Garnet samples from different mines in India (on 5mm square papaer)

These are examined on their chemical composition as usual and the results are compared with already collected data to improve the currently used method to assign the garnets origin by their chemistry. In addition, these samples, and other samples found in Sweden, using modern spectroscopic methods (UV-Vis, NIR-MIR and Raman spectroscopy) are examined to determine a dependence of the spectra-differences to the origin of the samples. In this manner discovered unique features may be useful as an additional discrimination criterion. Furthermore, inclusions are studied to show possible dependencies between the provenance and the other minerals.

Untersuchung von Granaten und deren Herkunft

Im Rahmen meiner Masterarbeit im Studiengang Geowissenschaften an der Johannes Gutenberg‑Universität in Mainz untersuche ich Granate mit Hilfe verschiedener Messmethoden auf ihre spektroskopischen Eigenschaften sowie ihre Einschlüsse. Das Ziel ist es, genauere Aussagen über die Herkunft der Edelsteine treffen zu können.

Ullmann_mikroskop
Arbeit am Spektrometer Nicolete 6700 FT-IR um Messungen innerhalb mittlerer und naher Infrarotstrahlung vornehmen zu können.

Der Begriff Granat bezeichnet eine Gruppe von verschiedenen Mineralen, die sich in der Kristallstruktur und -formel ähnlich sehen und untereinander teilweiße gut mischbar sein können. Unter Mischbarkeit versteht man hierbei die Tatsache, dass zwei oder mehr verschiedene Minerale der Granatgruppe aufgrund ihrer strukturellen und chemischen Ähnlichkeit gemeinsam auftreten und eine Einheit, also einen Kristall, bilden.
Im Wesentlichen gibt es zwei Reihen, die Pyralspit- und die Ugrandit-Reihe. Bei ersteren handelt es sich um Aluminium-Granate mit Magnesium (Pyrope), Eisen [dreiwertig](Almandin) oder Mangan (Spessartin), zweitere sind Kalzium-Granate mit Chrom (Uwarowit), Aluminium (Grossular) oder Eisen [zweiwertig](Andradit). Die Granateinlagen der Schmuckstücke gehören in der Regel der Pyralspit-Gruppe an.

Granatstruktur_aus_Novak_Gibbs_beschriftet_DE
Durch die große Masse an verarbeitetem Granat aus dem frühen europäischen Mittelalter stellt sich die Frage, aus welchem Abbaugebiet das Rohmaterial ursprünglich stammt. Zudem wurden die Granate zu dünnen Plättchen geschliffen und diese flächendeckend verwendet, Überreste von Granatschleifereien sind aber nicht zu finden. Wurden die Granate bereits geschliffen importiert? In welche Richtungen fand der Export des Rohmaterials, sowie der bereits bearbeiteten Steine statt?

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Beispiele von Granat-Plättchen von Funden aus Schweden. Foto: Michael Rychlicki

Um mögliche Herkunftsgebiete bestimmen zu können, werden mittels Röntgen-Fluoreszenz-Analyse die chemischen Zusammensetzungen von möglichst vielen Granat-Plättchen bestimmt. Durch dieses Verfahren lassen sich Gruppen von Granaten erkennen, welche ihren Ursprung höchstwahrscheinlich in Indien, Sri Lanka und Böhmen haben. Allerdings basieren diese Zuordnungen auf dem Vergleich zu Messergebnissen von Granaten, deren Herkunft nicht zu 100% gesichert ist, da oft exakte Details wie z.B. die genaue Fundstelle fehlen.

CaO/MgO-plot der gRanatfunde in Schweden und Indien. (In Anlehnung an Greiff 2010)

Daher ist es umso erfreulicher, dass nun Proben zur Verfügung stehen, welche garantiert aus verschiedenen Bundesstaaten Indiens stammen und von Borayin Larios selbst gesammelt wurden.

Granatproben aus Indien (auf 5mm kariertem Papier)

Diese werden wie üblich auf ihre chemische Zusammensetzung untersucht und die Ergebnisse werden mit bereits erfassten Daten verglichen um die aktuell genutzte Methode, Granaten anhand ihrer Chemie eine Herkunft zuzuordnen, verbessern zu können. Außerdem werden diese Proben, sowie weitere in Schweden gefundene Proben, mit modernen spektroskopischen Methoden (UV-Vis-, NIR-MIR- und Raman-Spektroskopie) untersucht, um eine Abhängigkeit der Spektren –Unterschiede zu der Herkunft der Probe festzustellen .Auf diese Weise entdeckte eindeutige Merkmale können als weiteres Unterscheidungs-Kriterium hilfreich sein. Ebenso werden die Proben auf Einschlüsse untersucht um eventuelle Abhängigkeiten zwischen der Herkunft und den fremden Mineralen aufzuweisen.
Diese werden wie üblich auf ihre chemische Zusammensetzung untersucht und die Ergebnisse werden mit bereits erfassten Daten verglichen um die aktuell genutzte Methode, Granaten anhand ihrer Chemie eine Herkunft zuzuordnen, verbessern zu können. Außerdem werden diese Proben, sowie weitere in Schweden gefundene Proben, mit modernen spektroskopischen Methoden (UV-Vis-, NIR-MIR- und Raman-Spektroskopie) untersucht, um eine Abhängigkeit der Spektren –Unterschiede zu der Herkunft der Probe festzustellen .Auf diese Weise entdeckte eindeutige Merkmale können als weiteres Unterscheidungs-Kriterium hilfreich sein. Ebenso werden die Proben auf Einschlüsse untersucht um eventuelle Abhängigkeiten zwischen der Herkunft und den fremden Mineralen aufzuweisen.

 

Literatur:

Novak, G. A. / Gibbs, G. V. (1971): The crystal chemistry of the silicate garnets, in: The American Mineralogist, vol. 56, 1971; 791-825.

Greiff, S. (2010): Zur Herkunft der roten Granate an Schmuckobjekten des Erfurter Schatzfundes, in: Ostritz, S. (Hrsg.). Die Mittelalterliche jüdische Kultur in Erfurt, Band 2, Der Schatzfund, Analysen – Herstellungstechniken – Rekonstruktionen; Weimar; 482-487.

Scientific and Technical Analyses of Garnet Jewellery

Deutsche Version | Team

Depending on their respective geological origin, the chemical composition of garnets varies significantly. Different areas of origin of ancient garnet can thus be distinguished by scientific analyses.

Analyses into the origins of garnet from the areas on the periphery of the Merovingian Empire, planned to be carried out within the scope of this subproject, will form the basis for an understanding of chronological changes of the distribution routes and recognising possible reasons for the “garnet route in crisis”. Continue reading Scientific and Technical Analyses of Garnet Jewellery

Garnet Cloisonné on the Continent during the 7th and 8th Centuries

Deutsche Version | Team

The usage of garnets on the continent changed markedly during the final decades of the 6th century. Objects with garnet inlay, which until then had been available to large parts of the population even in remote and rural regions, suddenly became scarce. The two general explanations are that cloisonné had either become unpopular, or that the trade links for the supply of raw material from Sri Lanka and India had been severely disrupted. Continue reading Garnet Cloisonné on the Continent during the 7th and 8th Centuries

Garnet on the North-western Periphery of the Merovingian Empire during the 7th Century

Deutsche Version | Team

The objective of this subproject is the archaeological and scientific analysis of garnet objects from England, Scotland and Scandinavia. The question is, why objects decorated with garnet went out of fashion on the Continent during the last third of the 6th century, while the style experienced a notable floruit in 7th-century England and Scandinavia. Continue reading Garnet on the North-western Periphery of the Merovingian Empire during the 7th Century

Archaeometric Investigation of garnet jewellery

Archaeological material from Hungary (late 6th- and 7th-century)

Deutsche Version | Team | Cooperations

Decorative use of garnet stones on fine metalwork has defined the goldsmith’s art in the Carpathian Basin for three centuries, although with varying intensity. After its main flourish between the early 5th and the middle the 6th century, its dominance was mainly limited to the material of some specific social groups. On the other hand, from the early 8th century, garnet inlay had completely disappeared from the design of fine metalwork. In the project we are focussing on one of the most interesting transitional periods, dated between the late 6th and the late 7th century, which in Hungary is called the Early and Middle Avar Periods. Continue reading Archaeometric Investigation of garnet jewellery

Garnet Cloisonné in the Rhinelands

Interdisciplinary Research on Trade, Workshops, Symbolism and Users

Deutsche Version | Team

The aim of the research project is to examine the Northern Rhineland as a model region for trade and distribution of imported goods by analysing origin, quality of cutting and garnets framing during the Merovingian period. Continue reading Garnet Cloisonné in the Rhinelands