Hadak Útján. A népvándorlás kor fiatal kutatóinak konferenciája (Szeged, 2000)

Fórizs István - Pásztor Adrien - Nagy Géza - Tóth Mária: Avar kori üveggyöngyök röntgendiffrakciós és elektron-mikroszondás vizsgálata. Alapadatok az üveggyöngyök genetikájához IV. Miből és hogyan?

Avar kori üveggyöngyök röntgendiffrakciós és elektron-mikroszondás vizsgálata

X-RAY DIFFRACTOMETRIC AND ELECTRON MICROPROBE STUDY OF THEAVAR AGE GLASS BEADS. BASLC DATA FOR THE GENETICS OF GLASS BEADS IV HOW AND WHAT WERE THEY MADE OF?

István FÓRIZS - Adrien PÁSZTOR - Mária TÓTH-Géza NAGY

INTRODUCTION

As a part of the monographic study of the Avar Age glass beads in Hungary, scientific instrumental investigations of the beads have been made beside the typological and typo- chronological ones. Different types of beads have been chosen for instrumental analyses from six cemeteries of 6th—7th centuries: Budakalász-Danube bank (sample name

APPLIED

Opaque and translucid (for definition see Fig. 1. 1-2) glass beads were analyzed by electron microprobe (JEOL Superprobe 733) for determining the microtexture and the chemical composition of the glass matrix. In few cases the

starts with B in Table 1), Csongrád-Felgyő (CsF), Szeg- vár-Sápoldal (S), Szentendre (Sz), Tiszaföldvár (Ti), Ti- szavasvári (T). As a control material late Roman (4th—5th c.) and Sarmatian (4th c.) beads from the same localities (see Table 1) have been analyzed as well.

METHODS

chemical composition of the crystalline inclusions were determined as well. The crystal structure of the inclusions were determined by powder X-ray diffractometer (Philips PW 1710).

RESULTS AND INTERPRETATION

Base glass (glass matrix)

The base glass (excluding the additives and colorants) was made of different components mostly in two ways in the period studied:

— sand-limestone-soda Roman type]

— sand-limestone-plant ash Mesopotamian type]

The first type was very characteristic for the glasses made in the Roman Empire, therefore it is called in the international literature as Roman type. The second type was invented in Mesopotamia, so we call it Mesopotamian type. Both types have been encountered among the studied beads with different abundance. About 80% of the beads was made from Roman type base glass, and cca. 20% was made from Mesopotamian base glass (Fig. 2. 1). No relation has been found between the opacity (opaque or translucid) and the type of base glass.

The classification of beads into the two types above is based on the chemical composition of the glass matrix. As two components (sand and limestone) of the Roman and Mesopotamian type glasses are the same, the distinctive feature of glass types comes from the different chemical characteristics of the third component (natural soda/trona or ash of halophytic plant). Although sodium is the main element of both the soda and the plant ash, the latter contains a considerable amount of calcium, magnesium and potassium as well (BRILL 1970; STAWIARSKA 1984). Furthermore plant ash always contain a few percent of phosphorus and sulfur, while it is not characteristic for the soda. Because sulfur escapes from the glassy material during melting, it is cannot be used for making distinction between the two types. On the contrary the phosphorus remains in the glass melt and proves to be a good tool for making distinction (Fig. 5-6). Stawiarska (STAWIARSKA 1984) developed criteria for the two glass types:

Roman type: K20 content <1.3%, K-index = K20/ (K20+ Na20)x 100% <7%

Mesopotamian type: K20 content >1.3%, K-index = K20/( K20+ Na20)x 100% >7%.

Using these criteria, areas of the two types are indicated on the K20-MgO diagrams (Fig. 2. 1 showing all the beads studied, Fig. 2. 2 showing the beads whose P205 and SO:i content was measured as well).

Wedepohl and his co-workers (WEDEPOHL ET AL. 1997) reported phosphate content of Mesopotamian type glasses to be over 0.20% by studying Medieval German glasses. Veritá (VER1TÁ 1995) reported mean phosphate content of 0.35% for Venetian glasses, and 0.25% for Islam glasses, Brill (BRILL 1970) reported 0.30% for Assyrian glasses. For the phosphate content of Roman type glasses there are only few data, Wedepohl and his co-workers (WEDEPOHL ET AL. 1997) published 0.15 ± 0.13%. So the phosphate content higher than 0.20% seems to be a good criterion for Mesopotamian type glass.

Using the above stated criterion we indicated the border between the Roman and Mesopotamian type beads on the P205-Mg0 (Fig. 3. 1) and P205-K20 (Fig. 3. 2) diagrams. The Roman type glasses lie in a narrow area, while the Mesopotamian ones spread over a wide area, and the separation of the two types seems to be significant. Note that samples SlOa and B27 on these diagrams are classified as Mesopotamian type, while on the K20-MgO (Fig. 2. 2) diagram they are Roman type (although close to the border), and sample Til is very close to the border on both kinds of diagrams. On one hand this observation indicates that the different criteria do not match each other exactly (the uncertainty

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