Petercsák Tivadar – Váradi Adél szerk.: A népvándorláskor kutatóinak kilencedik konferenciája : Eger, 1998. szeptember 18-20. / Heves megyei régészeti közlemények 2. (Eger, 2000)
X-Ray Diffractometric and Electron Microprobe Study of Avar Period Glass Beads. Basic data for the genetics of glass beads III. Inclusions in the beads
158 FÓRIZS ISTVÁN - PÁSZTOR ADRIÉN - TÓTH MÁRIA - NAGY GÉZA X-RAY DIFFRACTOMETRIC AND ELECTRON MICROPROBE STUDY OF AVAR PERIOD GLASS BEADS Basic data for the genetics of glass beads III. Inclusions in the beads István Fórizs — Adrién Pásztor — Mária Tóth — Géza Nagy Instrumental analyses were made on Avar period (6 t h7 t h centuries) glass beads (Avar period cemeteries at Budakalász-Duna-part, Csongrád-Felgyő and Szegvár -Sápoldal) in order to draw conclusions concerning the technologies and raw materials used in glass production. The detailed or even partial knowledge of both the raw materials and the applied technology can be used as a basis for the mapping of trading contacts (circulation of wares, the provenance of the geological (mined) raw materials) and the distribution of the applied technologies. There are two basic types of inclusions in the examined opaque glass beads: gas and solid (crystalline). The occurrence of gas inclusions (bubbles, Fig. 6/1) tells that the opaque glasses were treated on a lower temperature than the translucent glasses without gas inclusions and/or they were kept for a short period in the temperature zone where the gass could have left the glass melt. Their presence does not influence the outer appearance of the beads and, at the same time, they afford the use of less raw material and fuel during the production. From this respect, the bubble-rich opaque glass can be considered an „economic" glass. The crystalline phases can be divided into two groups regarding their origin: relict ones (particles that did not melt during melting) and euhedral inclusions crystallised from the melt. The most frequent crystalline phases of various composition and quantity depending upon the colour of bead can be characterised as follows: COPPER: It occurs in blue and red glasses as relict inclusions of 10-100 |im in diameter with rounded (Fig. 6/2) or uneven rims (Fig. 6/3). The latter one is usually surrounded by Ca-silicate crystals. Their composition determined by EDS spectra is copper in metal state, which means that the metal used for colouring was added in a metal state to the base glass, probably in the form of chips/turnings. Also we could identify copper ruby crystals of metal state measuring less than 1 |im in a more-or-less even distribution in the red opaque glass beads (Figs 6/4-5). These crystals lend the red colour to the glass bead. TIN: Inclusions with uneven rims and a diameters of 5-100 (im (Fig. 6/4) occur mostly in white, rarely in blue, red and black glass beads. Their chemical composition determined by their EDS spectra (Fig. 9) is Sn0 2. Their shapes indicate that they are relict phases and did not crystallise from the melt. It is not excluded that they were added to the glass in a metal state, but their oxidic form seems to be more realistic. Rarely euhedral Sn0 2 inclusions can be found in the red opaque glasses (Fig. 6/5), which crystallised from the glass melt. Their formation is probably related to the reductive environment applied for the production of red opaque glass. IRON: It was intentionally added to red and black glasses (FÓRIZS ET AL. 1996; 1997). Their shapes and chemical compositions are variegated. The shapes of the elongated straight or arched inclusions remind us of turnings (Figs 6/6; 7/1). Since iron can be cutted/chipped only in a metal state we think that iron was added in a metal state. This is supported by the observations that sometimes metal and partly oxidised iron inclusions also occur (Figs 7/2 and 11). The metal iron was probably used as an inner reducer. The way of red opaque glass production reconstructed by the above observatuions: it started with making raw glass, which was powdered and mixed with the metal iron chips, then melted until the glass powder melted down but the iron particles reacted only partially with the glass melt and did not entirely oxidise. Then it was slowly cooled down in an external reductive environment (inside a kiln rich in carbon monoxide). During cooling, the reduced-state iron helped the reduction of the copper in melt and the formation of copper ruby crystals. The result of this process was that the gas was trapped in the glass in the shape of bubbles, which means that the existence of the bubbles and inclusions does not indicate a primitive glass, it is a consequence of a technological constraint. (Pb,Sn)-OXIDE was used for the colouring of yellow opaque glasses (Fig. 3) (HENDERSON 1985; VERITÁ 1995). The 1-30 um large inclusions (Fig. 7/5) are evenly distributed in the homogenous glass matrix. Their rims are uneven, which suggests that they did not crystallise from the melt. Veritá (VERITÁ 1995,87) quotes the production of yellow opaque glasses from the Venetian book of glass production recipes from the 15 t h century. This description explains what we have observed. The reconstructed process is the following: Raw glass rich in Pb was produced, then it was powdered and mixed with the (Pb,Sn)oxide grains, which had been made in advance with the roasting of a mixture of metal lead and metal tin. The mixture was heated to a temperature where the glass
