Термомагнитно изследване на средновековна битова керамика от Плиска

The scope of the present paper is to define the firing temperature of the medieval household pottery from Pliska, using magnetic susceptibility measurements. Throwing light upon this topic is of a great importance for the study of the ancient pottery technology. The thermomagnetic method used for the study is based on the magneto-mineralogical changes during heating pottery. Different studies demonstrate that this method is more successful and more reliable than other archaeometric methods (XRD, SEM, DTA, TGA, Mossbauer spectroscopy). Sixty-six ceramic samples from three well-stratified sites in the Inner city of Pliska were analyzed (fig. 1). The samples are from two chronological periods – the Metropolitan (8th – 9th c.) and the Post-metropolitan (10th – 11th c.) (fig. 2). The pottery is set in four groups according to its function: kitchenware (I), tableware (II), storageware (III), and pottery with specific use (IV). The pottery was thrown on slow tournette wheel, fast tournette wheel, and kick wheel. The household pottery is divided in three categories according to the type of pottery wheel used (I.1, I.2, I.3). The lowest temperatures are recorded for the pots thrown on slow tournette wheel (I.1.) (fig. 4), while the highest temperatures – for the pots thrown on kick wheel (I.3) (fig. 6). The pots thrown on fast tournette wheel (I.2.) hold an intermediate position (fig. 5). The tableware is set in five categories – gray ware (II.1), yellowish-white ware (II.2), amphora-like pitchers (II.3), small pitchers/jugs (II.4), and glazed ware (II.5). The gray ware (II.1) is very common in Pliska. The vessels were fired in reduction atmosphere. The highest firing temperatures (830 and 850 °С) were recorded for two samples dicovered in the “Secret underground passage” near the “Krum’s palace” and dated in the late 8th – early 9th c. (fig. 7). Lower temperatures (between 710 and 790 °С) were recorded for the pottery found in common dug-out dwellings from the late 10th – first half of the 11th c. The yellowish-white ware (II.2) is not very common for Pliska. Four samples were analyzed. They are from the table set discovered in the “Secret underground passage” near the “Krum’s palace” and dated in the late 8th – early 9th c. The vessels were fired at temperatures varying from 790 to 830 °С (fig. 8). The data are very close and comparable to the firing temperatures of some of the gray ware discovered at the same spot. Amphora-like pitchers (II.3) are the most common tableware in Pliska. Twelve samples from three different chronological contexts were analyzed (fig. 9). The vessels were usually fired between 790 and 870 °С. The small pitchers and jugs (II.4) hold an important share among the tableware in Pliska. The five samples analyzed are dated in the late 10th – first half of the 11th c. The vessels were fired at temperatures between 780 and 850 °С (fig. 10). Nine samples of glazed ware (II.5), dated in the late 10th – first half of the 11th c., were analyzed. According to the results, the firing temperatures of the vessels range from 770 to 910 °С (fig. 11) with data clustering in to two narrower temperature ranges: 770–830 °С and 890–910 °С. Seven samples from amphorae (III.1) were analyzed. One sample was found in a context dated in the late 8th – early 9th c., while the rest are from contexts dated in the late 10th – first quarter of the 11th c. According to the results, the vessels were fired at temperature conditions largely varying from 730 to 870 °С (fig. 12). The sphero-conical vessels (IV.1) are very rare in Pliska. They were discovered in stratigraphic layers and contexts from the second third of the 11th c. Two samples were analyzed. They were fired at 730 °С (fig. 13). The thermomagnetic study of the household pottery in Pliska is attempt for a detailed research on the topic of pottery firing temperatures. The summarized data allow concluding as follows: • The use of different types of potter’s wheel, hand or wheel powered, is one of the main factors determining the development of the pottery technologies during the Early Middle Ages. During the 8th – 9th c. the pottery thrown on slow tournette wheel was fired at relatively lower temperatures (between 730 and 770 °С), while the highest temperatures (between 810 and 870 °С) (fig. 14) were recorded for the pottery thrown on a kick wheel. The pottery thrown on fast tournette wheel has an intermediate position with temperatures ranging from 730 to 850 °С. • There is a stable trend of increasing the firing temperatures of the vessels thrown on the three types of potter’s wheel during the Post-metropolitan period of Pliska (10th – 11th c.), but on a wider temperature range (fig. 15). • During the 8th – 9th c. tableware was fired better that kitchenware (fig. 16), while during the 10th – 11th c. similar division between the functional groups of pottery was not registered (fig. 17). • The average firing temperature of the pottery during the 8th – 9th c. is 792°С, while during the 10th – 11th c. reaches 814 °С (fig. 18) (Table 1). • There is a gradation in temperature according to the type of potter’s wheel used (see Table 2). • The highest temperature recorded for the 8th – 9th c. pottery is 870 °С, while for the 10th – 11th c. one is 950 °С (fig. 18). • The data concerning the firing temperatures of the analyzed pottery categories are summarized (see Table 3). The differences in temperature most likely reflect the existing specific production requirements for each category. The represented results from the thermomagnetic study conform to the archaeological observations until now. During the 8th – 9th c. the pottery was thrown mainly on a slow tournette wheel and was fired at lower temperatures. Technological changes, introduced during the Post-metropolitan period of Pliska, result in diversification of the production of pottery. Stratigraphic data from the site “Palace center – east” in Pliska point out that during the 10th c. pots thrown on a fast tournette wheel displace those thrown on slow tournette wheel. During the late 10th and the first half of the 11th c. kick wheel is very commonly used. Using more developed types of potter’s wheel increase productivity, specialization of the activities is also intensified. There is a stable trend toward increasing the firing temperatures of pottery.