INVESTIGATION OF ANCIENT CERAMICS

Transcription

1 INVESTIGATION OF ANCIENT CERAMICS Ľuboš Podobník Department of Physics, Faculty of Natural Sciences, Constantine the Philosopher University in Nitra, Tr.A. Hlinku 1, Nitra, Slovakia Correspondig author: Abstract We invetigate ancient ceramics, especially ancient pots and historical bricks. It is explained why the firing temperature is often studied and why it is only estimated. In the case of bricks we can find this estimate using the measurements by the thermogravimetry (TG), differential thermal analysis (DTA), and themodilatometry (TD). We use the knowledge about the degree of conversion. Several examples of the research of the pots are discussed. The measurements on samples from various parts of this ceramics are provided. This research allows one to obtain also information about the degree of the technological level. Key words: ancient ceramics, dehydration, dehydroxylation, firing, thermogravimetry 1 Introduction History of ceramics is very long. The oldest discovered ceramics was dated back to 29,000 25,000 BC (Ion et al., 2011). Ancient ceramics has been studied by thermal analysis for a long time. The results of the measurements are usually used for the determination of the firing temperature. In archeology the firing temperature is considered to be the property of the technological level of ancient society. We can discuss the application of thermoanalytical results. We can compare the freshly prepared ceramics fired at a high temperature to ancient ceramics primitively fired at relatively low temperatures (Drebushchak et al., 2007). Thermoanalytical work contains the results of experiments. The firing temperature is derived from this data. It is important to point out that we do not calculate an estimate on the firing temperature. Rather, we employ judging and reasoning (Drebushchak et al., 2007).The accuracy of the measurements is not of great relevance. So, it is questionable how valuable the estimate is. Thermoanalytical investigations of ancient ceramics provide useful information about the technology of pottery manufacturing. The comparison of the results concerning various ceramics or different samples of one ceramics provides scientists archeology information needed to draw the corresponding conclusions and arguments (Drebushchak et al., 2007). The research of the building ceramics is useful for the renovation of historical buildings. It could be desirable to assess the durability of applied materials in a specific building (Pavlíková et al., 2008; Anastasiou et al., 2005). 2 Ancient pots We now show an example of the investigation of ceramics from the transient age from Brass to Iron. Namely, by the carbon analysis it was dated back to IX-VII BC. They were subject to thermogravimetric measurements (TG and DTG) (Drebushchak et al., 2007; Drebushchak et al., 2011). The temperature range was 25 C 850 C and the heating rate was 20 C min -1. The mass of samples was from to mg. They were put in an open gold crucible (547 mg). The authors (Drebushchak et al., 2007) used 14 samples from the ancient ceramics (from the pot). The samples were taken along restored pot from the bottom to the neck (see Fig. 1a, b). 392

2 Fig. 1 Samples of ancient ceramics for the investigations: a scanning the inner surface of the restored pot from the bottom to the neck; b different parts of restored pots (A neck, B body, C bottom); c different parts of thick-walled sherd (O outer surface, I inner surface, C core) (Drebushchak et al., 2007). It was found that the total mass loss for all samples ranged from 5 to 7%. They started from about 120 C. Above 250 C the dehydration ceases and the mass loss becomes almost negligible up to 400 C. Then the dehydroxylation and the decomposition of carbonates takes place. All these reactions finish at about 750 C (Fig. 2). Fig. 2 The sample mass as a function of the temperature for all samples from the internal surface of the restored pot (the sampling is given in Fig. 1a) (Drebushchak et al., 2007). After the heating they compared the measurements from the inner, core, and outer parts of the pot (Fig. 1c). It was found that ancient ceramic pots could be exposed to two types of thermal treatment. First, the firing of a fresh pot transforms wet clay into dry hard ceramics. Second, the pot had been used for cooking. Water vapors escaped more from the outside surface than from the inside surface (Fig. 3). The degree of the thermal transformation of clay was maximal near the outer surface and minimal near the internal surface. The firing temperature was maximal at the outer surface and minimal inside the wall. So, we can conclude that the pot was used for cooking. It means that its outer surface was exposed to the heat of an open fire, as it was during the firing. Inside the pot meals were cooked at about C. This advantage can be very useful in the interpretation of archeological artifacts (Ion et al., 2011; Drebushchak et al., 2007; Ion et al., 2010). 393

3 Fig. 3 The DTG of the thermal treating of outer surface (O), core (C), and internal (I) surface of a thick-walled sherd (Drebushchak et al., 2007). Another example is an ancient ceramics dated from the transitional period of the late Bronze to early Iron Age (Novosibirsk region, VIII-VI centuries BC) and the early Iron Age (Russian Far East, VIII-IV centuries BC). The samples were investigated by the thermogravimetry, thermomechanical analysis, petrography, differential scanning calorimetry, and X-ray powder diffraction patterns (Drebushchak et al., 2005). It was found that the samples contained calcite. So, the firing temperature was below 800 C. The result of this research was the information that ancient pottery was produced at a very low technological level a low firing temperature. It also found that calcite contained in ceramics is a good informative parameter for the identification of the clay source for the pottery manufactured at a primitive technological level. The results are in good agreement with the assumption that the phase transitions taking place during the firing of fresh clay can take place again at the heating of the sherds. It has been several thousands of years since the pottery was produced (Drebushchak et al., 2005). Similar approach is used by investigating other historical artifacts, for example, historical plasters (Anastasiou et al., 2005). 3 Historical bricks The most frequent information of the thermoanalytical study of historical building ceramics (historical bricks) is the estimation of the firing temperature. The estimation is based on the comparison of the curves of TG, DTA, and TD. These curves are received from historical ceramic samples and from laboratory prepared samples (Podoba et al., 2012; Podobník et al., 2012). The degree of conversion characterizes the phase transformation in clay during firing. It depends on the temperature and time, f ( T, t), if it is measured for a small sample. In the case of a large ceramic body (brick) the situation is more complex. Experiments on large cylindrical green samples ( 80 mm) showed a clear dependence of the degree of conversion on the location of the sample taken for the TG. So, we have f ( T, t, r), where r is the distance of the sample from the axis of rotation. It was also found that after heating at 650 C for 10 h the degree of conversion reached 0.95 and almost did not depend on r (Ondruška et al., 2010). We studied historical bricks from the church in Pác, Slovakia. One might think that it is relatively simple to find the temperature of firing of the brick. The upper bound on the value of the firing temperature could be estimated as 700 C (due to the decomposition of calcite and the heating of wood). The lower bound could be about 650 C (when the total 394

4 dehydroxylation ends) (Podoba et al., 2012; Podobník et al., 2012; Štubňa et al., 2005). (Fig. 4). So, we obtain the estimate C. However, it is incorrect. Why is it so? Fig. 4 The examples of the DTA (gray) and TG (black) curves of the Gothic brick (Pác, Slovakia). Thermal analyses were carried out at heating 5 C min -1 from 25 C to 1050 C. The first endothermic minimum on the DTA curve, which belongs to the interval of C, is typical for porous building clay ceramics, which does not contain glassy phase. So it was fired at relatively low temperature less than 1000 C. It was also observed no signs of dehydroxylations of kaolinite or illite, which are completed at C. The sharp termination at 840 C is a typical feature of the TG curve of the decomposition of calcite (Podobník et al., 2012). The reason is that if we provide a sufficiently long time period for the firing, we do not need reach the temperature 650 C. After such a time period all reactions would take place even at lower temperatures and a similar degree of conversion would be reached in the center as well as near the surface of the brick. So, the estimate can be either C or C. (The temperature 500 C is the minimal temperature for the dehydroxylation to occur.) We thus see that it is very difficult to estimate the firing temperature (Drebushchak et al., 2011). 4 Conclusions For the measurements of the samples of ancient ceramics we used especially thermogravimetry. Measurements of samples from different points of the restored pot and from its different layers revealed differences in the results. The thermogravimetric analysis provided information about variations in the clay used for the (primitive) manufacturing of the ceramics and changes in the degree of thermal transformations in clay during their firing and usage. It was showed that this ancient pot was used for cooking (Drebushchak et al., 2007). The estimation of the firing temperature of the historical building ceramics is important as well (Podoba et al., 2012; Podobník et al., 2012). This estimation is very hardly available (Drebushchak et al., 2011). A suitable combination of analytical techniques used for the research of ceramics provided useful information. It helps to explain the nature of the raw materials used in the manufacture, possible origin, production, or firing technology. It is providing arguments to predict and confirm archeological hypotheses (Ion et al., 2011; Anastasiou et al., 2005). 395

5 5 References Ion, R. M. et al. Thermal and mineralogical investigations of historical ceramic. In Journal of Thermal Analysis and Calorymetry, Vol. 104, 2011, p Drebushchak, V. A. et al. Thermogravimetric investigation of ancient ceramics Metrolgical analysis of sampling. In Journal of Thermal Analysis and Calorymetry, Vol. 90, 2007, 1, p Pavlíková, M. et al. Hydric and thermal properties of materials used in historical masonry. In Proceedings of the 8 th Symposium on Building Physics in the Nordic Countries. Lyngby: Technical University of Denmark, BYG.DTU, 2008, p Anastasiou, M. et al. Comparative examination of historical plasters from Balkan Peninsula Byzantine monuments: TG-DTA and FTIR analyses. In Proceedings of the 7th Mediterranean Conference on Calorimetry and Thermal Analysis, MEDICTA, Thessaloniki, Greece, 2005, pp Drebushchak, V. A. et al. The mass-loss for the ancient ceramics. In Journal of Thermal Analysis and Calorymetry, Vol. 104, 2011, p Ion, R. M. et al. Thermal analysis of Romanian ancient ceramics. In Journal of Thermal Analysis and Calorymetry, Vol. 102, 2010, p Drebushchak, V. A. et al. The investigation of ancient pottery Application of thermal analysis. In Journal of Thermal Analysis and Calorymetry, Vol. 82, 2005, p Podoba, R. et al. The firing temperature of Romanesque brick from Pác. In Conference TUKE 2012, Košice, Slovakia. Submitted. Podobník, Ľ. et al. The firing temperature of Gothic bricks from Pác. In Conference Thermophysics 2012, Podkylava, Slovakia. Submitted. Ondruška, J. et al. Estimation of mass transfer parameters during dehydroxylation in a large ceramic body by inverse methods. In Ceramics International, Vol. 37, 2011, p Štubňa, I. et al. The influence of the sample size on low-temperature processes in green electroceramics. In Industrial Ceramics, Vol. 2, 2005, p