Achaeometrical Research in Hungary II., 1988

ANALYSES - POTTERY - György SZAKMÁNY: Insight into the manufacturing technology and the workshops: evidence from petrographie study of ancient ceramics

fragments the same techniques can be recommended as for accessories. However, the identification of schistosity metamorphic rocks is difficult due to differences in mineralogical and textural features between their bands or layers. Identification of the rock fragments is not difficult if grain size is large and quantities of fragments are present. Note that fragments of the igneous and metamorphic rocks occurred only in small amounts in our materials available for study. In some ceramics pottery fragment inclusions (chamotte) can also be found. These uncommon grains originated from earlier ceramics which had been broken and mixed with the raw material of the new pottery. These fragments may be

4 used in the identification of and distinction between ancient workshops (SZAKMÁNY, 1996). The majority of the aforementioned clasts did not alter at all or significantly during the firing procedure. The only grains that show alteration are the carbonate clasts, mafic silicates (mainly biotites) and opaque minerals (e.g. magnetite). However, previous studies have shown that the degree of alteration depends on the temperature (in the case the carbonates) as well as the oxidysing atmosphere of the firing (in the case of biotites and opaque minerals). Structural and textural study of the ceramics Careful structural and textural studies of the ceramics can provide unique information on ancient technologies of manufacturing. The results of our previous study (SZAKMÁNY 1996) are shown in Table 1. Under pétrographie microscope the following structural and textural characteristics should be observed: colour of the groundmass, isotropism of the groundmass, average grain size, distribution of grain size (sorting), roundness of the clasts, porosity. The colour of the groundmass can be seen in pétrographie microscope using techniques of lnicol and crossed niçois, respectively. If the colour of the groundmass is red and reddish brown at lnicol then firing the firing procedure took place in an oxydising atmosphere. In this case limonitisation and hematitisation took place in the matrix, due to the high iron contents of the raw materials. The biotite and iron containing opaque minerals (e.g. magnetite) occurring as clasts, also underwent alteration into limonite and hematite. If the colour of the groundmass is grey, greyish brown and brown at lnicol, then reducing conditions prevailed during the firing. If the contribution of organic matter to the raw material (basically in the clay) is high, the traces of limonitisation and hematitisation cannot be observed even under reduced conditions (KAKASY and SOMODI, 1979). In some cases the distribution of organic materials was inhomogeneous in the raw material, therefore these ceramics are spotted in colour. Observations under a pétrographie microscope, using the technique of crossed niçois, offers information concerning the isotropism of ceramics. Isotropism depends on the dispersion of fine grained clasts in the matrix, which is a predominantly carbonatic matter. The matrix is isotropic (actually black or almost black seen through crossed niçois) if there is no carbonatic material in the matrix, because the raw material did not contain carbonates or it was burnt out during the firing procedure. If the temperature of firing was low and originally the material contained high quantities of fine-grained carbonate matters, the colour of the matrix is clear and bright white or yellow and the matrix is prickled. If the matrix is coarse-grained (approximately 3-5urn), isotropism cannot be observed in the sample. Ceramics display considerable variability in terms of average grain size and the distribution of grain size (sorting). This variability helps us to assess the degree of sorting 80