The clays of Finland originated from glacial deposits formed during the deglaciation of the Weichselian ice-sheet and during the different stages of the Baltic Sea. The analysis of the fossil diatom flora is of central importance in the study of its developmental history. The diatoms in the clay indicate its sedimentary environment, which can be linked to the stages of the Baltic Sea. The siliceous valves of diatoms tolerate heating up to 1400°C and can con sequently be identified in ceramics fired at lower temperatures. Several diatom analyses have been made in Finnish Sub-Neolithic comb ware pottery in order to determine the origin of their clay materials. According to the results of these studies, prehistoric potteries used clays deposited during the Yoldia and the Ancylus stages. The criteria used in selecting raw materials were identical to those of modern ceramic industries. Further details as to provenance of clay materials have been illustrated in connection with research on pottery from the Åland Islands. In the example from Kymi, southeastern Finland, changes in the composition of the diatom flora have been found to correlate with changes in pottery decoration styles. See also AATA 18-1464 for related study.