Wood Ash, Olla & Egg: Chemistry Between Pottery and Science

Igor Bahor & Milko Novič - Pottery Center Bahor (Slovenia)

Roman potters are known as producers of exceptional pottery creations, of which their terra sigillata undoubtedly stands out. Their work is particularly remarkable given the fact that they did not know modern deflocculants, nor modern viscosity and density meters. What they had at their disposal were high-iron clay, wood ash (due to the burning of pottery kilns), and simple systems for concentrating the fine fraction of clay and simple density meters. It is known from the literature that wood ash can serve as a base for glazes, as it contains potassium carbonate, which acts as a temperature lowering agent for surface vitrification. The technique of glossing ceramics with ash has been known to the Chinese since about 1500 years B.C. It is also evident from the literature that the top layer of terra sigillata on Roman vessels is vitrified to a thickness of about 30 µm and that the vitrified layer is enriched in potassium. Based on these facts, we decided to study the potential benefits of adding wood ash to the clays in order to achieve the appropriate clay segregation and later to enable surface vitrification of a ceramic pot coated with terra sigillata produced in the above mentioned way. The concentration of the diluted fraction of fine clay particles we achieved by removing in a ceramic pot (biscuit fired, uncoated), where water seeps through the wall and evaporates, while terra sigillata concentrates in the interior. The density of the concentrated fraction was checked "by touch" (silky feeling), and by the "floating egg" method. Namely, at a certain terra sigillata concentration, raw egg begins to float on the surface. In the present paper optimization of the proposed terra sigillata production process and its application will be presented.
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