Jaffna traditional Mattu Vatti terracotta cooking vessels: sea-shell tempering and acid resistance?

Potters in Manipay and Chunnakam produce specialized earthenware cooking pots (Mattu Vatti / Chatti) designed to withstand the high acidity of northern tamarind-based seafood curries. Raw illite clay dug from lagoon margins is tempered with fine quartz beach sand and finely crushed marine bivalve and oyster shells (calcium carbonate). Thrown on manual wooden flywheels, the green pots are burnished with polished agates and fired in wood-and-palmyrah-husk clamp kilns. The calcium carbonate decomposes during firing, fusing with silicates to create an acid-resistant, low-porosity ceramic matrix that prevents sour tamarind juices from leaching raw earthen minerals.