Esipoltetaan

Esipoltetaan is a term used in ceramics to describe the process of bisque firing, the first firing of a ceramic object to a relatively low temperature before glazing. The term is commonly used in Finnish pottery and ceramic communities to distinguish this initial firing from subsequent glaze firings. The purpose of esipoltto is to drive off most of the chemically bound water in the clay, harden the form, and make the piece handleable and porous enough to accept glaze.

In practice, esipoltto is performed after the greenware has been thoroughly dried to a bone-dry state. The ware is fired in a kiln at a lower temperature than glaze firings, typically roughly in the range of cone 04 to cone 06, which corresponds to approximately 900 to 1000 degrees Celsius, though exact temperatures depend on the clay body and the intended glaze. The firing atmosphere is usually oxidizing, but the exact setting can vary with studio practices and material choices. After esipoltto, the ware becomes more durable for handling and can be glazed, re-fired, and matured at a higher temperature.

The results of esipoltto include increased strength, reduced risk of warping during glazing, and a porous surface that helps glazes adhere. It also prevents cracks and explosions caused by moisture trapped in unfired clay during higher-temperature firings. In commercial and artisanal settings, esipoltetaan is a standard preparatory step in the ceramic workflow, enabling reliable glaze results and consistent final products.