Terra sigilata is a mixture made from clay, water and a deflocculant such as sodium silicate. When brushed on the pot and polished, it creates a coating that is satiny smooth. In a high fire the terra sigilata acts like a satin matte glaze on the pot. This coupled with the soda can create some beautiful effects; like the colour on this cup. No glaze on the outside just a clear and a yellow terra sig that turned to grey and orange glazing with the soda.