Brown exterior glaze is from Antarctica sediment mixture applied to poreclain clay that was already fired once. In the glaze firing, sediment shrank and then relaxed down after getting to it's molten soft stage. A zone, looking like a line, divides a section that was oxidised and a section that was reduced -- referring to the presence of oxygen in the chamber during combustion. For some reason, on this mug, half was oxygen starved and half was not. The glaze colors are different because of this.