The mechanism for siliceous liquid-phase movement during sintering and thermal etching is investigated using single crystal alumina rods in a yttria-stabilized zirconia (YSZ) matrix. Bulk glass-phase extraction and intergranular movement during sintering is attributed to a chemically driven force; however, glass-phase expulsion is predominately due to thermal expansion differences in the glass phase, alumina fiber, and YSZ matrix. An increased understanding of the glass-phase mechanism will facilitate the reduction of the resistive grain-boundary phase, which consequently will decrease the operating temperature of high-temperature solid oxide fuel cells. In this paper, we demonstrated that scavengers such as alumina in combination with a suitable thermal treatment can be used to purify the grain boundaries from unwanted impurities, such as Si, through the expulsion of the unwanted liquid impurity phase. The driving forces behind the expulsion are mechanically and chemically driven capillary flow.