A reactive-ceramming process for making YPO4 ceramics involving the reaction between a YP-glass and a Y-source material. The invention can be used to synthesize, inter alia, phase-pure YPO4 ceramic material 5 at a relatively low temperature in a relatively short period of time and at a low cost. Invention can be used to make large piece of YPO4 blocks suitable for, e.g., an isopipe in a fusion down-draw process for making large-size glass sheets.