If you think of lava as clay, clay softens and fires in the range 700C-1300C. If you take a low-fire clay and take it all the way to 1300C then it will melt rather than simply soften. It's entirely down to the composition of the rock, so some minerals (often used as glaze) will be liquid at 600-700C. Glass firing is typically 600-800C, depending on the glass.