When sourcing firebricks, the most critical factor is the Refractoriness (Pyrometric Cone Equivalent), which determines the maximum temperature the brick can withstand without softening. For high-heat applications, prioritize bricks with a high Alumina (Al2O3) content (typically 45% to 85%) for better thermal stability. Additionally, check the Cold Crushing Strength (CCS) to ensure the bricks can support the structural load of a furnace, and verify the Apparent Porosity, as lower porosity generally indicates better resistance to chemical slag and erosion.