The infiltrationInfiltration of speissSpeiss—a copperCopper—and arsenicArsenic—rich metallic phase with a low melting point, high density, and low viscosityViscosity—has been identified as the primary cause of accelerated degradationDegradation of the sole in the flash smelting furnaceFlash smelting furnace at the Chuquicamata SmelterSmelter. This study examines the speissSpeiss infiltrationInfiltration process in the furnaceFurnace’s sole refractoryRefractories bricks, employing optical microscopy, X-ray diffraction, X-ray fluorescence, and SEMScanning Electronic Microscope (SEM)-EDS to characterize the infiltrated phases in a comprehensive and traditional “post-mortem” analysisAnalysis. Microstructural analysisAnalysis, alongside predictions of liquidus temperature and solidificationSolidification range for the multicomponentMulticomponent speissSpeiss system, enables a theoretical assessment of the infiltrationInfiltration phenomena and the chemical interactions between speissSpeiss and oxidized structural constituents of the brick, such as periclase and chromite. Findings reveal that speissSpeiss readily penetrates the open porosity of the brick, inducing structural damage due to spalling, with additional chemical interactions proposed. The implications of these observations are discussed for the designDesign of refractory materialsRefractory material that are more resistant to infiltrationInfiltration, including the consideration of alternative chrome-alumina bricks and low-porosity monolithic materials. A technical criterion for selecting refractory materialsRefractory material could be proposed to consider a simple infiltrationInfiltration test using the sessile drop technique. This work provides a scientific basis for improving the durability of the furnaceFurnace lining and extending its operational life.