While most metals are relatively inert at low temperatures, they can become aggressive chemically at high temperatures. Molten aluminium is aggressive in molten form and has no metallic but contains few [expensive] ceramic materials that have a high resistance.
Industrial processes that require constant monitoring can be inhibited by cost, for example, aluminium reduction is habituated to constant replacement of the thermocouple protection tube. Ferrous metals and Ni Cr alloys melt at high temperatures in addition to their chemical properties.
ECEFast manufacture thermocouples for non ferrous melts, in several designs, and use a variety of metallic and ceramic sheath materials. In many cases, the user has a preference based on long experience, for a particular style of protection tube.
Some materials include Sialon [SiAlN compound] metal ceramic, Al2O3 and Cr, and Sintered Silicon Carbide. In general, the expensive materials will be an economical solution provided they are not broken before they fail from ablation. In the absence of a successful material for the melting process, ECEFast can suggest new materials, and as an alternative, in some cases, infra red measurement may be used.