Slip casted, carbon bonded nozzles with active mullite coating arehave been simultaneously tested with uncoated nozzles in a steel casting simulator according to their clogging behavior against endogenous as well as exogenous non-metallic inclusions. After the tests with inclusions containing steel melt no clogging of particles could be observed on the active mullite coating after steel melt contact. The investigations show a position-dependent partially till completely decomposition of the mullite coating into coarse grained alumina-rich particles and a second phase with a main composition in the ternary system MnO–SiO2–Al2O3. It has been verified that these decomposition was caused by multiple reactions between the mullite coating and the substrate during heat-up as well as interactions between the coating and alloying elements of the steel melt during the tests.