The most common steel-making process in the world is the Lintz-Donawitz converter. During operation, a slag foam is created to improve the converter performance. Problems arise when the foam level exceeds the height of the vessel and overflows, causing metal loss, process disruption and environmental pollution. A warning system, based on advanced signal processing and key process parameter measurements is evaluated on data from 100 charges at SSAB Oxelösund. Slopping was timely detected in 80% of the blows.