An apparatus (2) for determining thickness of refractory material (4) lining a metal vessel (6) is disclosed. The apparatus includes a radiation source (16) for emitting radiation through a metal wall of the vessel and into the refractory material, wherein some of the radiation is scattered by the refractory material, and a radiation detector (20) for detecting radiation scattered by the refractory material through the wall of the vessel. A converter provides an output signal dependent on the quantity of radiation scattered by the refractory material through the wall of the vessel and detected by the radiation detector.