disclosed is a method and device for determining the loss on ignition of at least part of an iron and steel product during passage through a furnace upstream of a descaler . the device includes electromagnetic sensors , with at least one arranged to scan the product &# 39 ; s lower surface near the furnace outlet , the sensor oriented so the scanning plane of the electromagnetic radiation from the sensor is perpendicular to a direction of movement ; a set of at least two electromagnetic sensors upstream of the descaler , oriented so their scanning planes are substantially on a single plane perpendicular to the direction of movement of the at least part of the product ; and at least two electromagnetic sensors downstream of the descaler , oriented so their scanning planes are substantially on a single plane perpendicular to the product &# 39 ; s movement direction . the sensors determine the height of the product upstream and downstream of the descaler .