Apparatus for determining the temperature profile of the surface being sprayed or otherwise treated, the apparatus comprising four electric arc spray guns (1) spraying molten steel. The guns (1) are connected to a six-axis industrial robot (2) which is adapted to move them over the surface of the ceramic substrate (3). The metal deposited by the spray builds up a metal shell referred to as the sprayform. The temperature profile of the sprayform surface (3) is recorded periodically by a thermal imaging camera (4) positioned directly above the surface (3). The apparatus includes a pruning filter for receiving each pixel of an image captured by the camera (4) and for applying at least a lower temperature limit to it so as to reject or disregard any pixels determined to have a temperature less than the lower temperature limit.