A method of operation for an apparatus for the layer-by-layer manufacture of a 3D object from particulate material, the apparatus comprising a first heat source, a second heat source, a stationary overhead heater and a thermal sensor; the method comprising: carrying out a thermal calibration process comprising one or more calibration routines (100_1, 100_2, 100_3) for one or more thermal control components (72, L1, L2, 20) of the layer, and subsequently carrying out a build process to manufacture a 3D object; wherein the calibration process and the build process each comprise a layer cycle comprising the steps of: (i) distributing a layer of particulate material over a build area, the layer providing a build bed surface of the build area; (ia) heating the build bed surface by passing the second heat source over the build bed surface while operating the second heat source; (ii) depositing absorption modifier over at least one of a layer-specific region within the build bed surface and a surrounding area surrounding the layer-specific region; (iii) heating the layer-specific region by passing while operating the first heat source across the build bed surface; and (iv) measuring a temperature of the build bed surface using the thermal sensor at least once after one or more of steps (i) to (iii); wherein the layer cycle comprises heating the build bed surface of each layer by operating the stationary overhead heater in response to the one or more measured temperatures with respect to a target layer temperature (T3(target)) between the solidification temperature and the melting temperature of the particulate material; wherein the steps (i) of distributing each layer, (ia) of passing the second heat source over the build bed surface, and (iii) of passing the first heat source over the build bed surface are carried out in a first direction; wherein the layer cycle is repeated a number of times until the calibration process and the build process are complete; wherein an outcome of each calibration routine (100_1, 100_2, 100_3) is based on the measured temperature in step (iv) and applied to the thermal control component (72, L1, L2, 20) for the subsequent layer cycle; and wherein, for the build process, the step (ia) of heating preheats the layer to a preheat temperature between the solidification temperature and the melting temperature of the particulate material, and the step (iii) of heating causes the particulate material within the layer-specific region to melt. carrying out a thermal calibration process comprising one or more calibration routines (100_1, 100_2, 100_3) for one or more thermal control components (72, L1, L2, 20) of the layer, and subsequently carrying out a build process to manufacture a 3D object; wherein the calibration process and the build process each comprise a layer cycle comprising the steps of: (i) distributing a layer of particulate material over a build area, the layer providing a build bed surface of the build area; (ia) heating the build bed surface by passing the second heat source over the build bed surface while operating the second heat source; (ii) depositing absorption modifier over at least one of a layer-specific region within the build bed surface and a surrounding area surrounding the layer-specific region; (iii) heating the layer-specific region by passing while operating the first heat source across the build bed surface; and (iv) measuring a temperature of the build bed surface using the thermal sensor at least once after one or more of steps (i) to (iii); (i) distributing a layer of particulate material over a build area, the layer providing a build bed surface of the build area; (ia) heating the build bed surface by passing the second heat source over the build bed surface while operating the second heat source; (ii) depositing absorption modifier over at least one of a layer-specific region within the build bed surface and a surrounding area surrounding the layer-specific region; (iii) heating the layer-specific region by passing while operating the first heat source across the build bed surface; and (iv) measuring a temperature of the build bed surface using the thermal sensor at least once after one or more of steps (i) to (iii); wherein the layer cycle comprises heating the build bed surface of each layer by operating the stationary overhead heater in response to the one or more measured temperatures with respect to a target layer temperature (T3(target)) between the solidification temperature and the melting temperature of the particulate material; wherein the steps (i) of distributing each layer, (ia) of passing the second heat source over the build bed surface, and (iii) of passing the first heat source over the build bed surface are carried out in a first direction; wherein the layer cycle is repeated a number of times until the calibration process and the build process are complete; wherein an outcome of each calibration routine (100_1, 100_2, 100_3) is based on the measured temperature in step (iv) and applied to the thermal control component (72, L1, L2, 20) for the subsequent layer cycle; and wherein, for the build process, the step (ia) of heating preheats the layer to a preheat temperature between the solidification temperature and the melting temperature of the particulate material, and the step (iii) of heating causes the particulate material within the layer-specific region to melt.