A method of operation for an apparatus for the layer by layer manufacture of 3D obj ects from particulate material, the apparatus comprising a thermal sensor, a stationary heat source positioned above a build bed surface of each layer, and one or more further heat sources; wherein the method comprises a warm up process followed by a build process, and each process comprises processing a plurality of layers, wherein each layer is processed by a layer cycle (100, 200) comprising the steps (a) to (d) of: (a) distributing a layer of particulate material by moving a distributor over a build area, the layer providing the build bed surface of the build area; (b) heating the build bed surface using the stationary heat source, or a first heat source by moving the first heat source over the build bed surface while operating the first heat source; (c) heating the build bed surface by moving the first heat source, or a second heat source, over the build bed surface while operating it; and (d) measuring the temperature of the build bed surface at least once after one or more of steps (a) to (c), using the thermal sensor; wherein the layer cycle comprises, during one or more of steps (a) to (c), heating the build bed surface with the stationary heat source to a target layer temperature (T3(target)) between the solidification temperature and the melting temperature of the particulate material; and wherein the layer cycle of the build process further comprises, between the steps (b) and (c) of heating, a step of: depositing absorption modifier in the form of radiation absorber over one or more layer-specific regions; and/or depositing absorption modifier in the form of absorption inhibitor over a surrounding area surrounding the one or more layer-specific regions, such that the step (c) of heating causes the layer-specific region of each build layer to melt so as to form a cross section of one or more 3D objects. (a) distributing a layer of particulate material by moving a distributor over a build area, the layer providing the build bed surface of the build area; (b) heating the build bed surface using the stationary heat source, or a first heat source by moving the first heat source over the build bed surface while operating the first heat source; (c) heating the build bed surface by moving the first heat source, or a second heat source, over the build bed surface while operating it; and (d) measuring the temperature of the build bed surface at least once after one or more of steps (a) to (c), using the thermal sensor; wherein the layer cycle comprises, during one or more of steps (a) to (c), heating the build bed surface with the stationary heat source to a target layer temperature (T3(target)) between the solidification temperature and the melting temperature of the particulate material; and wherein the layer cycle of the build process further comprises, between the steps (b) and (c) of heating, a step of: depositing absorption modifier in the form of radiation absorber over one or more layer-specific regions; and/or depositing absorption modifier in the form of absorption inhibitor over a surrounding area surrounding the one or more layer-specific regions, such that the step (c) of heating causes the layer-specific region of each build layer to melt so as to form a cross section of one or more 3D objects.