An on-line apparatus and method of using particulography as a technique for detecting and locating overheated material such as insulation within a gas-cooled dynamoelectric machine is disclosed. This technique detects the presence of and identifies the origin of thermoparticulates in a gas stream which are produced by thermal degradation of materials. The gas cooling system is continuously monitored by a gas stream monitor and upon the detection of thermoparticulates in the gas stream, the gas stream is diverted through a particulate collection chamber for entrapment of a sample of the thermoparticulates. The sample is then isolated from the gas cooling system and a flow of clean gas is forced through the sample while the collection chamber is heated at a predetermined rate. Heating the collection chamber will cause reparticulation of the thermoparticulates which are carried to a gas stream monitor located downstream by the flowing gas. The gas stream monitor will monitor the gas and will produce a signal that is proportional to the intensity of the thermoparticulates entrapped in the flowing gas stream. The signal from the gas stream monitor is applied to one axis of a recording device and a signal that is proportional to the temperature of the collection chamber is applied to the other axis producing a particulograph. A particulograph is unique to each kind of material that is present within the machine due to the unique thermoparticulation temperature of each material. Thus, by knowing where different types of materials are located within the machine, the location that is overheated can be identified.