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Scientific investigation of nozzle ablation during arcing in gas-blast circuit breakers have hitherto been based solely upon pressure measurements along the length of the nozzle. Although such measurements have provided some insight into the ablation effects, a more detailed understanding requires the measurement of additional parameters. For this reason investigations have been undertaken of the nozzle ablation process using (in addition to pressure distribution) current, voltage, radiative emission, and absorption measurements. In particular, new measurements are presented of total and spectrally resolved radiation emission from high-current SF6 arcs burning through nozzles made from PTFE and copper. The arcs were sustained by alternating current waveforms with a frequency of 78 Hz and peak currents in the range 33-62 kA. Nominal upstream and downstream pressures of 8.5 and 1 bar absolute were used. By analyzing the complex interactions involving pressure transients and radiative transfer, an approximate indication has been obtained not only of the total amount of nozzle material ablated but also of the amount of ablated material present in the nozzle at a given instant and the average temperature of this ablated material. Insight has been gained into factors governing the different ablation behavior of copper and PTFE.