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In order to investigate the applicability of experiments on reduced size models to full size gas blast interrupters, a series of air blast experiments was performed in which peak currents and test frequency were varied together, keeping dl/dt at current zero fixed. With a strong effect of l thus eliminated, with current blocking avoided, and with the electrode vapour effect suitably controlled, the post current zero thermal recovery speed of the arc was found to be independent of the power or test frequency. Similar experiments were performed with several electrode materials. To these have been added measurements of electrode vapour contamination level and a theoretical model. In combination, they indicate strongly that a modest degree of vapour contamination (~ one percent) by a low ionization potential material (copper) can increase the electrical conductivity of an arc enough to halve its thermal recovery speed.