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Factors controlling the spread of smoldering combustion on solid wood (red oak, white pine) were examined in a configuration designed to enable self-sustained smolder. The sample was in the form of a U-shaped channel 74 cm long with 6.4 cm thick walls. A controlled flow of air was confined to the interior of the channel. Smoldering was initiated on the interior surface either of the upstream end of this channel (yielding forward smolder propagation), the downstream end (reverse smolder) or mid-length (coupled forward/reverse smolder). In separate tests the air flow velocity (referred to the initial cross section of the channel) was varied from about 9 to 22 cm/sec. At the low end of this range, the smoldering process was prone to extinction; at the high end it was increasingly likely to transition into flaming combustion. A simple energy balance model indicates a central role of radiative transfer in sustaining the smolder process.