In slagging gasifiers with crystalline slag, the fluctuation in temperature near the temperature of critical viscosity (T-cv) can rapidly increase the molten slag viscosity and cause an accumulation of slag inside the gasifier. In order to prevent operational problems caused by the slag, one therefore needs to obtain an accurate measurement of T-cv The effects of cooling profile and the extent of alumina dissolution on the experimentally determined T-cv were investigated. A synthetic slag was prepared using a composition of Alaskan Usibelli ash, which exhibited crystalline slag behavior, and T-cv was determined under constant cooling and equilibrium conditions. The T-cv measured under 2 degrees C/min of constant cooling was 33 degrees C lower than the T-cv under equilibrium conditions. The effect of alumina dissolution was determined using a platinum (Pt) lined crucible. At a 2 degrees C/min cooling rate, the T-cv measured using a bare alumina crucible was 23 degrees C higher than that from the Pt-lined crucible, offsetting the difference caused by constant cooling. In order to determine a safe gasification temperature range, however, the measurement of T-cv under constant cooling conditions should be avoided. (C) 2010 Elsevier B.V. All rights reserved.