This report discusses the results of sixteen high-temperature viscosity tests on slags derived from low-rank coals. The results of these slag viscosity tests led to the following conclusions: (1) the expected effect of alkali metal oxides in lowering slag viscosity may not in fact occur, and is probably dependent on the relative concentration of the alkali metal. (2) There can be significant differences in the compositions and viscosities of the ashes from two different low-rank coal samples taken from the same seam. (3) The effect of oxidation potential on slag viscosity is primarily related to the temperature at which the transition from the fluid to the plastic state takes place. The transition occurs at higher temperatures as the oxidation potential increases. The magnitude of this effect appears to be at least qualitatively related to the iron content of the ash, although the data from these studies were not sufficient to establish a quantitative correlation. (4) Overnight equilibration of a slag at temperatures at which it was in the plastic state failed to produce the expected decreases in viscosity. Instead, viscosity increases were observed, which were tentatively assumed to be due to gradual assimilation of solid material formed at the slag surface. Material balance calculations indicated that weight losses on the order of 7 to 13% of the initial ash weight occurred during ash melting. The extent of weight loss was primarily a function of ash sulfur content. Mineralogical analysis of the slags revealed plagioclase (anorthite) as the predominant major phase and pyroxene-group minerals (e.g., augite and enstatite) as common minor constituents. 6 references, 10 figures, 7 tables.