This paper describes a viscometer for measuring the viscosity of coal-ash slag up to temperatures of 2900 F. The investigations disclose that slags in only a relatively small portion of the field of compositions of coal ash have the melting characteristics of glasses and that, for most compositions, there is a distinct liquidus point. From the limited data available for this progress report, plots for viscosity are given for 2800, 2600, 2400, and 2200 F, as well as showing the liquidus lines for each temperature. The relation between “flow temperature,” as determined in earlier investigations for use with slag-tap furnaces, and viscosity and liquidus temperature is derived. When complete trilinear diagrams are available, accurate predictions can be made on the effect of mixing the ash of two coals or of the addition of the main constituents. The authors emphasize that predictions of the relative fusion and clinkering characteristics of coals from the softening temperatures of their ashes may give erroneous comparisons, but that the initial and fluid temperatures can be some additional guide.