Ashes from commercial wood fuel blends show wide compositional ranges that in part can be related to an admired soil component. To evaluate agglomeration potentials and to formulate predictive models, an experimental study was conducted of an urban wood waste fuel ash from an operating power plant in California. The melting relations were investigated from the liquidus at 1276 degrees C to the near solidus at about 1130 degrees C. The liquidus phase is melilite composed of a solid solution dominated by akermanite and sodium melilite. A phosphate phase appears at 1207 degrees C and can be described as a solid solution of calcium phosphate, sodium phosphate, and calcium disilicate. Garnet appears at 1159 degrees C and is composed of an andradite, pyrope, and grossular solid solution. The alkali metals are partially lost to the atmosphere during the experimental duration. Sodium loss is moderate and dependent on the liquid remaining. Potassium shows very low partitioning into the solid phases and is strongly lost from the slag. The experimental results have been used to formulate a calculation procedure for evaluating the high-temperature compositional behavior of urban wood fuel slag. The results predict strong potassium volatilization tendencies for relatively pure wood fuel ashes with less than about 47 wt % SiO2. Because the SiO2 content increases with increasing soil component, the effect of soil is to retain potassium in the solid residue, In addition, the effect of an admired soil component on wood fuel slag behavior is principally to increase viscosity and has little effects on the surface tension. Because of the inverse effect of viscosity on agglomeration, the result is that an admired soil to wood fuel will reduce alkali losses and agglomeration potential.