Thermal energy generation from biomass fuels usually leads to dense slag accumulation, causing combustion system problems. This study tested a type of local clay as a slag soothing additive on two biomass fuels: Acacia mangium (kratin-tree) and Manihot esculenta (cassava). Physical characteristics of the slags were studied. In addition, a laboratory method for determination of ash sintering was modified to assay the dense and loose slag accumulation. Compositions of the clay and biomasses were determined. While the clay contains high relative contents of Si (42.3%) and Al (24.5%), the cassava rhizome and kratin-wood contain high relative contents of K (46.8% and 47.2%) and Cl (10.8% and 7.54%), respectively. Gas phase transition of residual ashes was revealed as a result of slag inducing combustion treatments. Addition of the clay (0-5%) decreased the total slag accumulation, and the maximum effect was revealed at 3-5% clay. The modified method in this study requires a small amount of ash sample which is practical and convenient for laboratory testing of slag behavior. (C) 2020 Elsevier Ltd. All rights reserved.