Previous investigations have suggested that sand contamination to woody biomass fuels enhances slag formation in residential combustion appliances. The objectives were therefore to observe the effect of soil contamination in different forestry assortments on the extent of slagging and to gain increased understanding in the ash and slag forming chemical processes. This was accomplished by studying the bottom ash and the slag compositions after 19 2 h of combustion in a residential pellet burner. Melted ash reacted with the admixed sand particles resulting in an increased amount of melt with an increased content of silicon. The results confirm earlier experiences that melted bottom ash from combustion of woody biomass, upon cooling, forms silicate phases. In the corresponding melted ash, sand minerals as quartz, plagioclase, and microcline are not thermodynamically stable but will react and form a more silica rich melt. This melt has presumably lower liquidus temperature, explaining the increased amount of melt observed in the combustion experiments of soil contaminated fuels.