Toppenish’s fire chief knows what fueled the fire that destroyed the city’s public works building last month. He just doesn’t know exactly what sparked the paint-thinner fumes. “It’s either from a heat source or a spark from the combustion engine on the sprayer,” said Timothy Smith. The fire department’s report on the Aug. 23 fire at 8 Buena Way lists “flame/torch used for lighting” as the heat source for the fire. But Smith said that’s because the electronic report form does not give an option for sparks, heat or flame from an unknown source. A city employee was using paint thinner to clean the self-propelled paint sprayer around 2:40 p.m. when the fumes from the thinner he was using somehow ignited. The walk-behind sprayer was used for painting crosswalks and other street markings, Smith said.