Abstract: The present invention provides a method and composition for on-line treatment of furnace walls that are subject to deposition of highly reflective ash. The method involves exposing the walls to a darkening agent, or a combination of a darkening agent and a fluxing agent. The darkening agent, or the combined darkening agent and fluxing agent, may be mixed with the coal before it enters the boiler. Preferably, the appropriate walls of the boiler are treated directly with the darkening agent. A preferred embodiment involves direct application of the darkening agent to the water wall.