When it comes to the science of biomass combustion, no parameter is more complicated than ash fusion. When ash fusion characteristics are favorable, ash remains ash, and the only challenge is to remove it from the combustion system in such a way that it does not build up excessively. Conversely, when ash fusion characteristics are not favorable, strange things happen—ash can clump together and may need to be broken up, or even chiseled out of an ash pan. It can form a brick, look like a fused piece of glass, or even resemble a honeycomb.