Fig 1. Self-Hardened ash
Spontaneous stabilization after water addition (self-hardening) is the easiest and cheapest method of ash preparation. The ash is crushed after drying, but it is more difficult to get a uniform and adequate size of the ash particles than with granules or pellets (Fig. 1). If the carbon content is too high the stabilization process will not be successful, resulting in porous particles with high solubility in the forest.
Also, stabilization of large quantities is a slow process inside a large pile of ash. An impermeable layer of carbonates on the surface of the pile reduces the rate of CO2 and water transport into the pile which can lead to the incompletion of the stabilization process Steenari & Lindqvist, 1997.. In order to achieve acceptable results the ash and water have to be mechanically mixed. Lindkvist, 2000.