The densification process was measured in four amorphous coal ashes, as a function of temperature at constant time and as a function of time at constant temperature. Densification increased significantly if samples were sintered above the characteristic temperature T-cr. Above that temperature the sintering process is governed by the shrinkage of pores and the flow of melts out of the inter-granular spaces into the large and small pores. The characteristic temperature of sintering is defined as the temperature at which the sintering mechanism changes from the formation of necks and pores into pore-shrinkage and pore-filling by melt, which corresponds to low-strength deposits that respond to the pressure created by soot-blowing.