Coals with high ash fusibility temperatures are utilized in slagging gasifiers with the addition of a flux in order to lower ash melting temperatures and slag viscosity. Recently, thermomechanical analysis (TMA) has emerged as a technique that can characterize coal ash and slag behavior. The progressive shrinkage of heated coal ash samples mixed with calcium flux (CaO) has been measured using TMA. The resulting shrinkage has been correlated with the viscosity measured in a rotating bob viscometer for the same samples in order to quantify flux levels necessary for the viscosity required for coal fired slagging gasifiers (15-25 Pa . s). For this viscosity range, shrinkage levels in the range of 10-95% were obtained, indicating that the TMA technique has the potential to quantify the necessary flux additions.