In entrained flow gasifiers, crystallization inside the liquid slag can cause an increase of the slag viscosity, affecting flow along the wall and resulting in operational challenges. In the study reported here, a real Chinese ash was investigated for its crystallization behavior as well as the viscosity. Differential Scanning Calorimetry (DSC) and the Single Hot Thermocouple Technique (SHTT) were combined to measure the crystallization temperature and observe the crystallization process, constructing the temperature-time transformation (TTT) diagram. The ash was heated and then quenched to determine the crystalline phases. Unlike the synthetic slag mixed with oxides, the compositions of real ash had some variations after the long time heat treatment, which may attribute to the reducing minerals in the ash. Under inert atmosphere, gehlenite and spinel were the main crystals in cooling process. While under reduced atmosphere, the amount of crystals decreased especially gehlenite. Spinel was widely distributed inside the slag with irregular shape forming a network which caused the sharp increase of viscosity. The temperature of critical viscosity (T-cv) was around 1310 degrees C corresponding to the spinel proportion calculated about 20%. (C) 2016 Elsevier Ltd. All rights reserved.