In this study, five different ashes derived from Xinjiang lignite were tested for slagging at 1300-1400 degrees C in a weak reducing environment (1% CO in nitrogen), at different exposure time from 10 min to 2 h. The apparent viscosities for different slags were determined by using a modified inclined plane (M-IP) method with an inclination of 25 degrees based on the slag travel length per unit mass. In particular, the variation on slag viscosity upon the blending of additives including clay and MgO was examined in detail. As have been confirmed, the slags derived from the original basic and neutral ashes start to flow from 1400 degrees C onwards. The viscosities calculated based on the M-IP method at 1400 degrees C are in the range from similar to 1 Pa.s for neutral ash to similar to 5 Pa.s for the basic ash slags. For both neutral and acidic slags, their viscosities calculated by the M-IP method show a good agreement with some of the existing empirical models that have been validated for high-rank coal slags. Upon the addition of 10 wt% clay, the viscosity of basic ash at 1300 degrees C was reduced to 1.53-1.71 Pa.s, demonstrating a comparable flow-ability and viscosity to that of the original ash at 1400 degrees C. The addition of MgO promotes the slag flow-ability and reduces the slag viscosity to a relatively small extent in comparison to clay. Instead of melting with other elements in the ash matrix to form slag, the discrete Mg grains are observed in the slag matrix. It could even partially react with the alumina plate. The precipitation of Mg may promote the slag flow-ability by alleviating the slag penetration into the alumina plane and the resultant friction between them. Additionally, the viscosities for both the original slags and those blended with clay decrease quickly upon the increase of the exposure time, substantiating a non-linear slagging propensity and hence, probably non-Newtonian property A minimum exposure time of 40 min is essential to ensure these slags to settle down and reach their equilibrium viscosities in the M-IP method.