The behaviour of the weld characteristic of flux-cored wires is strongly influenced by the flux. The weld metal and weldability are determined by the flux composition and to a much smaller level by shielding gas used as well. The wide variety of components combined with the complexity of the welding process result in intricate mechanisms occurring in the slag. The slags of the different rutile flux-cored wires, designed for either position or standard downhand welding, were analysed in order to gain knowledge about the complex slag mechanisms and to carry out a metallurgical characterisation. Chemical analysis, differential thermal analysis and microstructural investigations of the slags were conducted to identify and characterise the formed phases. In addition, the viscosities of the slags were measured and correlated with the DTA results.