Viscosity is an important physical property, which influences the performance of slags, in metallurgical processes such as iron making, steelmaking and continuous casting. Viscosity of slags is strongly influenced by the chemical composition and the temperature. Experimental measurement of slag viscosity requires high temperature equipment and is time consuming. Empirical relations/calculations using certain chemical parameters have been attempted to predict the viscosity, which may then be used for controlling metallurgical processes. Metallurgical slags used in the steel industry are made up of different oxides and fluorides, and may often be treated as silicates. The presence of silica, an inorganic polymer, in slags, has a major impact on "viscosity". It is convenient to describe the silicate slag structure in terms of the network character of silicates-either the Oxygen to Silicon ratio (O/Si ratio) or the Non Bridging Oxygen (NBO).The effect of O/Si ratio (and temperature) on the viscosity of slags has been studied theoretically in this work. Data on composition and viscosities were taken from the literature and the trends have been analyzed in terms of Oxygen to Silicon ratio. Data on four different slag systems, consisting of 33 slags, covering a range of temperatures, have been used. In all cases, irrespective of the system and the temperature, the viscosity was found to decrease steadily with increasing Oxygen to Silicon ratio. This is due to the increased breaking of the silicate network. The suitability of this parameter for assessment of slags (viscosities) has been demonstrated. KEY WORDS: slags; viscosity; chemical composition; oxygen to silicon ratio; silicate structure.

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