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The shear viscosities of Cl-bearing melts in the system [Na.sub.2]O-[Fe.sub.2][O.sub.3]-[Al.sub.2][O.sub.3]-Si[O.sub.2] were determined at temperatures of 550-950 [degrees]C in the range of [10.sup.8.5] to [10.sup.12.0] Pa s using the micropenetration technique. The compositions are based on addition of [Fe.sub.2][O.sub.3] or Fe[Cl.sub.3] to aluminosilicate glasses with a fixed amount of Si[O.sub.2] (67 mol%). Although there was loss of [Cl.sup.-] during the glass syntheses, no loss occurred during the viscometry experiments. It is to be expected that [Cl.sup.-] takes the structural position of [O.sup.2-] and thus reduces the polymerization of the melt structure, and therefore the viscosity of the melt; as F does. Our measurements show that the presence of [Cl.sup.-] increases or decreases the viscosity of the melts as a function of melt composition. In the present melts, at least 10% of the Fe exists as network-modifying or charge-balancing [Fe.sup.2+] whereas the rest exists as network-forming [Fe.sup.3+]. It is proposed here that the different effects of Cl on viscosity are due to the preferred [Cl.sub.-]-[Fe.sup.2+.sub.NBO] bonding together with the different structure of peralkaline and peraluminous melts. In peralkaline aluminosilicate melts, the addition of [Cl.sub.2][O.sub.-1] will destroy 2 NBOs and create one BO if [Cl.sup.-] bonds primarily to the [Fe.sup.2+] creating non-bridging O atoms. This would result in an increase in viscosity. In peraluminous melts, the addition of [Cl.sub.2][O.sub.-1] may result in Cl- bonds to the charge-balancing [Fe.sup.2+], creating 2 new tri-clusters [assuming ([Al.sub.3+], [Fe.sup.3+])[Si.sub.2][O.sub.5] tri-clusters exist]. The preference of [Cl.sup.-] to form bonds to the NBO-forming [Fe.sup.2+] is indicated by the small amount of [Cl.sup.-] soluble in the peraluminous melt structure in comparison to that soluble in the peralkaline structure. Keywords: Viscosity, melt structure, micro-penetration, chlorine, iron, aluminium, peraluminous, peralkaline