In a rough simplification, we can say that in the Na2O(K2O)-CaO(MgO)-FeO-Al2O3-SiO2 system, there is the following effect of the content of components on the viscosity of the slag melt:

@Vadim Verlotski Can you please elaborate the difference between point 2&3. Also please explain in context of network former and network breaker. I have visited a bf where increasing basicity (increase in CaO) to 1.03 increases viscosity. Please explain.

There is no contradiction in paragraphs 2 and 3, I just accidentally wrote the word calcium instead of the word potassium (already corrected).

In silicate glasses of the Na2O/K2O-CaO/MgO-SiO2 system, the replacement of alkaline oxides by alkaline earth ones transforms "long" glasses into "short" ones, in other words, it creates a significant drop in melt viscosity at the liquidus temperature. If silicate glasses with a high content of alkali oxides begin to soften already from 600°C and a further decrease in the viscosity of the melt occurs gradually with increasing temperature, then glasses in which alkali oxides are replaced by alkaline earth ones begin to soften almost at the liquidus point (1300-1400°C), in which there is a sharp drop in the viscosity of the melt. So it turns out that the viscosity of melts with a high content of alkaline earth oxides at temperatures below the liquidus point is very high, and above the liquidus point it is very low.