How can the stability of the melted glaze be increased?

If materials which contain a third category of oxides, commonly called glass stiffeners or stabalisers, are added to the glaze recipe the melting range of the mixture is extended such that it gradually softens passing through a treacle-like or highly viscous stage before eventually becoming water-like and too fluid to hold its position.

Viscosity is the term used to describe the fluidity of the molten glaze. The more viscous the melt the more treacle-like it is with a tendency to move very slowly on sloping or virtical clay surfaces.