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R2O:RO Ratio

A ratio that compares the alkali fluxes (Na2O, K2O, Li2O) with alkaline earth fluxes (CaO, MgO, SrO, BaO).

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A comparison in the total amount of alkali fluxes vs the total alkaline earth fluxes. This is often understood as an indicator of durability in pottery glazes.

However, this recommendation may have taken on a life of its own, being applied far more broadly than intended. It is incorrect to assume that "if my glaze has 0.3 R₂O and 0.7 RO, it is durable." The 0.3 R2O : 0.7 RO ratio did not arise from a general theory of durable glazes. It came from the work of Matthew Katz and later Peter Berg, who were trying to formulate glazes that would better retain copper. They observed that glazes dominated by alkali fluxes (Na₂O, K₂O, Li₂O) generally leached copper more readily than those having a larger proportion of alkaline earth fluxes (CaO, MgO, SrO, BaO). So they standardized on a flux balance of approximately:

30% alkali fluxes (R2O): Na₂O, K₂O, Li₂O
70% alkaline earth fluxes (RO): CaO, MgO, SrO, BaO

The important point is that this was one design constraint among many, not the secret to a durable glaze. Alkali oxides are excellent fluxes, but they are also classic network modifiers. They break Si-O-Si bonds in the glass network and create non-bridging oxygens. Calcium and magnesium do the same thing, but they generally produce glasses that are harder, more chemically durable and less soluble in acids (e.g. laboratory glassware contains lots of calcium and alumina instead of being a simple sodium silicate glass).

An R2O:RO ratio that falls within a certain guideline says nothing about the amount of Al₂O₃ and SiO₂ present. Yet durability is much more about producing a strong, homogeneous glass having complete melting (but not over-fluid), minimal phase separation and surface crystallization, good body fit and absence of large amounts of easily dissolved colorants. In the above reference work, copper changes the equation, at 5% it is not merely a colorant but a substantial glaze component (that can
change melt viscosity, crystallization behavior, phase equilibria and resistance to leaching.

Rather than aiming for a magic flux ratio, I think it is better to ask: "Does this glaze have enough silica and alumina to build a durable glass while still melting completely?"

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