Why are K2O and Na2O often combined as KNaO in glaze unity formulas?
Insight-Live displays the chemistry of glazes as shown here. In oxide formulas it is typical to express the total of K2O plus Na2O as KNaO. The reason goes to the heart of why viewing glazes as formulas of oxides is so much better than seeing them as just recipes of materials. K2O and Na2O are oxides contributed by feldspars and frits. They impart very similar characteristics to the fired glaze or glass. Pretty well all feldspars source both of them but frits generally have just Na2O. Using common materials it is most often impossible to match both when doing formula-to-batch calculations, so their combination is targeted instead (that being said an approximate match is still desireable).