Q: What is a Seger/Unity formula?
A: One way to help evaluate a glaze recipe is through the Seger or Unity Formula named after Hermann Seger who a century ago arranged glaze components into a particular order. He called one group the “flux” oxides—usually the oxides of Lithium, Sodium, Potassium, Magnesium, Calcium, Strontium, Barium and sometimes Iron. In another group, called glass-formers, he placed the oxides of Silicon, Boron, Phosphorus and Titanium, although most glazes consist chiefly of silicon oxide. Seger also described a third group, called “modifiers”, which included the oxides of Aluminum (aka aluminium), Boron, Iron and Phosphorus, the dominant one being Aluminum.
The kind of glass (quality) is decided by the “relative” amounts of each type of basic oxide put into the batch, and to make the proportions easier to recognize, Seger set the total number of flux oxide molecular equivalents (“moles”) equal to unity. This is done by summing all the flux oxide moles, and then dividing all numbers by this flux-oxide total, thus arriving at a “formula” (unified formula) for the recipe.
When the basic oxides are so arranged, direct comparisons become of value, especially when other factors are concurrently interpreted. Over the years successful recipes, in Seger form, have been collected and arranged in a summary chart called “Flux Unity Formulas”. The table thusly cites what proportions of basic oxides make good glass at specified temperatures. Such a list of basic oxides, when converted to a mix-batch, are known as “balanced” recipes.