the flux oxides only maximum levels are given. For alumina and silica
minimum and maximum amounts are suggested.
originated from the work of Hermann Seger who established a recipe
(unity formula) for his pyrometric cones which softened and bend
within specific melting ranges. Seger focused on alumina and silica
levels for each cone / temperature while maintaining a consistent
flux content in his recipes.
Since that time
research has established broad guidelines for flux oxide maximums
and for boron.
software such as MATRIX use graphic
displays to display the oxide limits for the firing temperatures.