The same glaze with MgO sourced from a insight-live.com/glossary/43">frit (left) and from talc (right). The glaze is 1215U. Notice how much more the fritted one melts, even though they have the same chemistry. Frits are predictable when using glaze chemistry, it is more absolute and less relative. Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide in a glaze a different system with its own relative chemistry is entered. But when changing form one frit to another to supply an oxide or set of oxides, the melting properties stay within the same system and are predictable.
Pages that reference this post in the Digitalfire Reference Library:
Frit, Talc, Ferro Frit 3249, G1215U - Low Expansion Glossy Clear Cone 6, Frit, Flux, Glaze Chemistry
This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.