|Monthly Tech-Tip |
After comparing the chemistries of an original feldspar and a tentative substitute, these melt flow tests are an excellent way to confirm physical similarity also. These were done at cone 6 (2200F). Each feldspar is mixed with 15% Ferro Frit 3195. Some things to note: Nepheline Syenite is the champion melter. Mahavir is very similar to G200. Kingman and Custer are very similar. Our Minspar substitute is very similar to Minspar itself.
Why do this? We did not have it in stock and customers needed to mix recipes. When the chemistries of the two feldspars are very similar substitution is often not a problem, especially when a recipe only calls for 5 or 10%. However, when a recipe calls for a significant percentage the situation becomes much trickier (in our cone 6 test recipe, "Perfect Clear", 40% Minspar is needed). Feldspars are almost a glaze in themselves, just needing silica and alumina to shift their chemistry toward 'glazedom'. In this project I calculated a mix of materials, in my Insight-live.com account, that sources the same chemistry as Minspar. I made a cone 6 GLFL test comparing the Minspar and Minspar substitute (left) and comparing the Perfect Clear glaze with each feldspar (right). As you can see, the similarity in melt flow is stunning! This is a real demonstration of just how practical and valuable glaze chemistry calculation can be.
Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
|Materials||Mahavir Potash Feldspar|
|Materials||Covia Nepheline Syenite|