|Monthly Tech-Tip |
Formerly called NC-4 Feldspar. Very similar chemistry to the no-longer-available F-4 feldspar.
A 200 mesh flotation-grade soda feldspar from Spruce Pine, NC. Used in the ceramic whiteware industry for sanitary ware, dinnerware, floor and wall tile, artware and glazes.
Refractive Index: 1.53
Mohs Hardness: 6.0-6.5
Mean Particle Size: 12 microns
Specific Surface Area (sq-m/g): 0.9-1.2
Dry Brightness: 94
Raw color: white
% Moisture Content: 0.1
Bulk Density (lb/cu. ft)
Bulk Density (kg/cu. meter)
Particle Size: 100 140 170 200 325
% Retained tr tr 0.1 0.3 3.6
U.S. Sieve Series
*This is from a data sheet from Jan 2013
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.
Pure MinSpar feldspar fired at cone 6 on Plainsman M370 porcelain. Although it is melting, the crazing is extreme! And expected. Feldspars contain a high percentage of K2O and Na2O (KNaO), these two oxides have the highest thermal expansion of any other oxide. Thus, glazes high in feldspar (e.g. 50%) are likely to craze. Using a little glaze chemistry, it is often possible to substitute some of the KNaO for another fluxing oxide having a lower thermal expansion.
Laguna Clay sells a substitute for the no-longer-available G200 feldspar. G200 HP is higher in K2O and lower in CaO than G200, Minspar is an ideal addition since it's K2O is much lower and CaO much higher. A 7:3 G200HP:Minspar mix produces a chemistry that is remarkably close (on paper) to G200. They label this blend "Old Blend". They also list a product called "New Potash" in their pricelist, that is G200 HP.
|Materials||G200 HP Feldspar|
|Materials||Kona F-4 Feldspar|
The most common source of fluxes for high and medium temperature glazes and bodies.