Custer and Kona tests

I pulled some gorgeous test tiles out of the kiln on Friday, the results of my first tests on Custer and Kona F-4 feldspars.  These are the glazes that I wanted to find by the end of the semester, so I'm beyond psyched.  I know that this project throws me squarely in the middle of the Pastel Porcelain Crowd, but I have no problems with that, or with producing functional pieces.  Hey, I'm actually thrilled when a piece is truly functional; my skills are still fairly rudimentary. 

I really, really love these glazes!  I've been testing my buttery Matte 12 base glaze on three kinds of clay: grolleg porcelain, white stoneware (Dan's Four Equal Parts), and regular stoneware.  The porcelain is the purest I could manage, so there is only a minute amount of iron in it to color the glaze.  Even at that, the base glaze comes out slightly blue.  The white stoneware has a tiny amount of iron, but it makes the blue a little greyish.  The regular stoneware, of course, has enough iron to completely change the glaze color and ends up being a fairly bland, nasty grey.

Matte 12 base with Custer feldspar, no added FeO, on three clays.

Here is the same base recipe, with Kona F-4 feldspar on the same clays:

Matte 12 with Kona F-4 on grolleg porcelain, white stoneware, and regular stoneware

I wanted to see what a half-percent of black iron oxide would do, and the results were pretty spectacular:

Matte 12 with Custer feldspar and .5% FeO on grolleg porcelain, white stoneware, and regular stoneware

The speckling is considered a fault, but I think it will work well for some applications.  It especially looks good on the stoneware because it mimics the clays natural mottled appearance.  For porcelain, I will need to ball-mill the glaze.

I was surprised that changing the feldspars had so little effect on the color, because Custer is a high-potassium spar and Kona is a soda spar.  But as you can see, they seem to be interchangeable:

 

 

 

 

 

 

 

 

 

 

Matte 12 with Kona F-4 feldspar and .5% FeO on grolleg porcelain, white stoneware, and regular stoneware