This is a high feldspar glaze. As such it would normally craze. But the zinc content lowers the thermal expansion and increases the elasticity of the glass, enabling it to better tolerate a mismatch with the body. But that likely did not occur because of the high quartz content of bodies made using Saskatchewan clays (they were easier to fit glazes to because they put them under compression during the cooling cycle in the kiln).
Mixed and used in cone 10R firing on Medalta Ware.
I tried the glaze again in Apr/1990 on H550 crocks. The results were excellent, the glaze fired clear no matter what thickness. The very thick areas did have an few micro-bubbles. However, the clay underneath tended to have wide areas of darker and lighter reduction color development (depending on glaze thickness). Thus, this glaze should be applied very thin at cone 10R for best results.
MEDALTA GLAZE ALTERATION 1
This is the one I tested, it is marked 1129 #1.
------------ CUSTER FELDSPAR.. 169.30 ZINC OXIDE....... 47.75
WHITING.......... 50.64 BALL CLAY........ 53.61
FLINT............ 178.70
*CaO .42 7.34%
*MgO .00 .02%
*K2O .14 4.12%
*Na2O .00 .05%
*ZnO .42 10.57%
*Fe2O3 .00 .10%
*TiO2 .01 .18% Al2O3 .32 9.98% SiO2 3.65 67.65%
This is a my own independent development, it is 1129 #2.
I opted for #3 and mixed 10000 grams of it for the first firing of crocks. The glaze was not mature enough at cone 10 in the electric kiln, however it was good at cone 10 oxidation in the gas kiln. There is some cloudiness in the thick sections of the glaze.
It seems to work fine on a 580 casting body I made with a little iron.
This was taken from the Medalta building. Fired bars were made.
They fired whiter in color than Plainsman M340 or H550. The clay surface is completely free of fired specks. The color is stable through the cone 6 to 10 range (likely because this clay does not vitrify until past cone 10).
The drying shrinkage is low, this body is not very plastic. That means it contained a minimum of ball clay (Saskatchewan ball clays had higher iron content and thus fired darker in color). While this means it would not be suitable on a pottery wheel it would have been good for machine-forming and fast drying (where low drying shrinkage is important in avoiding cracks).
