I was asked by another student at the studio to give some examples of stoneware glaze recipes, how to mix them up, what does what in the glaze. The recipes I chose were I think suited to his tastes, and the first one was from this book:
It is kind of old school, but at stoneware temperature, not much has changed.
I Started with Gordon Baldwin’s Dolomite ^7
Potash Feldspar 41.5
China Clay 19.5
Unusually in this book, the totals add up to 100, which I was taught was a good thing and something called a batch recipe. It is easily multiplied to bigger quantities.
As we mixed it up, I explained what the ingredients did, and what kind of surface we were expecting. A calcium semi matt.
We were testing the glaze on Scarva Earthstone ES5, which is a pretty robust and forgiving body we use in the studio.
All of the tests were fired pretty hot… 1260°C with a 10min soak at the top
The first result is the plain glaze:
Soft matt surface… Expected. The next is the classic double dip… This shows what happens when the glaze is really thick:
Really too thick in areas but the bulbing at the bottom is showing that it melted and travelled down the test. The glaze does not fit the clay as you can really see the pronounced crazing.
The third and final test was to introduce an opacifier, Zircopax at 6% to make it white:
The Zirconium oxide is going to make the melt slightly stiffer, but it’s one of the simplest ways to make a white glaze. The crazing is less apparent, but it’s still there and needs to be addressed.
Now we melted some glaze, now we have to fix it to the body correctly. What I’m going to do is a Flint increase line blend without changing the other ingredients. 2,5,7,8% should fix the crazing, but what else will it change. There is plenty of flux and clay, so I think there is room to maneuver, but only testing will make it right. I have found that most prepared stoneware clays in the UK have about the same fit, excluding porcelain, so if you fit the glaze to one body it will ‘probably’ fit another. I’ll get back to this post after the line blend and a colour are investigated.