Tackling Stoneware

After a joyous romp with earthenware, the time has come to tackle the inevitable.. This is where I started, when I first got my kiln, with stoneware. Stoneware is fired to a fierce temperature and I needed to know if my old, second-hand kiln could even reach 1260°C. So with no experience, this is where I set of and immediately got mediocre results :(

By the time I had finished building and testing my kiln controller (please comment below if you are interested in how to build and code a kiln controller, I'll post about it if there is interest) I had become a little more accustomed with the whole affair and had a few acceptable pieces. As I had established that the kiln is functional I turned my attention to the earthenware bracket of pottery and oh my. Instant great results with gorgeous glaze colours and effects. I had kinda conveniently put the whole stoneware thing aside. But alas, the time has come.

I realize that I am in fact writing this post to the aid of other potters out there who struggle with this issue. A field report of sorts.

Stoneware... The problem has been bubbling/blistering in the glaze. I have read all the advice on-line and had spoken to many "experts" so it was frustrating to experience the same problem no matter the attempted remedy. "Maybe it's the clay". Nope, tried a different clay, same issue. "perhaps the glaze". Also not the glaze. "How about the temperature? 1260?! No!, Too high, try 1200" Blisters at 1200. Vent the kiln, don't vent the kiln, Bisque twice or Bisque higher. Ramps, holds and what not?

In the end the news is bad, well good in the sense that it's the solution but definitely bad for my pocketbook. I am still fine tuning my so far successful theory here but in short my kiln is too leaky. It is no longer as well insulated as it may have been decades ago when it left the factory so this old cracked and worn kiln has seen better days for sure!

I mean I was desperate,so I took desperate measures. An ice cold 2 liter Coke, fully charged vape, kiln glasses, clipboard and pencil were my armaments. I sat down next to the kiln and fired it. I must say, I took a lot of unnecessary notes! Also, it gets pretty hot spending so much time next to a blazing kiln on a hot day. But I persevered and here is the essence of my observations;

Shortly after the glaze melts the bubbles start. And they don't stop. I had 3 different test pieces in view of the peep hole and apart from varying melting temps, all three glazes bubbled and bubbled away. All the way to target.

That's it: glazes (stoneware, ^6 and up) bubble. It's apparently normal and inevitable. So you can imagine, that if the kiln reaches target temperature and shuts off and like mine, is a leaker, the glaze cools so rapidly that the current bunch of bubbles freeze in place. They don't have time to "heal".

So my scheme was and is to "Ramp Down". I did code such a ramp but never tried. So I fired the kiln again this time ending the firing schedule with a Cooling Ramp Down to 1120°C target and a 60°C/hour slope. It worked! Only e few minor bubbles but no more of the sharp edged craters! I am so happy but that cooling ramp added another 3 hours worth of firing time, 30% more electricity used :(