From the Ashes... Success

Well, I had a chance to do a firing before the Thanksgiving weekend and grabbed it. The plan was to re-fire the wood ash glaze objects from the previous firing which had been under-fired by about 300° C. Since this time I had to reach 1263° C, it became a great opportunity to test the limits of the kiln, to see how high it could get.

10:15 - start

11:30 - 400°

1:30 - 800°

2:30 - 970° (temp increasing much slower now)

3:30 - 1120° (vent now totally open)

4:15 - 1147°

4:30 - turned off fuel and plugged up the kiln to let the temperature decrease slowly.

As you can see, we maxed out at 1147° - and it took an entertaining hour to get those last 27 degrees. Since we were still a ways off from Cone 10, I figured the glaze wasn't going to work. But when I took out the pieces I found to my surprise...

...they have a delightful glossy shine to them.

what a difference 300 hundred degrees makes

I'm not sure why the one mixed with wood ash, feldspar, clay powder and silica turned dark (and is really no shinier than the other two with a glaze mixture of ash and feldspar only). It could be because it was getting a lot of direct flame from the blower, and somehow got 'burned', whereas the other two were more protected.

Also, the bowls were higher up in the kiln, away from the flame, and these have much less shine to them. Not sure why, since the temperature in the kiln should be pretty even (I think, but maybe not?).

In any case, it was wonderful to see how the glaze worked, even though we were 120° lower than apparently necessary. And this effect from ash and feldspar alone: very primitive, bare bones, but with some lovely colourings and effects.

According to my research (namely 'Clay and Glazes for the Potter'), wood ash contains alkalies like potash and soda ash, in addition to silica and some alumina. These glazes came about through improvements to kiln technology made by the Chinese. The downdraft kiln which used the hot air from the wood fire, allowed them to fire at around 1200º, compared to the earlier Near East temperatures of about 1050º or less.
  

We'll try this glaze again, this time with the ash more finely ground, and then we can also test the positioning of the objects to see how the flame affects things.