Fire, fire again
Jun. 7th, 2020 07:00 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
offcntrMy last two firings were a struggle. The kiln didn't want to heat evenly, and when I did finally get the cones to drop more-or-less together, I had horrible amounts of oxidation. Around the door, along the sides, pretty much everywhere but the very top. So this time, I decided to try a different tactic. Rather than adjusting the airflow via the chimney and damper, I'd cut it off at the source.
A venturi gas burner uses the pressure of the natural gas flowing through it to draw in air through the back of the burner into the mixer, where it (duh!) mixes with the gas, then ignites at the tip of the burner, shooting flame into the kiln. This is primary air, and you can adjust it by turning a rotating shutter at the back of the burner. Secondary air comes in around the burner (or through leaks in the kiln door or walls), drawn by the draft of the chimney. With the big gas kiln, we normally set the primary air early in the firing and leave it, doing any later adjustments with secondary air and draft. But I'd been using less and less gas the last few firings, and it occurred to me that I might have in fact had too little gas in the burners for the amount of air I'd entrained. So this time, when I set the shutters at the start of the firing, opened them only two turns, rather than my usual two-and-a-half.
It wound up being a struggle. The top still got ahead of the bottom in temperature, and I still had to mess with the damper to even it out. The firing took about three hours longer than usual, and about 5 units more gas. But for the first time in ages, I had a glaze firing that wasn't a mess of oxidation.
It was a mess of over-reduction, instead.
The pots were much browner than I like. Some were still passable; others, you could barely make out the pattern. Fortunately, I have a solution.
A few months back, Mason Stains stopped making my preferred black, and the replacement tended to blister when it went on too thick. I eventually solved the problem by changing the ratio of stain to Gerstley Borate (1:2, rather than 1:1), but in the meantime, I had pots with bubbles and blisters. I tried grinding them down, and dabbing on a little Gerstley, and refiring, but while the bubbles would be fixed, the background glaze got browner, and on vertical surfaces, mugs, for instance, the picture would start to slide. What I really needed, I decided, was the ability to fire cooler, in oxidation, maybe cone 9?
Then I discovered (by blowing circuit breakers) that my new kiln was in fact, a professional model, rated for up to cone 10. So I tried refiring some blistery pots, and not only did they smooth out nicely, several of them brightened up considerably. It's still not perfect--the background glaze looks a little dirty, to me--but it's a huge improvement. And it only costs around five or six bucks per firing. So far, I've done two loads of brown pots, and I think I'll do at least one more. Here's a before and after comparison.
I wish I had a solution for the opposite direction.
A venturi gas burner uses the pressure of the natural gas flowing through it to draw in air through the back of the burner into the mixer, where it (duh!) mixes with the gas, then ignites at the tip of the burner, shooting flame into the kiln. This is primary air, and you can adjust it by turning a rotating shutter at the back of the burner. Secondary air comes in around the burner (or through leaks in the kiln door or walls), drawn by the draft of the chimney. With the big gas kiln, we normally set the primary air early in the firing and leave it, doing any later adjustments with secondary air and draft. But I'd been using less and less gas the last few firings, and it occurred to me that I might have in fact had too little gas in the burners for the amount of air I'd entrained. So this time, when I set the shutters at the start of the firing, opened them only two turns, rather than my usual two-and-a-half.
It wound up being a struggle. The top still got ahead of the bottom in temperature, and I still had to mess with the damper to even it out. The firing took about three hours longer than usual, and about 5 units more gas. But for the first time in ages, I had a glaze firing that wasn't a mess of oxidation.
It was a mess of over-reduction, instead.
The pots were much browner than I like. Some were still passable; others, you could barely make out the pattern. Fortunately, I have a solution.
A few months back, Mason Stains stopped making my preferred black, and the replacement tended to blister when it went on too thick. I eventually solved the problem by changing the ratio of stain to Gerstley Borate (1:2, rather than 1:1), but in the meantime, I had pots with bubbles and blisters. I tried grinding them down, and dabbing on a little Gerstley, and refiring, but while the bubbles would be fixed, the background glaze got browner, and on vertical surfaces, mugs, for instance, the picture would start to slide. What I really needed, I decided, was the ability to fire cooler, in oxidation, maybe cone 9?
Then I discovered (by blowing circuit breakers) that my new kiln was in fact, a professional model, rated for up to cone 10. So I tried refiring some blistery pots, and not only did they smooth out nicely, several of them brightened up considerably. It's still not perfect--the background glaze looks a little dirty, to me--but it's a huge improvement. And it only costs around five or six bucks per firing. So far, I've done two loads of brown pots, and I think I'll do at least one more. Here's a before and after comparison.
I wish I had a solution for the opposite direction.