Saturday, 5 June 2010

kiln talk

Pictures for Soub. The burners go horizontally into the kiln and there is a deflector right at the back. I've added an extra one two thirds along each side, as it fires more evenly with them there.
the flue exit is horizontal from the bottom at the back, damper about a foot later just before it begins to curve upwards.
I have been using two smaller bottles, changed to one big (47kg) one part way through the last firing. Normally we'd use two per burner on the salt glaze kiln I share. My kiln seems to be doing fine on one (and I can't stretch to the cost of another yet!)

There is a thread on clayart forum started yesterday about firing propane fuelled gas kilns- could be useful!


A Green Spell said...

What a fun blog! I love ceramics! Thanks for visiting mine!

gz said...

good to hear that you visited.
Come back again!

soubriquet said...

Sorry about the delay, life's a bit fraught, with some bad stuff like allergies, so half the time I'm zonked on antihistamines.

Okay, that's a nice tidy, simple burner setup, the paper-clip's a little unorthodox...
Bullfinch burners are efficient, cheap, easily replaceable, the red button is atop an electro-magnetic flame-failure device (ffd), and the little probe on the end of that thick copper wire is a thermocouple. When its tip is in the flame, the heat at the hot junction of two dissimilar metals produces a tiny current, about 15-20 thousandths of a volt, just enough to hold the other end of the button down, and the gas valve thus open. If that current stops, or goes below about twelve millivolts, the spring under the red button becomes more powerful than the magnet, and the valve pops shut. Simple and efficient, but be warned. If the kiln is hot, but there is no flame, the valve can't tell the difference, and may allow gas to flow. Not good. Take care with that.
More modern systems use ultra-violet optical sensors, or pass a current through the flame to verify it's actually there. But these systems are complex and expensive. Just be very careful if the flame ever blows out, ventilate well before relighting.

On gas bottles. The gas, stored in liquid form, needs energy to vapourise. As it does so, it uses ambient heat, and the liquid in the bottle cools.... The more gas you're using, the more cooling occurs, and you may see frost on the bottle. The colder the bottle, the slower it produces gas. the bigger the bottle, and the more liquid in it, the more surface area it has to absorb heat, and thus keep the pressure up.
I have, in the past, had cylinders with hot air blowers aimed at them, or boiling water poured over them, to try keep a kiln going!
I'm thinking that if too fast a firing is the problem, you could put in a smaller pair of burners above the main pair, and use them for the first few hundred degrees, then switch to main burners as needed.
The regulator you have is a reliable, robust, but not very informative type. A pressure gauge on the regulator would make adjustments easier and more intuitive.
I like the way the kiln builder has used an exhaust clamp to hold the burner, good thinking!

When you're firing, do you adjust the burners by regulator pressure or by lever valve or both?
How often do you adjust the damper?

gz said...

I use both, using the lever valve as more fine tuning.
I probably need to learn more about using the damper. I'm ok with using it at higher temperatures, not so sure to start with.

20th Century Woman said...

I enjoyed looking at your blog and your pots. I know very little about pottery, which is a shame, since my oldest and best friend is a potter, and the Giffins (of the Giffin Grip) and good friends and neighbors.

Stacks of unfired pots look beautiful.

Isle Dance said...

I love these photos and am amazed at what you're doing.

soubriquet said...

Leaving the damper quite well in might slow down the early part of a firing, I'm wondering if your flue is exerting too big a pull on the kiln?

20th century woman... I've used Giffin grips, they're great, a really neat piece of engineering and an amazingly clever idea, like a lathe headstock redesigned to hold fragile clay on a potter's wheel.
Unfortunately, they're also rather too expensive for me to justify buying one for myself, I carry on holding pots in improvised chucks with wads of clay. (I don't begrudge the price, they're well engineered, and I'm sure, not cheap to make).
Say Hi to the Giffins, please!

gz said...

soub, it is an 8" diameter flue, and the hole at the back is quite big...I think you have a point there.
I should be seeing Pete Goodridge and Mel Brown on Saturday to talk kilns and saltfiring (hopefully Rosemary Cochrane too)