Gas firing misunderstandings

[Monty Z]

Hello! I'm not a potter but my wife is and I'm trying to learn how to fire the propane downdraft kiln we built. It's 15 cu ft and built out of IFB with a slight arch on top. The burners come in from the bottom along the outside edges of the floor. The bottom shelves form a flue that goes back to the chimney.

So with this firing I was mainly struggling to keep it from going up too fast, and too try to even out the temperature, the top was much hotter than the bottom.

Mistake number one was not adjusting the baffles on the venturi burner after body reduction. I left them fairly closed with a blue flame. I assumed that if we had the damper in and were getting reduction then we didn't want more oxygen coming in through the burner baffle. I'm thinking that was a mistake.

Mistake number two was keeping it in heavy reduction after the body reduction was over and we opened the damper a little bit. I did this to try to even out the heat and also to slow it down.

The most perplexing thing was that the entire time, I had to keep the LP pressure under 0.25" wc. in an attempt to keep it from climbing too fast, and I'm wondering if I should get a gauge that reads in smaller increments if I'm always going to be at pressure that low.

Next time I'll go for a neutral flame after body reduction and see what happens, and try to have it in less reduction too. But how to even out the temperature difference?

If I do a test firing without pottery but using heavy firebrick to simulate the mass of the pottery, how many bricks should I use? An equivalent weight to a normal pottery firing?

Thanks for any ideas!

Monty

[Bill Kielb]

1 hour ago, Monty Z said:

So with this firing I was mainly struggling to keep it from going up too fast, and too try to even out the temperature, the top was much hotter than the bottom.

 

Generally affected by damper and speed

1 hour ago, Monty Z said:

Mistake number two was keeping it in heavy reduction after the body reduction was over and we opened the damper a little bit. I did this to try to even out the heat and also to slow it down.

Reduction flame will be yellow, very inefficient so if in reduction it ought to be going slow

1 hour ago, Monty Z said:

The most perplexing thing was that the entire time, I had to keep the LP pressure under 0.25" wc. in an attempt to keep it from climbing too fast, and I'm wondering if I should get a gauge that reads in smaller increments if I'm always going to be at pressure that low.

Lp tanks have hundreds of psi so two stage regulation to get down to 0.25” which is very small, pressure for a kiln that likely is intended to operate at 8”-14” maximum.

2 hours ago, Monty Z said:

Next time I'll go for a neutral flame after body reduction and see what happens, and try to have it in less reduction too. But how to even out the temperature difference?

You are firing propane, not a whole bunch of blue flame available really. Definitely not like natural gas for sure.

 

so lots of things seem a bit amiss.

I suggest pictures of everything we can see: operating gauge, operating valve, regulator(s), burner, burner orifice size, damper, flue height (approximate), how fast do you go in degrees per minute or hour, what speed are you trying to achieve,  etc….

From there we can see how this kiln ought to fire then discuss typical reduction techniques and flame coming out of the spyhole, etc… right now many things you are experience seem to contradict the physical world. Not saying it is not acting up, but I suggest a good baseline would clear some of these things up.

[Mark C.]

You have alot of potters in southern Or who could help you. Thier was a southern Oregon potters guild as well and was very active at one time

[neilestrick]

Are you sure it has the correct burner orifices for propane?

With venturi burners you should be able to set the air flaps and leave them, controlling the speed and reduction with just the damper and gas valve.

[Mark C.]

I never adajust my air flaps only gas pressure and danmper setting.and 98% is only the damper setting no presure after about 1800 degrees