How to stack gas kiln properly?

[Antoinette Brown]

I have a new kiln and my first firing was awful. Everything was severely underfired and it took a lot of gas. I know from my old kiln that stacking the right way makes all the difference, I thought I got it right but obviously it was all wrong. No matter how I changed the air, the flue and the pressure I would not reach cone 10. I probably change to cone 6 but still it makes a lot of difference in gas consumption. 

So here's the question: The kiln has the flu on the left and two burners at the right.  How do I stack: 1) shelves at the same height and a gap in the middle? 2)shelves lower towards the flu and higher towards the burners? 3 ) shelves lower towards the chimney and higher towards the burners? 

Overall should the load be more top heavy or has more air space at the bottom or should it be even?

As seem on the photo the right shelf probably was way too low.

Also the it was completely weird the temperature went up when I closed the air more and the flue at least three quarters!

In my old kiln I used a bag wall (half a shelf upright went nearly through half the kiln. My guess is that I placed the right shelf way too low.

What do you think of stacking? I attach a couple of photos. Unfortunately there seems to be no manual around anymore and the factory is long gone.

[neilestrick]

Are the burners blowing up under the shelf, or is there a space between the shelf and wall where they come up? What is the perforated floor? Is the flue under there?

[Antoinette Brown]

The burners are sitting under the kiln (that piece of in is against spiders when not in use) and there is a tube out of kiln brick going through the fibre. The perforated floor is way  left from were the flame goes in, visible in the first picture. I did not stack any shelves over the flame path and left that area free. thank you for asking!

BTW your artwork is great (did you know that your shop pages are not loading?)

 

[neilestrick]

Is the entire perforated portion connected to the flue? Like it draws in through that entire area?

5 minutes ago, Antoinette Brown said:

BTW your artwork is great (did you know that your shop pages are not loading?)

Thanks! I don't have an inventory at the moment, so nothing to load.

[Antoinette Brown]

yes. this is how the floor looks like. The flue is right left from the perforated floor and you can see were the flame gets in on the right. I know from my old kiln that stacking makes all the difference but I could not find some "rules" and did it more intuitively.

[Kelly in AK]

My two cents, staggering your shelves is good, keep doing that. Space for combustion is good, I think raising that bottom shelf is the right instinct.

I’m still not quite clear on the exit path of the flame, I think you’re saying it goes down through the perforated floor.

A couple questions, is it forced air or natural draft? And, was there a great difference in heat from top to bottom? 

[Bill Kielb]

It appears from your pictures this is an atmospheric downdraft. Leaving a clear space for the burners is likely very necessary so shift things to the left seems appropriate. Stacking things so there is a clear patch for the combustion gases is also good so reasonable spacing from the bottom, and sidewalls necessary. I think your instinct is great there. Forcing the air path will help but, late in the firing the kiln is depending on radiation. All that combustion air does not hold a great deal of heat, it’s too thin. You cannot heat your wares to two thousand degrees f by blowing hot air at them really.

So this seems to be a low mass fiber kiln which should be easy to fire to temperature. I would try an empty firing and get a feel for gas pressure and damper opening. If you have a pyrometer you can learn pretty easily by setting a minimal pressure to start and closing the damper until you see the highest rate of temperature rise. Maybe it will be one degree every second or two seconds, etc… then slowly raise the gas pressure and find the next damper position for best rate at the new pressure. Your damper adjustments are likely to be slight movements wider as you go up in gas pressure. Note the pressures and damper openings as you go. Efficient use of the gas and damper is essential to firing a gas kiln and damper movements are often very small to yield best performance.

This is hard to do on your own, so if lucky, you have someone to show you. If not it often takes an enormous amount of patience to see the slight variations that help so much with saving fuel. If you test fire, take some pictures along the way ……. of a burner, spy hole, flue if flames are visible and post with gas and damper settings.

Edited November 21, 2022 by Bill Kielb

[Antoinette Brown]

1 hour ago, Kelly in AK said:

My two cents, staggering your shelves is good, keep doing that. Space for combustion is good, I think raising that bottom shelf is the right instinct.

I’m still not quite clear on the exit path of the flame, I think you’re saying it goes down through the perforated floor.

A couple questions, is it forced air or natural draft? And, was there a great difference in heat from top to bottom? 

It is natural draft. I should have used more than one cone pack, but the whole kiln was underfired. If staggering shelves is important then which way is best? And is it best to have the kiln top heavy or bottom heavy?

Bill thanks for the input, I have a multimeter it does not show every degree it shows a number and then you read the degrees in a table. Yes I was told that opening and closing the damper should be done in small intervals, the lady I bought it from had a lovely red clay on her property which probably was low fire and she left the flue open. I did the same,but then realised that closing it half is better. But to my astonishment when I reached the temperature to reduce, and closed nearly the flue (maybe left 1/4 open or less the temperature was rising. And there was clearly a reduction atmosphere in the kiln!

As said I had the same problem with my old kiln once and it was clearly the stacking which made the difference, much more than anything air or pressure.

Edited November 21, 2022 by Antoinette Brown

[Bill Kielb]

53 minutes ago, Antoinette Brown said:

have a multimeter it does not show every degree it shows a number and then you read the degrees in a table.

If your meter doesn’t have a temperature scale you can get a sense by watching the millivolt change per second just to have an idea if you are rising , slowing or stalling. Many meters have a temperature setting which convert the reading for you. Their own built in lookup table if it’s there. Not all have this.  Stacking definitely has an effect on being even. And excessively restrictive stacking can affect achieving peak temperature. If you can’t make cone six or cone ten then I would solve that first, not necessarily a stacking issue unless overly restrictive. Can you make temperature in an empty kiln? You could always stack a kiln with empty shelves and see how much difference it makes with getting to peak temperature.

I believe you were in reduction, but was the kiln pressurized and did you have a jet out any of the spyholes. Pictures help. When I do this with folks and we use an O2 meter, first firings are usually spent in amazement of how sensitive things are to minor adjustment. I would suggest that empty firing to temp with and without shelves solves the issue of can you get to temperature. Closing the damper a bunch and still seeing a big temperature rise is contrary to the physics of reducing the flame efficiency so it would be interesting to see what that is occurring.

So I would follow all your stacking instincts but test the kiln and the intuition to see if you can make temperature.

Picture of a nicely formed heavy reduction flame from a spyhole below. Many Folks are are surprised when they see this for the first time.

 

 

Edited November 21, 2022 by Bill Kielb

[oldlady]

the only time i fired a gas kiln was when a friend broke her arm and i did it all while she was not in the studio.   i staggered the shelves so the flame path made a repeat S pattern moving sideways as well as up and down.    she said the results were better than she had experienced and she was going to load that way afterward.

is the previous owner close enough to give you a lesson in exchange for lunch or such?

[Antoinette Brown]

Thanks all the kiln does make cone 6 but not cone 10 probably it would. The previous owner did not fire very high and yes I had a nice reduction (if you burn your fringe then you have reduction no need for a hairdresser). And a multimeter does not show all these fancy stuff it shows a figure that's it, no voltage...That S is probably the right thing feels right. I will tell how it goes next time. I will probably do a lot of white glaze and De Boos oatmeal, they are very forgiving and then stray around some other trials.

[neilestrick]

If the kiln is working properly then you should not need to stagger the shelves. I've never fired a gas kiln with the bottom shelves staggered. I think the main concern is getting enough space around the shelves for the kiln to breathe properly. You should have at least 2" clearance between the shelves and any walls. My other concern is the the floor grid flu is so close to the  burners, which could be pulling heat right out the flue before it ever has a chance to circulate in the kiln. If this were my kiln, I would set the bottom shelves up about 2" off the floor, and block off the space under the shelf along the burner ports.

[Babs]

22 hours ago, Antoinette Brown said:

I have a new kiln and my first firing was awful. 

Also the it was completely weird the temperature went up when I closed the air more and the flue at least three quarters!

 

 

 

I feel you need a bag wall along the kiln level with the lower shelf after you raise it a bit.

It isn't weird that the temp goes up when you alter the air  intake and close off the damper. Playing with this can be key to temp rise after other adjustments made. How high is your stack?

Lots of years since i fired with gas but these points come back as pertinent.

[Antoinette Brown]

dYes it's weird with the perforated floor. My old port-o- kiln didn't have thadt. It was otherwise a very similar design. And I used to use half a shelf upright as a bag wall. As a first step I try to rise the lower level. In fact you're right I used to use a full shelf at the bottom, maybe that will improve it! But I will stagger the top ones. I had that clearance of say 6 cm. And basically what you say use a bag wall, how about half a shelf upright?

Babs, the temperature ROSE while reducing!! and I did not reduce a bit so this is weird.

[Babs]

Well adjusting the air flow to produce reduction has made the air circulate to the outlet in a less direct path????less suck straight up the chimney so heat interior rises????? But requires more adjustment to bring about reduction????? Just thoughts from s.one missing their gas kiln:-)

I guess you could put an old shelf over most of the floor to block off the suck????

Edited November 22, 2022 by Babs

[Antoinette Brown]

That kiln is named high-tech kiln. That makes me assume that they thought something when they did this ( things were usually thought through back then) . But what I will do is to sacrifice one kiln stilt and cut a notch in it to have a half shelf standing up. I attached a pic on this it worked well. 

[Bill Kielb]

Looks like they still might be in business. Maybe worth a call to get the old manual / user guide.

 

[Antoinette Brown]

no I rang them!

 

[Kelly in AK]

I think is @neilestrick is solidly on point here. “I would set the bottom shelves up about 2" off the floor, and block off the space under the shelf along the burner ports.” 

The fuel needs room for combustion. Space around the edges of the shelves is critical. I imagine the space above the burners is designed as combustion area, so don’t extend shelves into that zone. It may well be that the back shelf was too high, making that space a primary combustion zone. It would make sense to me that opening the damper cooled the kiln, heat pulled off the floor right out the chimney. I wouldn’t block the inlet to the flue at all, use the damper to regulate that.

The height of the bagwall you made may be overkill (gut feeling tells me it very much is), if the kiln stays too hot on top, that’s the sign. If you have high and low peepholes you should be able to see it. 

This looks like a touchy kiln to fire, small, well insulated, short chimney. Little adjustments can make big differences. Sometimes turning down the gas can make your kiln hotter. 

[Babs]

Can you get an old kiln brick and slice it to make a bag wall which will run along level with the bottom shellf?

That's all you need.

I didn't stagger shelves but space around them and down the middle I did have.

Can you contact the prev owner, or put a shout out for anyone with experience with this type of kiln?

Remembering, I also put the shelves smack against the outlet side of the kiln...my thinking was to not allow the flame to shoot down there and out of the kiln fast.

Change one thing at a time I guess.

 

[Babs]

On 11/22/2022 at 2:15 PM, Antoinette Brown said:

That kiln is named high-tech kiln. That makes me assume that they thought something when they did this ( things were usually thought through back then) . But what I will do is to sacrifice one kiln stilt and cut a notch in it to have a half shelf standing up. I attached a pic on this it worked well. 

Thinking, maybe, because that floor IS perforated, rhe bottom shelf is ok to be lower as in first photos

[Antoinette Brown]

I did fire a second time. I did not get the bagwall in because it takes too much space. first I stacked the kiln with a whole shelf 7 cm from the kiln floor, but after reaching 400 or so C I stopped because it went up way too slow. I did let everything cool down unloaded and then loaded once again but this time the lower shelf only 2.5 cm from the floor and voila it climbed nicely! The firing is finished in due time and tomorrow I'll see weather the pieces came out nicely.  Cone 10 went down when my pyrometer said around 1235 C or 2255 F for those  on the other side of the planet, but it did so before. 

The weird thing is that when the kiln hits around 840 C  (1544F) I reduce, but when I wanted to open the flue it goes down. Picture 2 shows how far the flue is closed (well the picture is crap). It went up very nicely when I opened the air at the burners fully and the flue at least 3/4 closed. 

I wonder whether there is a problem if after 900C it's fired in reduction all the time.

It's the second time this happened to me. I had the same issue with my old kiln once changed the way I stacked and it would not go up. IMO one of the most important issue in gas firing is the right stack! 

Edited November 24, 2022 by Antoinette Brown
add

[Babs]

I am not seeing the flu anywhere.

What are your flames looking like as they enter the kiln 

Think you need to get the bagwall in level with bottom shelf. Direct suck out the bottom if not.

Readjusting the airintake after reduction.

Reduction all the way, some bodies wont like it, some glazes wont be as attractive...

John Eagle had a great article in Journal of Australian Ceramics 2008.

[Antoinette Brown]

The flue is basically in the middle of the picture. These fibre kilns don't have a real flue, my old one was the same. At least in Australia. The flames are blue with little specks of yellow every now and then. I left the bagwall out in the end.

[Babs]

Yeh well, aussie here, I built a vent above the stack and suspended a flue pipe  above the short stack with enough room to operate a damper. It reached up a few fluepipe lengths into the hood then to outside I was firing inside a shed so  needed to draw the fumes up into the hood a ways.

Fibre kiln. .fired fast up, (only used tge pilot lights for first couple of hours), and dropped down  very fast unless sorted.