How to Minimize Temp Difference Top to Bottom in Small Gas Kiln

[Mudfish1]

Folks

Can I get some advice on the best loading configuration to get the most even firing (top to bottom) in my small gas kiln? I am getting too large a temp difference from top to bottom.

It's the same kiln I've been working on for months - inside dimensions are 24" diameter and 28" high. 5 gas burners coming up from the bottom, "X" pattern with one burner in the middle.

1. Should I have the shelves spaced closer together lower in the kiln, and further apart towards the top of the kiln? Or the reverse - more space between shelves towards the bottom, and more tightly spaced towards the top?

2. Should I leave a gap between the shelves at each level so that central burner can shoot flame all the way up the middle of the kiln? Or should I have no gap and make the central burner flame bounce off the bottom pair of shelves and travel around the outer edges of the shelves like it does for the other 4 burners?

3. Other shelf configurations I could try?

I am asking for advice because my test firing had a big temp difference - Cone 11 at the bottom, Cone 6.3 at the top. Ug.

8 hour total firing. Damper closed at 50% and the kiln atmosphere was in oxidation until Cone 022 dropped at 4 hours. Increased gas pressure and moved into reduction with a 6-8" flame out the vent and both plug holes. Cone 7 on the bottom shelf dropped at 5.5 hours, followed by Cone 8 and Cone 9 at 6.5 hours. At the top level, Cone 7 still hadn't dropped, so I closed the damper to 25% and reduced the gas pressure to maintain a 6-8" flame. My goal was to slow the firing down and let the top catch up to the bottom. However, at the 8 hour mark I saw Cone 11 on the bottom starting to drop, while Cone 7 on the top was just starting to bend, so I shut off the gas, closed the damper, and went to bed feeling unhappy. Copper red test tiles showed good/great reduction on all shelves except for the top shelf, which had slight reduction.

Shelf spacing looked like this:

--------------Top of Kiln with central vent hole

7"

--------------Shelf (Reached Cone 6.3)

6"

--------------Shelf (Reached solid Cone 9)

3"

--------------Shelf (Reached solid Cone 10)

6"

--------------Shelf (Reached solid Cone 11)

3"

---------------Bottom of Kiln with 5 burner ports in "X" pattern

 

As you can see, the biggest temperature difference was between the top shelf and the one right below. That made me wonder if having a 7" space above that top shelf was a bad idea, and if should have tried 3" spacing for that top shelf instead.

As always, I appreciate any help, advice, or suggestions! I'd love to find a configuration that kept me at 1-2 cone difference top to bottom.

[Bill Kielb]

Ok, I will bump this a bit and start with firing as evenly as practical.

First thing that comes to mind is firing speed. Firing faster amplifies the uneven nature of many kilns. Finding your best target speed is important, what firing speed provides the best even temperature distribution in your kiln?

Second thing that goes hand in hand with speed, are you optimizing the damper and gas pressure to maintain a speed for the loading of the kiln? A wide open damper is just a giant hole to let heat out, so it stands to reason that we want the minimum sized damper opening to achieve a speed  and type of firing while using our gas as efficiently as practical. Are you familiar with sequencing your damper to provide just enough draft for a given gas pressure to achieve a speed and firing type (oxidation / reduction). Opening the damper wider increases heat loss at the top of the kiln so using the minimum opening necessary to maintain the atmosphere at a reasonable speed is critical to maintaining an even firing.

Your best speed for even results in a small overpowered kiln often is very slow, so probably 25-50 degrees per hour nearing finished cone using minimal damper opening and necessary gas pressure  to maintain the desired speed and  atmosphere. Good news is once you find that speed, then you can block out the last hour or two of the firing to even out the kiln temperature as much as practical by reducing firing speed and minimizing the damper opening.

 

Edited August 31 by Bill Kielb

[Dick White]

In addition to what @Bill Kielb said, I have found with smallish updraft kilns that it is best to end the loading with an empty full shelf a few inches below the top. As Bill noted, that vent is just a big hole to let the heat out. Setting a final shelf inside a few inches below it blocks the rapid exit of the heat. The heat can still get out, it just has to stay inside a bit longer as it goes around another barrier. This shelf will not affect reduction - that is managed by sliding a brick or piece of broken shelf partially across the vent hole to reduce the outflow of the exhaust. This in turn reduces the inflow of secondary air at the burner port, thus producing reduction in the kiln.

[davidh4976]

I have read that, for an updraft kiln, it helps to have a full top shelf that is one or two inches from the top of the kiln with no pottery on it. (Edit: Ha! It looks like Dick White and I hit send at the same time!)

Edited August 31 by davidh4976

[Dick White]

8 minutes ago, davidh4976 said:

(Edit: Ha! It looks like Dick White and I hit send at the same time!)

@davidh4976You get the points, it is several hours earlier in the morning there.

[Mudfish1]

Hi Folks! Apologies for the slow response to your kind suggestions - one day I will be retired and not have to worry about the end of the FY at my company! It's been about 6 weeks since I had any pottery fun.

The suggestions about a slower firing speed and also a baffle plate right at the top make sense - I will try it and see if it helps!

I was also reading a 2017 post by Neil about someone with a similar issue - huge temp difference top to bottom in a small gas kiln. It made me wonder if my shelves are too large, and are restricting the flames/gas flow of gas? When I looked at the 23 3/8" Olympic gas kiln online, I noticed that the furniture kit had 19" shelves. That would give 2" of open space between the kiln wall and the shelves.

I have a 24" inside diameter and I am using 22" five sided half shelves. With 22" shelves, I only have an average of 1" of space between the kiln wall and the shelves. I am wondering if that's just not enough space around the edges, and if I need to change to 20" half shelves to give more room between the kiln wall and the shelves.

Thoughts/advice?

[neilestrick]

1 hour ago, Mudfish1 said:

It made me wonder if my shelves are too large

Yes, your shelves are too large. Gas kilns need room to breathe. I would go even smaller than 2" space around, because the Olympic kilns are notorious for firing unevenly with that setup. You can cut down your shelves with a masonry cutting disc on a circular saw or angle grinder.

[Mudfish1]

Neil! Thanks for responding so quickly on a Sunday. My kiln isn't an Olympic, it's that weird mystery kiln that everyone was trying to help me identify with 5 burners on the bottom. However, I noticed that the inside diameter and height of my mystery kiln was very similar to the Olympic 2327G gas kiln, and your comment on that post from 2017 made me check out the furniture kit that Olympic sells for their kiln. That's when I realized my 22" shelves might be way too big!

So would you go with 18" shelves, and leave a good 3" gap all the way around between the kiln wall and the edge of the shelf? I've reread the Kiln Book and checked around online, and I haven't found any good discussions about how big a gap you need between the kiln wall and shelves in a small updraft gas kiln.

Thanks!

 

[Bill Kielb]

4 hours ago, Mudfish1 said:

So would you go with 18" shelves, and leave a good 3" gap all the way around between the kiln wall and the edge of the shelf? I've reread the Kiln Book and checked around online, and I haven't found any good discussions about how big a gap you need between the kiln wall and shelves in a small updraft gas kiln.

Gas kilns really require constant management of damper and gas pressure to fire most consistently. It generally means one picks a firing rate, sets a gas pressure and adjusts or coordinates the damper as much as practical to maintain a best rate of heating or later to maintain a reduction level and proper pressure and rate in the kiln. Setting it a fixed amount generally results in very uneven results as the flue becomes a hole in the top that just lets too much heat out too quickly.

I would suggest learning this dance with your kiln before cutting anything up. Shelf Clearance appears too small but learning to effectively hand fire the kiln is extremely useful.  To teach this, I usually make students pick a rate then adjust gas and damper to maintain the rate and type of atmosphere (oxidation or reduction) through the entire firing. Often results in 15 - 30 minute adjustments until one learns the kiln and just where it can be left alone for longer periods. Having someone teach this dance is often the only way for clarity as optimal adjustment is often much smaller than intuitive.

 

[neilestrick]

You could start with 19" and see how it goes.  With the big shelf it's not going to work regardless of how you fire it, it's blocking too much air flow. All of these kilns tend to behave about the same regardless of manufacturer. 

Another thing to try is putting a shelf at the very top about 3" down from the flue opening.

 

[Mudfish1]

Bill and Neil, thanks! I'm going to follow both suggestions - move to 19" shelves, and I am going to keep doing test firings while I learn Bill's dance! I fired a large West Coast gas kiln many, many times, and I felt pretty confident with that dance partner! This small gas kiln is a whole different world.

What's your advice on shelf spacing? Do you keep even spacing between the shelf levels all the way up, as I saw in the Olympic gas kiln manual, or do you pack more shelves in at the bottom? Or more shelves at the top? Or in this small kiln does it matter as long as I have more space around the outside of the shelves?

I tried a shelf about 3" down from the vent opening, I think it helped, but based on your advice I'd say nothing is really going to help until I use smaller shelves and really open up some space around the edges of the kiln.