Gas kiln deflector brick placements

[tomhumf]

I'm firing my gas kiln with one burner , input and chimney output shown in blue. It's not an option to add another burner at this stage. 

https://photos.app.goo.gl/PjcvCarps8tqn2tA8

I've always had a problem that the back lower and middle section is reduced more than elsewhere. Even with damper adjusted to wider and lower settings, I can't get properly oxidised bisque there, and glaze is a little over reduced compared to rest of kiln. 

https://photos.app.goo.gl/EhiX3FfJJHWfwZwV6

I recently added the bricks in green to deflect some flame into the front section more, I'm happy with how those helped. The back section here is how I've had it set for ages, it gives too much reduction at back bottom and middle. 

https://photos.app.goo.gl/w3zRqkvzDfTe9VLN7

Last firing I added the bricks in red. The middle section improved slightly I think, but the bottom back got super reduced, the tankards in red I probably won't even attempt to glaze. So this seems like I've made things worse overall at the back. 

https://photos.app.goo.gl/ttvPXqK7Px6MbN7S9

I'm wondering now if I should just removed everything at the back in yellow? Just make more space there, have a tiny one brick deflector on the back wall, and keep the green bricks at front. Any thoughts? Thanks

 

 

[Mark C.]

one burner kilns are problematic 

Go back to what worked best and maybe leave that issue space blank with no wares

[neilestrick]

You've got a lot going on there. I would try a short bag wall, just a row of bricks on end with small gaps between them right along the shelf, and one target brick in the firebox about 2/3 of the way to the back wall.

[tomhumf]

Thanks @Mark C. Yes I know it's a bit crazy having one burner but I've used this kiln working full time for the last couple of years. If I could get this working a bit better I think I'd be happy never having another burner. I will put it back to previous setup for the next glaze firing like you suggest.

@neilestrick For the next bisque I'm going to try your suggestion. I layed it out today like I think you meant? There is about a 1" gap between each brick in the bag wall. Not sure if the target brick is how you meant? There isn't much gap for flames to go around. 

https://photos.app.goo.gl/6Hnu1hFAVMF4wToK6

https://photos.app.goo.gl/AfYRrtvkjUV2G8yh6

I find it nearly impossible to cut heavy bricks with my saw, I do have a couple of thin bits I could use stacked up but that may be a bit wobbly.

https://photos.app.goo.gl/FyaP2BD2tzBeKC6L9

I'm right in thinking the bag wall and target brick should be heavies? I could cut some HTIs in half so flame channel is wider if that's an option. I can't really move the shelf left as I'd loose stacking space under the arch. 

 

 

 

 

[Bill Kielb]

I am with Neil, but maybe use a shelf on spacers instead of the brick to give yourself some space. The target brick can be 1/2 target and the Coanda effect will make 1/2 the flame wrap around the 1/2 target so a nice way to still get about 1/2 the capacity to the rear wall and get some energy and turbulence towards the front of the kiln.  Since you are reducing so easily, it indicates not enough secondary air. The opening around your burner might need enlargement, general practice or to bell these or enlarge them so about 50-60% of your air comes as secondary air. Oxidizing firing would be 10:1 air fuel ratio. If you get your hands on an oxygen sensor you could tune this much easier.. Old Alpine designs used to include additional secondary air ports in the rear of their flame trench(s). It did even things out as their burners were fully imbedded in the kiln restricting the secondary air opening so it was sort of necessary. Just some thoughts for experimentation.

Edited September 30, 2022 by Bill Kielb

[neilestrick]

Yes, just like that. If you can cut everything a little narrower to give some more space that would be good but this is a good start. Like Bill said a 1/2 width brick for the target would be preferable. You can cut hard brick with a masonry disc on a circular saw pretty easily.

[tomhumf]

Ok, I'll use the two cut bricks I showed in the last photo as target brick to start with then, I think that makes the 1/2 size you refer to? I don't know why but the masonry disc I have really struggled with these bricks so want to avoid cuts if possible. 

@Bill Kielb I have actually considered adding a hole on the back wall like you suggest. So this would have a different affect to opening the chimney damper more? The damper is currently about half open, it seems any more and I just need to use more gas to maintain climb. 

I was thinking of cutting a half brick out at red marks. I'll try the new bag wall setup before and see what happens with that first though.

https://photos.app.goo.gl/cTStyTC1FYd6cpsN6

[Bill Kielb]

2 hours ago, tomhumf said:

@Bill Kielb I have actually considered adding a hole on the back wall like you suggest.

Careful, would rather enlarge around your burner first. Any hole cut ought to be 90 degrees to the burner and on the same centerline or elevation of the burner. Flames will begin to emerge as you go into reduction so if this is not low enough it will hinder how you can reduce. Some pictures below to maybe give you ideas. Ignore the upper hole it is for soda injection.

Also. Pic of Example of shelf as your bag wall as well. Just some ideas. The goal would be to get you enough secondary air so your damper could be closed as customary or even a bit more to help retain heat but make sure your flame is oxidizing still. You can always close the damper more to drive it into reduction pressurizing the kiln from top to bottom. Enough pressure and flames start to jet out your lower spyhole. This tiny pressure also hinders secondary air from entering so the mixture becomes very rich and reducing. Often really really small damper adjustments are made, like 1/16” at a time to fine tune this back pressure.

Edited September 30, 2022 by Bill Kielb

[neilestrick]

Don't go adding any holes until you try the bag wall arrangement first. One change at a time. Plus the holes are unique to Alpine kilns, so they may or may not actually help you. I've never found them to be necessary in any of the kilns I've built. Plus updraft and downdraft behave differently.

[Mark C.]

One burner kilns will always have cold and hot spots-the trick is find glazes and clay that can work in those areas.

 

[tomhumf]

I didn't see those last two comments until just now. I realise this is a mistake but I did change the burner port opening and bag wall setup. I can put them both back how they were but I understand changing more than one variable isn't a good idea.

I made the burner port an inch bigger 

https://photos.app.goo.gl/WdD8L4Yemrier7ba9

And setup the bag wall like this. 

https://photos.app.goo.gl/1GyoYCLKiGkGaHKV7

The bisque was good, a tiny bit of reduction at the rear but much better than previously. 

The glaze firing wasn't very good. The top half was fine, if a little overfired at the back. The bag wall bricks have just been moved to see bottom shelf in this photo. The red area was a bit underfired.

The firing took 8 hours instead of the usual 10. Strangely I was getting more reduction flame from the bottom peep than usual with same gas and damper settings.

I didn't see any flame from the chimney though in later stages which I usually do. 

https://photos.app.goo.gl/nwK3KboUvWmM5VJg7

The bottom shelf was really underfired and oxidised. The front section will all have to be refired. 

https://photos.app.goo.gl/6sefBNYdgjrSvRQf9

https://photos.app.goo.gl/wkC6oE7MMyXafNxu7

So I guess it's hard to say what's going on as I changed both things at the same time. 

My idea is to do the next bisque the same way as this but to do the glaze with the burner port blocked to the same size as I had it previously. Feels like I should take one of the bricks out the bag wall and have bigger gaps but I guess I shouldn't do that as well. Any ideas?

Edited October 13, 2022 by tomhumf

[Bill Kielb]

5 hours ago, tomhumf said:

My idea is to do the next bisque the same way as this but to do the glaze with the burner port blocked to the same size as I had it previously. Feels like I should take one of the bricks out the bag wall and have bigger gaps but I guess I shouldn't do that as well. Any ideas?

My feeling is the burner port size confirms you were going in reduction because of lack of secondary air. And the better performance seems to support that result as you were able to fire faster because your burners put out more.
subjectively, To me I like the use of a shelf as your bag wall so it becomes a glowing red hot radiant panel and enlarges your flame trench a bit especially in a small kiln. I like the wall to be significantly less mass that the wares in the kiln. Also to me my next try would be with the target brick to the right or up against the right hand wall to provide some deflection forward and because of the coanda effect it still will drive about 1/2 the flame to the rear and to the right wall of the kiln.

One thing for sure, I have rarely been able to solve by changing multiple things. It’s often too dynamic so unless you get lucky with first placement it makes things really hard to solve. So whatever changes are made I think one at a time is the most effective way to creep up on a solution.

[neilestrick]

6 hours ago, tomhumf said:

Strangely I was getting more reduction flame from the bottom peep than usual with same gas and damper settings.

Are you using the same gas and damper settings, or are you making adjustments based on the how the kiln is behaving? I wouldn't expect the same settings to work. When I put the kiln into reduction, I do a good heavy reduction for about 45 minutes with the kiln stalled out- little to no temperature climb. Then I open things up into a light reduction/neutral atmosphere and climb from there. At that stage the damper should be set so you get a big flame out one peep, and just a little puff out the other. That means you've got pressure in the entire kiln but you're not choking it off. It's going to behave differently with your design changes, to make adjustments as necessary.

[tomhumf]

1 hour ago, Bill Kielb said:

My feeling is the burner port size confirms you were going in reduction because of lack of secondary air. And the better performance seems to support that result as you were able to fire faster because your burners put out more.
subjectively, To me I like the use of a shelf as your bag wall so it becomes a glowing red hot radiant panel and enlarges your flame trench a bit especially in a small kiln. I like the wall to be significantly less mass that the wares in the kiln. Also to me my next try would be with the target brick to the right or up against the right hand wall to provide some deflection forward and because of the coanda effect it still will drive about 1/2 the flame to the rear and to the right wall of the kiln.

One thing for sure, I have rarely been able to solve by changing multiple things. It’s often too dynamic so unless you get lucky with first placement it makes things really hard to solve. So whatever changes are made I think one at a time is the most effective way to creep up on a solution.

Thanks, yes I will definitely only change one thing next firing, but will keep the bisque setup the same. I think the burner port change was good. Now when I candle the kiln at the start the flame seems like it's really happy to go into the kiln, it was pretty cautious before at that stage.

1 hour ago, neilestrick said:

Are you using the same gas and damper settings, or are you making adjustments based on the how the kiln is behaving? I wouldn't expect the same settings to work. When I put the kiln into reduction, I do a good heavy reduction for about 45 minutes with the kiln stalled out- little to no temperature climb. Then I open things up into a light reduction/neutral atmosphere and climb from there. At that stage the damper should be set so you get a big flame out one peep, and just a little puff out the other. That means you've got pressure in the entire kiln but you're not choking it off. It's going to behave differently with your design changes, to make adjustments as necessary.

My method is to start reduction at 900C and just have damper in enough to get positive pressure at bottom peep - this still has temp climbing slowly. I open up the damper a little at stages as the kiln climbs maintaining positive pressure at the bottom peep. This has given good results with pretty even temp and reduction throughout the kiln.

In the latest firing I was expecting to keep the damper closed more due to the expanded burner port size. I started the damper and gas at the same as usual but was surprised  at getting more flame from the bottom pee  - Schedule below.

As firing progressed and I saw larger flames than usual at bottom peep, this continued through the firing. For the first three stages below I keep to the same settings even though the flame from the bottom peep was bigger. At 10.40 I opened to the largest damper I usually use about an hour ahead of my usual schedule as I as concerned I was getting too much reduction.

9 damper 1.5" 900

9.40 damper 1.6" 1030

10.15 damper 1.75” 1100

10.40 damper 1.8" 1145

11.09 1184

11.40cone 8 down

12.15 kiln off

[neilestrick]

Ignore the numbers on the damper. Go by what's happening with the kiln. The numbers wan be helpful for repeatability once you get things dialed in, but ultimately they're just approximate positions and will have to be adjusted according to the behavior of the kiln, which can change with every firing depending on the load, weather, etc.

[Bill Kielb]

49 minutes ago, tomhumf said:

My method is to start reduction at 900C and just have damper in enough to get positive pressure at bottom peep - this still has temp climbing slowly. I open up the damper a little at stages as the kiln climbs maintaining positive pressure at the bottom peep. This has given good results with pretty even temp and reduction throughout the kiln.

Probably the most important part - your method works for you. I will say that excess pressure out the lower burner port lines up with a bit too much restriction or reflected flame in the flame path, especially if your kiln is not pressurizing from the top down. Under normal circumstances folks find a suitable gas pressure then begin closing their damper very slowly until some pressure (flame)  exits the lower spyhole and they are in heavy reduction. With the restriction in the flame path it sounds like you will need to free this up a bit, it likely is too restrictive. One thing at a time though.