Uneven firing in small gas kiln

[georgeperrypots]

Apologies for quite a common topic but here goes. I've done 2 gas firings so far in this small Laser kiln. I've managed some relatively quick firings (under 7 hours) with good reduction mostly. I've found the bottom shelf can be quite patchy. With some pots coming out half reduced/ half not. Also not as hot, not reaching cone 9. What's the main resolution? Less tight pack? Longer/ slower firing?

It's an updraught with two burners, fired with propane. Fired it to cone 9 on the middle shelf which you can see in the pictures.

I've attached two pictures to show how they've been packed so far.

Any thoughts/ recommendations welcomed!

Edited May 23, 2022 by georgeperrypots

[Bill Kielb]

Slower is generally more even. For reduction and speed as well actually, how well you manage your damper, gas pressure and internal kiln pressure  to keep oxygen from entering.  Oxygen reoxidizes your already reduced piece if let it in and it’s not thoroughly diluted with the reduction atmosphere. So the normal way to tell how pressurized your kiln is is to ensure you have a positive jet coming out the lowest spy hole. If a flame is not coming out, air is going in. 

Pressurizing happens from the top down so it’s common to find a damper position where the lowest part of the kiln is slightly pressurized to ensure there is reduction throughout. (As little as 1/16” movement might be required)

for areas of the kiln that fire faster or hotter regardless of speed, loading these areas more densely can help requiring more energy to heat in that area.

picture below - really healthy jet at spyhole and really nice even flame out the top of an updraft. How do yours compare?

 

Edited May 24, 2022 by Bill Kielb

[neilestrick]

What Bill said. There must be pressure out both spy holes. One will have a lot of flame, the other just a flicker, but that indicates pressure throughout the kiln. 7 hours is probably too fast for an updraft kiln. Take your time. 9-10 hours would be good, and should give you more evenness. In my experience, top and bottom may be uneven for a good portion of the firing, but should start to even out by cone 7 or so.

[georgeperrypots]

Thank you both for your responses. I'm reducing from 950C to the end of the firing and maintaining a decent flame out the top and spy hole (there's only one spy hole in the middle). I don't think I'm getting flames quite like the ones in your pictures though. I guess I should try a slower firing as well as some healthier flames. 

[neilestrick]

5 hours ago, georgeperrypots said:

Thank you both for your responses. I'm reducing from 950C to the end of the firing and maintaining a decent flame out the top and spy hole (there's only one spy hole in the middle). I don't think I'm getting flames quite like the ones in your pictures though. I guess I should try a slower firing as well as some healthier flames. 

When I fire gas reduction, I do a 45 minute reduction around cone 012 during which I stall out the kiln with little to no temperature climb. Then I put it into a lighter reduction for the climb. That way I'm sure everything reduces before it gets too hot to take reduction well.

[Bill Kielb]

11 hours ago, georgeperrypots said:

I'm reducing from 950C t

I agree with Neil, If you are looking for better body reduction, you may want to go in heavy reduction at 880c to get to a more open body. I wouldn’t go less than 850c though, just for safety.