How do I even out the kiln firing?

[jrgpots]

I just finished my first firing of my new down draft gas kiln.  The bottom fired one cone hotter than the top.  So, how do I even the thing out?  Normally, I thought the top was hotter than the bottom.  If the bottom is hotter than the top, would I open the damper up a bit more to even out the temp?

 

Also I got bubbles on one of my glazes. Could this be from a too rapid of ramp or did I need to let the kiln run for longer at the end? The glaze did not bubble with electric kiln.

 

Jed

[Bill Kielb]

Hmm, several issues and lots of dynamics in a gas kiln so hard for me to provide a specific answer. If you provide more details of how you fire, what you fire, how you load, a little about the kiln, are you reducing, etc… folks here will likely have ideas.

To name a few issues re: firing evenly

• Number one on our list of even firing would be final speed you fire in the last two hours?
• Number two on out list would be flame centerline and flame path which may be influenced by damper operation.
• Number three would be where are the kilns greatest losses from the shell

Bubbling glaze / pinholes

lots of causes but peak temperature and a faster than normal cool down with a possible remedy of  down firing or it’s equivalent may help with problem glazes.

lots of speculation here, share more info and folks will likely provide constructive feedback. Maybe a picture or two.

 

Edited October 24, 2022 by Bill Kielb

[jrgpots]

The firing chart is uploaded.  Temp(Celsius) vs Time (Hours).   the initial few hours were with venturi burners.  Power burners were started at about 300 degrees.  The damper was at 1.5 " open.  (it burns well at that setting).  Short time of reduction at 900 degree ( 20 minutes or so).   The kiln's rate of climb was slowing in the last hour or so, but I think I should have slowed down a bit more. The last 20 minutes , the kiln was again placed in reduction.   

The lowest kiln shelf was 4.5 inches off the floor. Fire brick slits lined the outer shelves edge creating bag walls just on the inside edge of the flame troughs.    2 burners at the back on either side facing forward.  I did not put deflecting bricks in the flame trough.  Flame troughs were 4 " wide.  Hard fire brick splits on their sides were used to support the shelves and make the bag on the inside edge of the  fire troughs.  The second shelf was  8 " above the first and the third shelf 8" above that. I did not bag wall the front of the first shelf.  I will do that next time.

 

Jed

Document 6 (1).pdf

Edited October 24, 2022 by jrgpots
typo

[jrgpots]

Explain "kiln losses from the shell."  If you are referring to heat loss, The sidewalls and front walls are 9" IFB.  The arch is 9" also.  The floor has a 4" hard brick foundation, then 4.5" IFB,  then topped with 2.5" Hard brick.  I could throw wool on top of the arch.  But it really di not get too hot.  The greatest loss was from the front door.

 

Jed

 

Edited October 24, 2022 by jrgpots
clarification

[neilestrick]

Raise the bag wall a few inches.

[jrgpots]

10 minutes ago, neilestrick said:

Raise the bag wall a few inches.

The bag walls were 9" plus the thickness of one shelf.  How high should I take them?  I could rais the bag wall  to the third shelf ( 13.5").

 

Edited October 24, 2022 by jrgpots

[Bill Kielb]

1 hour ago, jrgpots said:

 

Explain "kiln losses from the shell."  If you are referring to heat lossThe sidewalls and front walls are 9" IFB.  The arch is 9" also.  The floor has a 4" hard brick foundation, then 4.5" IFB

I would assume the top has the most surface area and is less insulated than the sides so likely larger shell loss than the sidewalls and maybe first or close second to the floor. More surface area still I would think than the floor. The 4” raised shelf on the floor should also help a bit as well.

it’s really hard to know your speed from the graph, any idea what you fired in the las two hours.? I don’t understand the units off time on the graph actually.

I would like to see a picture of the firing setup, one power burner, two?

Flame centerline will determine the heat distribution as almost all the heat at the end of the firing is radiant even though you are moving a lot of hot air, the volume of air and it’s ability to retain substantial heat is very small in relation to the radiation.

The effective centerline will be a function of the burner physical location and it’s effective height depending on operation. Generally when going in reduction It raises a bit.

Post some pics of the setup, hard to speculate on position and clearance without seeing.

[neilestrick]

10 hours ago, jrgpots said:

The bag walls were 9" plus the thickness of one shelf.  How high should I take them?  I could rais the bag wall  to the third shelf ( 13.5").

 

If they have gaps you could try closing up the gaps.  Otherwise just a couple inches will probably make a difference.  Slowing down the firing at the end can also help, like for the last 2-3 cones.

Given that the kiln is built and you're not going to be adding insulation or moving the burners, at this point changing the bag wall and/or target brick and making adjustments to the burner settings are your only options. And honestly, if you're within one cone, then there's no reason to make and significant changes to the kiln design itself. Bag wall and settings should be able to dial it in just fine.

[jrgpots]

In my excitement, I pushed it too fast.  Next time I will have a higher bag wall and slow down the firing in the last few cones.   I think I just need to get acquainted the my new girl. Thanks everyone for the insights.

 

Jed

[Kelly in AK]

New kiln. It’s part of the joy. I’ve taken pictures of the insides of many people’s kilns, a lot of them similar to your design. The majority have a target/deflector brick (usually near the end of the the flame trough). So, in addition to the bagwall height, and adjustments to burners, that’s another arrow in your quiver.

My initial reaction was, “Oh, downdraft too hot on the bottom? Lucky.” I’ve had much easier time fixing that problem than the other way around. For me raising the bagwall 4-1/2” went from a cone too hot on bottom to even. It almost felt unnatural how even it fired after that. Also, though you’re already keen to this, hurrying things tends to expand the differences.

[Mark C.]

I’m on the other side of the world now for awhile but your very close with only one cone apart. Do as Neil says raise the bag wall to move heat up and fire slower especially at end of fire.