[Laser Gas Kiln Help]

On 1/20/2024 at 1:25 AM, Mark C. said:

1260 in my world 2300 degrees  which is just at the temp you are firing to so 1st its going to shrink and crack

You need higher temp materials for cone firing in kilns for repeated firings. I would use 2600 degree fiber (you can convert as I had to to see what this temp is )

Fiber board and fiber rolls shrink and they shrink the most at maximum limits and thats where you are right at cone 10.

Even bricks at their limit will spall over time-say like acone 10 brick kiln with K2300 soft bricks they spall after reapeated firings. I have laerned this about 40 years ago in my kiln building life.

Repalce all the 1260c with higher limit materials-they will cost more.

The metal in that photo (photo needs to show a larger area so one can see what it is (a chimmney stack or kiln lid? )It looks like part of kiln frame and if thats true you need it.

I would need to see the inside of kiln ceiling and outside better to advise on the that metal bar and whats it doing. better and more photos

If you have used up the 1st tank and its low pressure less gas ,well yes it can affect reduction very much

25mm is what we call 1  inch and you need in my mind a minimum  of  6 inchs or 16 centimeters for a kiln ceiling of fiber for cone 10 firings. More would be better

You only need the hot face to be 2600 degree material-say the 1st 2 inchs thgen the cheaper 1260 material can be the rest of the make up to get -9 inchs of ceiling in total you need

My guess is you are losing the heat through kiln ceiling.Lots of heat

Fiber board for me shrinks  and cracks more and I prefer fiber blanket with buttons to hold it up but this will fail over time.. It's not the best for ceilings because of this.. You can fold it in U shapes and compress it with stainless rods and steel ends and it works well for flat ceilings this way.

I have a salt kiln made this way with a 9 inch thick ceiling and the rods are about 6 inchs up from hot face. I folded  the  2 foot wide blanket  in half and cut it to make the 9 inch folded tacos(this takes about 22 iches of 2 inch thick fiber as tghe fold trakes up length  and compressed them in a wood form and rammed a sharpened treaded stainless rod (use nuts on ends ) through them with a flat bar on each end to compress the tacos into a tight flat roof that is a uni solid one piece unit that sides on kiln walls.

Hi Again Mark (an y one else!)

I have a couple of follow-on questions from the previous discussion, if you don't mind.

1. The fibre board supplier has 25mm (1 inch) and 50mm (2 inch) 1440C / 2600F fibre board available.  I assume I should I be going for the thicker one to replace the interior ceiling but could you let me know your thoughts?

2.   If I'm going to replace the metal rod at the back of the kiln, is there a particular type of metal I need to be using?

Many thanks.

 

Clare    

[Laser Gas Kiln Help]

2 hours ago, Bill Kielb said:

Just looking at this, where is the back of this kiln?  Hmm think I actually see it now. I would make sure the flue passage extends to top - no leaks here.

Also I think the missing member is just a tie bar to keep the sides of the kiln together and leave room for the rigid (rear) top of this kiln also missing in the picture.

I think if you plan for the shrinkage (the rigid will shrink during first firing, it has LOI - they all shrink). My initial thought would be: the top, and floor extend wall to wall, the sides rest on the floor and help hold up the top. Fire to shrink everything and stuff all remaining gaps. Readjust buttons so all are as straight as practical and not overly tight, just straight. Add a tie bar and maybe make a new top out of rigid if that is missing and make sure no gaps between top and flue so the tie bar stays out of direct heat.

Finally after all shrinking and stuffing, you could just lay in a second piece of rigid across the bottom, replaceable and helps with heat loss on the bottom anyway. Nice project I think definitely could perform better than new in the end!

Thanks again, Bill.  Appreciate all these suggestions.  

[Laser Gas Kiln Help]

5 hours ago, Mark C. said:

I would check the burners and ckean the orfices first as Neil suggests.

The kiln fiber board is cracked but not yet in bad shape.Except those massive opening arount the edges-you need to stuff thise with high temp fiber (loose stuffing) . At max temperature of 1300C  for that kiln means the interior layer needs to be well above that to keep from cracking and coming apart. My guess is you have lost lots of heat thru thosew ceiling spaces around edges which also will cook the button wires holding this together very fast now, The idea is keep the button wires cool and away from hot face heat. 

The lid has shrunk and the edges now have space exposed and you need to fix before firing again. That shows well in your photo of kiln ceiling.

That kiln is made of fiber board and I'm not a fan as it shrinks and cracks more than loose fiber unless its the very high temp stuff.

I would use material that rated to 1426C for all hot faces. For me thats 2,600 Fahrenheit. Fiber or soft Bricks As one who fires to cone 11 a lot it pays off in the long run.No spalling or excessive shrinking.

Many thanks, Mark.

[Laser Gas Kiln Help]

7 hours ago, Bill Kielb said:

Here are some thoughts
A couple things, rigid insulation can be more conductive than loose fiber so replacing loose fiber with rigid can result in higher losses. What is the thermal conductivity of your rigid and how does that compare with loose fiber?

Air gaps - huge problem, so most fiber has published shrinkage after first 24 hours - often in the range of 2-5%. Air gaps can cause crazy loses if the gaps leak enough. Those gaps look significant so unless the fiber is rigidized behind them they likely are increasing the losses through the shell. Account for the published shrinkage and layup your initial cuts with sensible overlap. Then after final shrinkage the overlapping joints can be infilled to further improve them. If they don’t overlap to form a clean joint  though this is likely not possible.

Compression - insulation works by trapping air. Often why loose fiber is rated as good or better per inch than rigid. Rigid however is structural so it has that advantage. It looks like the rigid is compressing the fiber though so if that is the case, this will reduce the fibers insulating value significantly. Sandwiching or compressing the fiber with the rigid will derate the fiber.

I think all doable however, just need to be mindful of achieving goor air sealing and as much or greater insulating value as the kiln was designed with. So some real lookup of specifications, existing thicknesses and then adding enough thickness to meet or exceed.

Finally air leakage is an energy waster, but also critical to keeping an even reduction environment. Minimizing unwanted leakage should help reduction consistency a bunch.

Thank you so much, Bill.  The manual which accompanied this kiln, which was second hand, was thin on specifications and internet searches haven't turned up anything either.  Not sure where else I might look!  But from the feedback provided here I think we definitely need more than what we've used to date.

[Laser Gas Kiln Help]

10 hours ago, neilestrick said:

Even if the materials aren't rated high enough, they shouldn't change the insulation factor all that much until they've really shrunk up. If you're getting gaps, then that could definitely be a problem, though. Only the hot face would be an issue, not anything above it. Looks like you could put another layer or two of blanket on the top.

Have you checked to make sure the burners didn't get some crud in them during the rebuild?

Thanks, Neil.  Yes, cleaned out all the burner with some compressed air before we refired.

[Laser Gas Kiln Help]

12 minutes ago, cspindler said:

Mark,

Thank you so much for your quick and thorough reply.  I had only found out after doing the repair that the materials I'd been given were 1260C and thought that wasn't enough.   But it's good to have your confirmation here and your suggestions on how much total thickness of insulation we should be aiming for.   

I'm attaching here some general images of the kiln showing 1) front on view of the exterior and hood; 2) interior view (post fix, where you can see the shrinkage and cracking in the ceiling) and 3) close up showing flue opening and hood.   

I will send separate messages with images of the roof, which will hopefully better explain the positioning of the corroded metal bar. 

 

Here is a top down picture of the roof with the metal cap piece removed. 

The other picture is an attempt to show where the corroded metal bar was positioned.  The  blue arrow is pointing at the flue wall / back wall of the kiln chamber.  The pink arrow is pointing at the hole where the metal bar was attached to the kiln wall with a screw (there is another hole on the opposite side of the kiln).  Given that the position of this hole is very close to the top of the kiln roof, I don't think its purpose is to provide a support for the kiln chamber ceiling.  But maybe is was a place for the ceramic pin wires to be attached?

[Laser Gas Kiln Help]

On 1/19/2024 at 10:50 PM, neilestrick said:

@cspindler  How was the original ceiling constructed?

Hi Neil,

The original ceiling was fibre board with ceramic pins inserted which then were attached with wires to metal cross bars that spanned across the top underneath the metal roof piece.   The attached pictures show 1) the interior post repair, but which is very similar to what it looked like before and 2) the top of the kiln with metal roof piece removed showing the cross bars where the pin wires were attached.

[Laser Gas Kiln Help]

On 1/20/2024 at 1:25 AM, Mark C. said:

1260 in my world 2300 degrees  which is just at the temp you are firing to so 1st its going to shrink and crack

You need higher temp materials for cone firing in kilns for repeated firings. I would use 2600 degree fiber (you can convert as I had to to see what this temp is )

Fiber board and fiber rolls shrink and they shrink the most at maximum limits and thats where you are right at cone 10.

Even bricks at their limit will spall over time-say like acone 10 brick kiln with K2300 soft bricks they spall after reapeated firings. I have laerned this about 40 years ago in my kiln building life.

Repalce all the 1260c with higher limit materials-they will cost more.

The metal in that photo (photo needs to show a larger area so one can see what it is (a chimmney stack or kiln lid? )It looks like part of kiln frame and if thats true you need it.

I would need to see the inside of kiln ceiling and outside better to advise on the that metal bar and whats it doing. better and more photos

If you have used up the 1st tank and its low pressure less gas ,well yes it can affect reduction very much

25mm is what we call 1  inch and you need in my mind a minimum  of  6 inchs or 16 centimeters for a kiln ceiling of fiber for cone 10 firings. More would be better

You only need the hot face to be 2600 degree material-say the 1st 2 inchs thgen the cheaper 1260 material can be the rest of the make up to get -9 inchs of ceiling in total you need

My guess is you are losing the heat through kiln ceiling.Lots of heat

Fiber board for me shrinks  and cracks more and I prefer fiber blanket with buttons to hold it up but this will fail over time.. It's not the best for ceilings because of this.. You can fold it in U shapes and compress it with stainless rods and steel ends and it works well for flat ceilings this way.

I have a salt kiln made this way with a 9 inch thick ceiling and the rods are about 6 inchs up from hot face. I folded  the  2 foot wide blanket  in half and cut it to make the 9 inch folded tacos(this takes about 22 iches of 2 inch thick fiber as tghe fold trakes up length  and compressed them in a wood form and rammed a sharpened treaded stainless rod (use nuts on ends ) through them with a flat bar on each end to compress the tacos into a tight flat roof that is a uni solid one piece unit that sides on kiln walls.

Mark,

Thank you so much for your quick and thorough reply.  I had only found out after doing the repair that the materials I'd been given were 1260C and thought that wasn't enough.   But it's good to have your confirmation here and your suggestions on how much total thickness of insulation we should be aiming for.   

I'm attaching here some general images of the kiln showing 1) front on view of the exterior and hood; 2) interior view (post fix, where you can see the shrinkage and cracking in the ceiling) and 3) close up showing flue opening and hood.   

I will send separate messages with images of the roof, which will hopefully better explain the positioning of the corroded metal bar. 

[Laser Gas Kiln Help]

Hello.  I’m a UK potter looking for help in diagnosing problems with a Laser J12N downdraft kiln, which runs off LPG and has 4 burner ports.  Following a recent repair to replace a collapsing ceiling, the kiln now requires more gas to make it to temperature (cone 9/10) and the reduction has become very patchy.

The interior ceiling was repaired using a piece of 25mm ceramic fibre board with a temperature resistance of 1260C.  Above this, under the metal roof,  we inserted a layer of 25mm ceramic fibre blanket plus a layer of 25mm ceramic fibre board, both with a temperature resistance of 1260C.   

When we removed the old insulation under the metal roof, we discovered the remains of a metal bar at the back of the kiln just in front of the flue opening (see photo showing position) which had corroded away.  It looked like it may have supported a thin layer of ceramic board/blanket.  We didn't replace this metal bar when doing the repair.  

Prior to our “fix” it would take us about 8-9 hours to reach cone 9/10 using one 19kg tank of LPG.  We typically put the kiln into reduction at about 1000C using ceramic bricks in the flue opening.  In terms of reduction achieved within the kiln, pieces on the bottom shelves would be slightly less reduced due to proximity to the burner ports but pieces above would be nicely reduced with those on the top shelves achieving the heaviest  reduction.

Following our “fix” we found that one 19kg tank of LPG was no longer enough to get to cone 9/10 so we purchased a changeover valve to allow for a second tank.  Also the levels of reduction within the kiln are now very unpredictable.  But  generally, pieces on the bottom shelves are not reduced at all and neither are pieces on the top rear shelf.  Pieces on the middle shelves are the only ones showing any amounts of reduction.

After we had done a couple firings, we noticed that the piece of hardboard we'd used to replace the interior ceiling had shrunk from the back wall and was already showing signs of cracking.    Also, our first few attempts of using the change-over valve were a bit bumpy.  In both instances, the pressure/flow of gas from the first tank had already dropped quite low when we initiated the changeover.  We didn’t completely lose flame before engaging the next tank but it did drop quite low for about 5 minutes.

So before embarking on another fix I'm looking for some guidance on how to do it correctly.  Specifically, I have the following questions:

• What could be the cause(s) of the need for more gas and the change in nature of the reduction within the kiln?
• Should we be using ceramic fibre board / blanket that has a temperature resistance higher than 1260C?
• Should we be using ceramic fibre board / blanket that is thicker than 25mm?  Or is there a total thickness of insulation that we need to achieve?
• Does anyone know the purpose or importance of the metal bar at the back of the kiln?
• Could the issues we encountered with the cut over valve have contributed to the lack of reduction on the bottom shelves?  Is there a different/ better way for us to manage the change over?

Any other thoughts/comments/suggestions beyond the above are certainly welcome!  Many thanks in advance for your help!

Clare