Electric kiln lid repair

[CactusPots]

I'm taking a shot at repairing a 3" lid for a Skutt kiln.  Questionable, I know.  It's back from 3 pieces to 1 with Alsey kiln mortar.  It's rated to 3000 degrees.  I think I got it from Ward for coating and repairing my fiber kiln.   I think I need to put the band on before I do anything else with the lid.  It won't have any strength without it.  If I put the band on and put the lid in my gas kiln, how hot do you think I should go to set the mortar, but not adversely affect the stainless band?  If no one has an opinion, I'll call the mortar company in the morning and see if I can get support.

Meanwhile, I have a 2 1/2 " replacement, so this isn't mission critical.

[neilestrick]

Chances are the mortar won't hold. Old bricks mortared together in a long line across a big slab just won't hold. Putting it in the gas kiln won't help. If it's going to work, the  joints should be strong before firing, and the heat from a firing will do little to help it. If the floor is still in one piece, I'd swap the slabs, putting pice of sheet metal under the floor to keep it rigid.

[Bill Kielb]

Take a look at the L&L video. As I recall they repair a 3” lid that is broken into four or  five pieces. The firing at the end I believe is slow bisque to cone 5 of the kiln with the new top on ........ I believe. Haven’t watched it in a long time.

 

[CactusPots]

ALL of the structural integrity of the electric kiln lid is in the band.  Until my band broke, I never even knew I had a broken lid.  The video is some help, thanks for that.  I might have bought the kiln cement they referenced, but I have a bucket of this kiln mortar, so we'll see how it goes.  I think I will just put it on the kiln and run a bisque load.  Can't really hurt anything.  The lid is supported all around the edge of the kiln and the band will keep it from going anywhere no matter what.  I usually run my bisque with a fiber blanket on top, but I'll just run one without to see if we get some burn out.

If you see a heavy oxidation on the band right above the top spy hole, replace the band before it burns all the way through.  Don't be stupid like me.

[Bill Kielb]

The band is super important! The firebrick is terrible in tension and easily breaks with a few pounds of force. The band simply keeps everything in compression not allowing the brick to  ever go into tension and fail. Same technique we use to design prestressed concrete except our tension members run through the concrete.

You are absolutely correct, the band is everything and keeping it properly tensioned will prolong the life of the lid.

[MFP]

Why don't you just buy soft brick and make a new lid instead of trying to mortar together  broken brick? You can cut the soft brick easily with a key hole saw....I know because I built an arch that way.  You already have a "blueprint" of how to put the new brick together.

[CactusPots]

Better than a keyhole saw, I have have a floor bandsaw.  Cuts soft brick like butter.  But really, what's the point?  If I'm joining 3 pieces or 30, I'm still mortaring soft brick.  Like I said, I have a working lid for the kiln.  I hate throwing things away that I can fix.  I may or may not be successful, I'll give a report when I'm done.

The 2 1/2" lids are easily available used for low cost.  Just eyeballing the project with new soft brick strikes me as $100 bill at least.  So now the question on the table is, what is the value of a 3" lid over a 2 1/2" lid?  This is a 3" brick Skutt.  Especially since I fire it with a 3" fiber mat on top, I bet it's not worth the price of a new lid.

[neilestrick]

There's a big difference between mortaring a long crack in old brick vs. mortaring up new brick. Like I said before, the long crack most likely won't hold. Laying up new brick will give you a good strong lid. But I agree- if you can find a used lid in good condition that would be great.

[CactusPots]

The long crack can't go anywhere because the band won't let it.  I bet I fired the kiln with a cracked lid for years.  It was only when the band failed and I tried to lift the lid that the problem was evident.  Plus this is a 06 bisque kiln only.  I'm pretty sure my repair is going to work just fine.  My local dealer had a pile of (good enough) used kiln lids.  All 2 1/2"

Do you have an opinion of the use of 2 1/2 and 3 inch bricks on electric kilns?  For my use, there isn't much point in getting crazy about the lid.  IMO

[liambesaw]

Since you're firing with a fiber blanket on top it doesn't matter how thick the lid is.  Seems like overkill for a bisque kiln, but I'd just pop the used lid on and maybe keep the old one around for spare parts.

[CactusPots]

That's good advice, except I can't resist a fix it job.  The used lid is already in service.  I may just finish the repair, stash it away and call that the spare.

The funny thing about the electric kiln for only bisque.  The elements don't take the beating, but the rest of the kiln does.  The burnout is really tough on the band and handle especially.  I don't know why Skutt puts the spy port that's going to belch corrosive gas right onto the handle.  That's right where my band burned through.  The lifting handles for the individual rings also are corroded. 

 

 

[liambesaw]

1 hour ago, CactusPots said:

That's good advice, except I can't resist a fix it job.  The used lid is already in service.  I may just finish the repair, stash it away and call that the spare.

The funny thing about the electric kiln for only bisque.  The elements don't take the beating, but the rest of the kiln does.  The burnout is really tough on the band and handle especially.  I don't know why Skutt puts the spy port that's going to belch corrosive gas right onto the handle.  That's right where my band burned through.  The lifting handles for the individual rings also are corroded. 

 

 

That's nuts, is it because it's unventilated or something? Mines just outdoors but there's always a breeze, so I haven't seen any corrosion.  My kiln is pretty old too.  Maybe it's that fresh California air getting to it 

[neilestrick]

Unvented kilns corrode much faster than those with a downdraft vent.

Most kiln lids will flex when the crack goes all the way through, regardless of how tight the band is. To some degree it depends on what direction the crack runs. When they flex they drop little bits into the kiln onto the ware. With only using it as a bisque kiln it's not an issue. In a glaze kiln it's best to replace the lid rather than mess with mortaring it.

[Bill Kielb]

I would suspect that your repair will be fine. Three joints versus  many individual joints is probably insignificant in the scheme of things.. The band is essential though, new or cracked,  Any old lid I ever removed usually would separate easily at the joints  when the band was removed. Very few  if any newly constructed slabs can be picked up and moved about without their own weight fracturing the brick and joints If not handled carefully.. Once banded folks put pots on top for drying etc... amazing how strong they are when kept in compression. 

The difference between two and one half inches vs. three inches thermally is about 16% more loss for the top which is only a percentage of the kiln losses. Five inches of fiber saves energy, if ya got it use it.

The extra trapped heat from the fiber and of course the corrosive fumes will likely degrade the band sooner though.  Even Stainless steel used in home kilns is not the best ever though, but market prices and margins generally set an acceptable design life.

The lid repair is pretty simple (watch the video)   I would do it in a heartbeat and use it or keep it as a spare. That’s just me though.

[CactusPots]

Bill,

Is that to say that a 3" kiln lid would save 16% over the cost of using a 2 1/2 " lid?  Electric kiln bisque firing is easily the most expensive part of my production.  So much so, that in anticipation of my retirement, we had solar installed on the house.  My kiln isn't new enough to do the calculation of kw used.  My last attempt to figure the cost came up with $15 per load.  Seems high?  That's California for ya.  Adding the fiber mat over rides the debate, I understand.  I would like to know the answer because the question has been asked of me to advise 2 1/2 vs 3 inch on a new kiln purchase.

[Bill Kielb]

@CactusPots

The 3” lid will have 16% less loss than the 2-1/2” lid. They are the same material homogeneous so  straight percentage in R or U value. As to the  amount of energy just lost through the lid it will be approximately a surface area calculation. Add up all your areas and figure what percent total the top is and yes the top will have some infiltration and will radiate a bit more so losses through it so will be slightly higher.

As a general example for a 36” diameter kiln that is 32” high losses end up approximately 18% top, 18% bottom and 64% sides. The reality is the sides are probably banded with stainless which will help a little with radiation, and the top is a little leaky so the educated swag:

20% Top

15% Bottom

65% sides

so realistically the 2-1/2” lid will actually have 1.16 X .20 or about 23.2 % loss if you use it, or roughly 3.2% more total loss. Which is why I said if ya got the fiber, use it. It  is saving you probably 3-5% each time you use it no matter which lid.  I know folks that have wrapped their kilns just to squeeze a few more firings out of worn out elements. Now controlled cooling in a well insulated kiln, generally becomes a controller issue. The additional heat trapped by the blanket will likely accelerate corrosion of the stainless though.