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Jackpots

Question about Skutt electric kiln

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Hi - I am attempting my first bisque firing ever.  I started the kiln this morning at 7:30 am on slow cone mode and added a 2 hour preheat.  The slow cone mode was to take about twelve hours but it is still going around 1940 degrees F at 10:30 pm.  I do have a vent running. What concerns me is that I have to lift the lid a tiny bit to hook the latch.  When I bend down evenly with the bottom of the lid I can see the red heat all the way across.  Is it supposed to close more tightly than that? 

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21 minutes ago, Jackpots said:

Hi - I am attempting my first bisque firing ever.  I started the kiln this morning at 7:30 am on slow cone mode and added a 2 hour preheat.  The slow cone mode was to take about twelve hours but it is still going around 1940 degrees F at 10:30 pm.  I do have a vent running. What concerns me is that I have to lift the lid a tiny bit to hook the latch.  When I bend down evenly with the bottom of the lid I can see the red heat all the way across.  Is it supposed to close more tightly than that? 

 No, that's how my lid closes too, and it even shoots a laser beam out in all directions.  

So a slow bisque with a 2 hour preheat is gonna run you about 16ish hours total without accounting for venting.  The program itself is 13 hours, not including the preheat (also the preheat is a slow ramp to 200 and then the hold for however many hours). So let's say a preheat of 2 hours will take about an hour/hour and a half to reach 200 depending on ambient temperature.  Then hold for 2 hours, then slow bisque schedule for 13 hours.  Tada!  I usually only slow bisque my kiln if it's full of red clay, might be prudent to test whether your clay responds well to a fast bisque as it shaves 3 hours off of the schedule.

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If you are using the Bartlett control standard cone fire schedule both schedules include a first segment  slow heat to get to 250 degrees to help things dry out slowly.  Slow bisque = 2.25 hours and fast bisque = 1.5 hours to 250 degrees.

After that the slow bisque is slightly slower than the fast for an extended burnout of your clay. I’ve attached both below for reference. Any preheat you add will add to the program  time total. As your elements wear and get older these times will creep up as the kiln loses power.

its good to know the time your kiln typically takes as this will be a good indicator your elements are beginning to wear.

BAE00B24-06AE-414E-B6CD-D39DE100E1FD.jpeg.c95055e00fd77035de3c05fe0602d540.jpeg

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Thanks for your help.  

Going by the graph in my Skutt manual, the cone 04 slow speed takes a little over 2 hours to get to 250 - but being a paranoid beginner I decided to add the preheat. I think next time I'll just make sure everything has had plenty of time to dry out - but with Georgia humidity does that ever really happen - and try it with no preheat.

My next question is - would it be a terrible idea to put a layer of ceramic fiber material on the top rim of the kiln to close/insulate the crack between the kiln and the lid? It sort of seems like a refrigerator without a gasket.

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The problem there is that you're actually cutting off oxygen which is an important ingredient when burning out the organics.  Your kiln isn't firing slow, it's firing according to the schedule, so it won't help with anything.

I do at least a 1 hour preheat on all of my bisque loads just for safety.  It is humid here in the Pacific Northwest and I like the insurance it affords.  Cutting an hour or two off the firing time doesn't do anything for me since I fire overnight, and the preheat is probably the least energy intensive portion of the firing, so not much savings on electricty either.

That said, I encourage you to experiment and see what works best for you!

Edited by liambesaw

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I am not in favor of this but can’t find a negative to it other than if it binds in anyway then distortion becomes an issue and of course damaging your pots with debris. When this kiln is cold  does the lid fit well?  When we refinish these we use something very square and straight (Air-file makes quick work of this)  as well as check the surface contact all the way around after milling with a piece of paper or thin shim stock or even a laser guide if the kiln is level.

when kilns heat up they can deform and gaps widen. Fortunately at 2000 degrees the atmosphere is so thin exfiltration is rarely an issue even if you pull a port plug. The infrared energy is still a problem, so much so that in our larger L&L rectangular pretty much all the joints glow at top temperature. The projection or line of site  of  one of these joints happened to be across the power plug which eventually overheated the molded plug and I had to replace and shield it from this line of sight.

Edited by Bill Kielb

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Bricks expand as they heat. The hot face expands more than the cold face, so everything distorts. On a lid, the center expands more than the edges, and it ends up dishing downward in the middle, which has the effect of causing the edges to go up a bit. Think of the shape of a very shallow contact lens. On my large rectangular top-loader, which has an opening of about 30x44, the middle would dish down almost a full inch.  So unless whatever you put around the edges will expand as the lid moves, it won't do any good. If you put fiber there when you load the kiln, it will probably just crush and not expand to fill the gaps when they happen. Plus fiber is nasty stuff and shouldn't be handled that much. Kilns move, gaps happen. It's completely normal and completely unavoidable.

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Thank you all for your help.  I also sent the question to Skutt technical support and the technician agrees with all of you - fiber is a bad idea and the lid does go concave up when it gets hot - explaining what I had thought was just a loose fit. 

Now I will know a little more what to expect.  On to trying to mix a glaze!

 

 

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13 hours ago, neilestrick said:

Bricks expand as they heat. The hot face expands more than the cold face, so everything distorts. On a lid, the center expands more than the edges, and it ends up dishing downward in the middle, which has the effect of causing the edges to go up a bit. Think of the shape of a very shallow contact lens. On my large rectangular top-loader, which has an opening of about 30x44, the middle would dish down almost a full inch.  

All the more reason, to *not* dry wares on a firing kiln lid...

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