Bad bottom

[ronfire]

Why is it that if I place a large bowl or platter in the bottom of the kiln they usually break on a bisque fire, i don,t dare on a glaze.The last time I thought I would fool the kiln and placed a 1/2" stilt and a small shelf on top of the bottom shelf to help insulate more from the bottom and still breaks, if I have the piece anywhere else in the kiln even close to the top it is fine.

The Gremlin must live under the bottom shelf but I never see him when I clean the kiln.

 

 

[Mark C.]

Put the large pieces in middle or nerar top of kiln.

I put smalls in bottom for bisquing

[neilestrick]

Refresh my memory, is the kiln manual or digital?

[ronfire]

It is sitter kiln that I added a Bartlett controller with solid state relays. You would think that it should not matter then where you load the large pieces but it does. I seldom load a large piece on the bottom but with the stuff I had there was not a choice. 

[Bill Kielb]

Is the bottom shelf very rough? Very strange. Ought to be a physical reason though.

[ronfire]

I even coated the kiln washed shelf with alumina,and added a small shelf 1/2 above the bottom shelf... definitely Gremlins

 

[Sorcery]

I guess a draft.

Is there a bottom vent?

 

Sorce

[ronfire]

Yes the kiln has a vent, that was why I added an extra shelf to stop a rapid cooling. I had the kiln on a slow cool of 200f per hour to 1000 then shut down including the fan. The kiln then still takes 18-24 hrs to cool enough, the 3" bricks sure hold the heat in.

 

[neilestrick]

@ronfire is the controller set up for multiple zones or just one thermocouple?

[Babs]

My thoughts, same prob, is tjat as my thermocouple is towards the  top when the rate of rise i ramped up to full, the bottom, lagging in temp by design, whoops up much faster ...big platters close to row of elements gets a big wallop at a lower temp and cracks because of this.

When does the greatest shrinkage ,temp wise occur?

Apologies for scientific lingo

[Mark C.]

My guess is single zone. 

[oldlady]

ronfire, is the shelf you are using perfectly flat?   if it is not, try a layer of sand or fine grog that you "comb" onto the shelf with one of those tile setting tools.  settle the pot securely in its bed of sand so it is evenly supported by twisting it slightly as you put it down.

[neilestrick]

@ronfire New theory: If your kiln is set up with one zone, the middle zone where the thermocouple is located is the only place that the controller knows the temperature in. If the top and bottom elements are made to run hotter than the rest of the elements, then those areas are going to heat up a lot faster than the other sections at the beginning of the firing. Later on it's necessary due to heat loss out the lid and floor, but at the first half of a firing heat loss is not really an issue. If you've ever turned the elements on full blast, you've seen that the top and bottom elements glow a lot sooner and a lot brighter than the others, so when the controller is cycling the elements on and off to achieve a certain rate of climb based on the temperature in the middle of the kiln, the very top and very bottom are actually heating a lot faster. The top section is probably less of an issue sine the lid seal isn't particularly good in top loading kilns.

One other thing to try, if you haven't already, is to turn off the vent once the firing is done, before it gets below 1200F. I once had a batch of big 20 pound pots that were cracking in cooling, and turning off the vent solved the problem.

Or gremlins.

[ronfire]

The kiln does have a single thermocouple and is at the midpoint of the kiln. All the elements will fire at the same rate as it is set to a single zone. Even if I set the kin for a slow bisque of 16 hours I still have the breakage on the bottom shelf of large items. The shelf is flat and the items placed on the shelf does not rock at all, also placed on alumina to help with drag issues and try to reduce breakage. On the large items firing I have  a slow cool of 200f per hr to 1000f and then have the fan shut off,  it usually takes at least another 12 hours to cool to 200-150f.

I would think that with the controller set to SSR mode that the bottom element would not get that far ahead of the firing schedule. Now I wonder if I should have built the controller for 3 zones instead but being a wall mount it would get a lot more wiring.

Now I get worried and just load a shelf of mugs or bowls on the bottom but at time I will only have large 14"-20" bowls or sinks to fire so I try again and have breakage.

 

 

[Babs]

I was trying to say what Neil said so eloquently.

I guess reality....don't put platters on bottom shelf. 

 

[Mark C.]

Turn off the vent  before it looses red heat as it adding air when you do not want it as neil says-you can also put some mass (small bricks around base as a heat sink) to slow down the cool on bottoms