Firing Tall Pot

[KarenB]

Hi, I Bisqued this pot to 04 slowly. As you can see in the picture it goes almost full height of my kiln. I had half shelves stacked with ware from bottom to top next to it. The cone at the top of the kiln bent to touch the shelf, the cone at the bottom of the kiln bent to 3 o’clock.  

I am concerned that when I go to Cone 6, there will be too much stress at that temperature difference and this will cause a crack. I thought maybe I should do a hold somewhere in the firing to give it a chance to even out. The top will be off when I fire.  It is on a thin bed of grog because I’m paranoid about the flat bottom cracking.

It is glazed with a clear shiny inside and a clear mat outside over the underglazes.  It is hand built from 2 slabs, then shaped on wheel.  What do you suggest? Thanks so much..

[liambesaw]

Just fire slowly

[KarenB]

Okay, thanks Liambesaw.

[liambesaw]

3 minutes ago, KarenB said:

Okay, thanks Liambesaw.

Do you have a 3 section kiln with zone control? If so you could adjust the TC offset on the bottom two or top one to try to fire more evenly, but I dont think there's a lot you can do besides that... Other than fire slowly.

[KarenB]

Hmm, I’ll have to check the manual. It’s a pretty new L&L with a fancy new software and a lame manual. I didn’t even think of that..  but that would be ideal.

I’m back to pottery after a couple years of inability to work. It seems I’ve forgotten soo much. 

Could you answer another quick question? I am about to mix some test glazes in small amounts. I don’t remember if the powder goes in the water, or the water goes in the powder? I can’t seem to find any statement of one or the other.  Thanks!

[liambesaw]

Always add powder to water when mixing.  You can start with less water than you'll need and adjust after it's wetted.

[KarenB]

37 minutes ago, liambesaw said:

Always add powder to water when mixing.  You can start with less water than you'll need and adjust after it's wetted.

Thanks again.

[neilestrick]

Sounds like you need to calibrate the zones of your kiln, but half a cone isn't that bad. You don't need to worry about the piece cracking from the difference in temperature between the top and bottom. It's a gradual effect over the height of the pot, so it can handle it. It's not like there's a dividing line and above that line is one temp and below it is another and where they meet it it'll crack. I've seen really tall pots with a 2 cone difference between the top and bottom.

[KarenB]

Oooh, will do. I’ll look in the manual for that process. Thanks much Neil. 
Karen

[Chilly]

Fire it all alone.  No other shelves or pots.  I did the same with the large pots I fired this summer.  I'm sure that way, the temperature will be more even.

[KarenB]

On 11/4/2020 at 6:40 PM, neilestrick said:

Sounds like you need to calibrate the zones of your kiln, but half a cone isn't that bad. You don't need to worry about the piece cracking from the difference in temperature between the top and bottom. It's a gradual effect over the height of the pot, so it can handle it. It's not like there's a dividing line and above that line is one temp and below it is another and where they meet it it'll crack. I've seen really tall pots with a 2 cone difference between the top and bottom.

Neil,

The TC1 temp is 63F,  

TC2 temp 60F,

TC3 temp 53F.  

Should I do a cone offset? 
Would it be best to adjust TC1 - 5 degrees  and TC3 +5 degrees?

Is that the correct logic?

[neilestrick]

40 minutes ago, KarenB said:

Neil,

The TC1 temp is 63F,  

TC2 temp 60F,

TC3 temp 53F.  

Should I do a cone offset? 
Would it be best to adjust TC1 - 5 degrees  and TC3 +5 degrees?

Is that the correct logic?

How long has the kiln been sitting at room temp? It can hold a little residual warmth for a day. If it's been sitting for a while and it's not reading the same, then first check that the thermocouples are all touching the ends of their protection tubes, and check their condition. If they're pretty warn, I would replace them and do a firing to see how they do before doing any adjustments. Also check what the current thermocouple offset is. It should be 18 degrees.The issue isn't the 3-5 degrees you're seeing at room temp, though, it's the half cone you're seeing at the high end. If all else is good, you need to lower the offset in the section that is firing too cold, and increase the offset in the section that is firing too hot. See HERE. I'd start with 5-10 degrees on the top and bottom and see how that does.

[Bill Kielb]

The difference between a tipped cone and three o’clock cone is really small.

[KarenB]

20 hours ago, neilestrick said:

How long has the kiln been sitting at room temp? It can hold a little residual warmth for a day. If it's been sitting for a while and it's not reading the same, then first check that the thermocouples are all touching the ends of their protection tubes, and check their condition. If they're pretty warn, I would replace them and do a firing to see how they do before doing any adjustments. Also check what the current thermocouple offset is. It should be 18 degrees.The issue isn't the 3-5 degrees you're seeing at room temp, though, it's the half cone you're seeing at the high end. If all else is good, you need to lower the offset in the section that is firing too cold, and increase the offset in the section that is firing too hot. See HERE. I'd start with 5-10 degrees on the top and bottom and see how that does.

Several weeks at room temp.

It’s always had 3 different numbers, I didn’t realize it wasn’t normal.  Thermocouples are good!  Thanks!

[KarenB]

19 hours ago, Bill Kielb said:

The difference between a tipped cone and three o’clock cone is really small.

I guess, but with this tall pot, I’d rather balance the temp. Thanks!

[Bill Kielb]

48 minutes ago, KarenB said:

I guess, but with this tall pot, I’d rather balance the temp. Thanks!

No worries. Just an FYI, cones that are bent midway are usually only a few degrees from full tip. I believe the chart below says 1-2 degrees actually.  So you may have difficulty making this more even.

For large and or heavy walled items firing slow is excellent advice. 400 degrees per hour generally considered medium  speed which means 150 - 200 degrees per hour would be considered  relatively slow. If it were not to survive that then alternatives to how it’s made likely should be explored.