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Witness cone stopped at 3:30, not 4:00

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Hi. I fired underglazed earthenware bisqueware in my kiln to 04 hotness. Unfortunately, after my kiln had cooled and I looked inside, my 04 witness cone had bent only to a clock-angle of 3:30. It should have bent to a clock-angle of 4:00 at minimum, according to Ortonceramic.com.

 

 

 

It typically takes 15 to 25 minutes for a cone to bend, depending upon the cone number. The cone bends slowly at first but once it reaches the halfway point, it bends quickly. When the cone tip reaches a point level with the base, it is considered properly fired; however, the difference between a cone touching the shelf and a cone at the 4 o'clock position is small and rarely affects the fired results
.

 

 

To make sure my kiln heats to 04, should I start using an 03 pyrometric bar under the kiln sitter's actuating rod, like it mentions below (also from Ortonceramic.com)? Is there a downside to that solution?

 

 

 

The Kiln Sitter® shuts off the kiln when a small cone (or bar) placed under the sensing rod receives enough heat for it to fully bend. Bending is caused by the weight of the sensing rod. Because the cone in the Kiln Sitter® is located at the kiln wall (closer to the heating elements), it frequently receives more heat than witness cones, causing the kiln to shut off early. Using the next hotter cone/bar may be necessary.

 

One more thing: my kiln has no timer, just a kiln sitter, so I can't add extra minutes to a firing. I'm assuming adding extra minutes would solve this sort of problem.

 

Thanks for any help!

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Hi. I fired underglazed earthenware bisqueware in my kiln to 04 hotness. Unfortunately, after my kiln had cooled and I looked inside, my 04 witness cone had bent only to a clock-angle of 3:30. It should have bent to a clock-angle of 4:00 at minimum, according to Ortonceramic.com.

 

 

 

It typically takes 15 to 25 minutes for a cone to bend, depending upon the cone number. The cone bends slowly at first but once it reaches the halfway point, it bends quickly. When the cone tip reaches a point level with the base, it is considered properly fired; however, the difference between a cone touching the shelf and a cone at the 4 o'clock position is small and rarely affects the fired results
.

 

 

To make sure my kiln heats to 04, should I start using an 03 pyrometric bar under the kiln sitter's actuating rod, like it mentions below (also from Ortonceramic.com)? Is there a downside to that solution?

 

 

 

The Kiln Sitter® shuts off the kiln when a small cone (or bar) placed under the sensing rod receives enough heat for it to fully bend. Bending is caused by the weight of the sensing rod. Because the cone in the Kiln Sitter® is located at the kiln wall (closer to the heating elements), it frequently receives more heat than witness cones, causing the kiln to shut off early. Using the next hotter cone/bar may be necessary.

 

One more thing: my kiln has no timer, just a kiln sitter, so I can't add extra minutes to a firing. I'm assuming adding extra minutes would solve this sort of problem.

 

Thanks for any help!

 

 

Most of the literature on the Orton bars and mini cones indicates that often one will have to use a higher temp bar or mini cone to get the witness cone to right temp. Some people also speed up their firing schedule to get it to match the witness. I have even force fired once or twice by holding up the Kick off, until my witness cone went to desired temp. The easiest by far is to use the higher temp cone.

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I always use one cone hotter when I use the small witness cones in a sitter. While it might not matter that much in a bisque firing, it makes a big difference in a high firing. You still need to use wittness cones to double check temps inside the kiln.

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The exact placement of a small cone in the Kiln Sitter mechanism will affect the end point. You have to have the little cone sitting on the two support prongs with the trip bar in exactly the same place to have the exact same firing results from firing to firing. Great care needs to be take in this regard is you are looking for precise results like you are mentioning (very slight heat work difference).

 

That might be a factor here.

 

 

best,

 

....................john

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The exact placement of a small cone in the Kiln Sitter mechanism will affect the end point. You have to have the little cone sitting on the two support prongs with the trip bar in exactly the same place to have the exact same firing results from firing to firing. Great care needs to be take in this regard is you are looking for precise results like you are mentioning (very slight heat work difference).

 

That might be a factor here.

 

 

best,

 

....................john

 

 

Hm. After reading your post, I did some research. I think I placed the pyrometric bar too close to the kiln wall on the two support prongs, where it's extra-hot. I'm going to place the next bar outward a little, level with the edge of the prongs, and re-fire the pots. Hopefully that will get me the correct angle on my next witness cone.

 

If that doesn't work, then I'll try a one-increment-hotter bar for the kiln sitter, like Chris and Pres suggested.

 

Thanks for all the help!

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"Best practice" is to use a three cone pack. For 04 put cones 05 04 03, in that order, left to right, in a wad of clay. That way, when you open the kiln, you will know if you under or over fired the load. If you are unsure of the accuracy of the kiln sitter, put that cone pack in front of a peep hole and watch. (with safety glasses. . .)

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"Best practice" is to use a three cone pack. For 04 put cones 05 04 03, in that order, left to right, in a wad of clay. That way, when you open the kiln, you will know if you under or over fired the load. If you are unsure of the accuracy of the kiln sitter, put that cone pack in front of a peep hole and watch. (with safety glasses. . .)

 

 

Thanks!

 

Why safety glasses? Can looking through a peephole be any more dangerous than looking at a glowing element in an oven? I've heard this a couple times now and don't understand the need for caution.

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One more thing: my kiln has no timer, just a kiln sitter, so I can't add extra minutes to a firing. I'm assuming adding extra minutes would solve this sort of problem.

 

 

 

The timer has nothing to do with how long a firing takes. It is simply a countdown and shutoff device. Setting it at 8 hours will not make the firing take 8 hours. It will shut off the kiln at 8 hours whether the firing is done or not. The timer should be set to 15-30 minutes longer than the firing is expected to last.

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Thanks for the extra information. I have so much to learn.

 

This forum is such an excellent resource for inexperienced kiln-users like me because of people like you and all the other posting potters.

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If you check the Orton cone chart you will see that same cone numbers have different measuring temperatures. There is a 63 degrees difference between Self-Supporting Cone 04 and Junior Cone 04. Your Self Supporting cone should not have bent over to the 4 o'clock position.

Here is a link showing the temperature cone chart. http://www.glazemixe...one%20Chart.pdf Try firing by temperature rather than cone number. You can use cone numbers as a reference for purchases of pyrometric cones.

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If you check the Orton cone chart you will see that same cone numbers have different measuring temperatures. There is a 63 degrees difference between Self-Supporting Cone 04 and Junior Cone 04. Your Self Supporting cone should not have bent over to the 4 o'clock position.

Here is a link showing the temperature cone chart. http://www.glazemixe...one%20Chart.pdf Try firing by temperature rather than cone number. You can use cone numbers as a reference for purchases of pyrometric cones.

 

 

My self-supporting cone should not have bent over to the 4 o'clock position? I don't understand. :(

 

To summarize and add some extra information: My self-supporting cone is 2 1/4" tall and is for temperature 04. My pyrometric bar for the kiln sitter is 04. My box of clay reads "RED CLAY W/ GROG C/06-4." I wanted the kiln to get to 04 hotness because I used 04 underglazes. The witness cone bent to 3:30, indicating that the kiln didn't get hot enough.

 

Also, as far as I know, I can't fire by temperature because my kiln has only a kiln sitter.

 

Thanks for posting. I didn't know there were different sized witness cones with the same numbers that do not coordinate their temperatures (which doesn't make sense to me).

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Here are few questions- What is the maximum temperature that your kiln can reach?

What 'temperature' do you want to fire to?

When you set up your kiln did you do an empty kiln test fire, using witness cones on the 'ware empty' shelves to see how your kiln fires?

Does your kiln manual tell you about test firing?

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Not to be grumpy, because this is important stuff, but it seems we're getting really picky about the cones. I agree that we should all understand how they work, and should be able to adjust our firings to achieve some level of accuracy and consistency. But the difference between 3:30 and 4:00 is negligible, IMHO. My personal attitude towards cones is:

 

1. With gas kilns, we generally accept 1/2 cone difference within the firings. That's acceptable to me in an electric, too.

 

2. Being off so little is not at all important in a bisque firing.

 

3. If your glazes look good, then it doesn't matter in a glaze firing, either.

 

4. If you have a well formulated glaze, it will have a broad enough firing range to deal with a 1/2 cone difference.

 

Once or twice a year I get a kiln repair customers who are using the KISS computer software to track the firings. They are always freaked out that one thermocouple is behind the others, or the peak temperature wasn't exactly what the controller said it would be, or the rate of climb was 10 degrees slower than the program, etc. They are always concerned about the lack of accuracy, and my response is always the same: "How do the pots look?" And their response is always the same: "Fine, but....."

 

Kilns are not 100% accurate. Some degree of variation will always be there. If the pots look good, then everything is working fine. You can drive yourself nuts trying to make it perfect. If I told my customers that 5 witness cones placed throughout their kilns should all be bent to 4:30, they would all think their kilns are broken.

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