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Kiln reaching correct temp on sitter but underfiring in the kiln


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When doing a glaze fire with 06 Cone , the sitter shows that it reached temperature but the glaze is under fired.

With 04 cone bisque fire, everything seems fine.

I adjusted the sitter as per manual and replaced the rod as well.

Kiln was working fine until it overfired by a few hours one time.  Since then, it keeps shutting off earlier than it should even though the cone in the cone sitter looks like it fired correctly.   I'm not sure what else to adjust, would really appreciate anyone who might know what could cause this.

Thanks,

Paulo

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Thanks for your quick reply! Used a piece of duck tape to hold up the release trigger and glaze came out looking perfect so seems it is the sitter and not the kiln.

Use to fire for about 6 hrs but shuts off at only 3 hrs now.  Cone in sitter looks a bit over, so i dont think there is any small adjustment on the trigger that I can do to increase the firing time.

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1 hour ago, PauloG said:

Use to fire for about 6 hrs but shuts off at only 3 hrs now.  Cone in sitter looks a bit over, so i dont think there is any small adjustment on the trigger that I can do to increase the firing time.

You were doing six hour glaze firings and things were even and now you are doing three hour glaze cycles? Maybe slow down back to six hours, three hours is very difficult to get the whole kiln heated evenly. The tiny little low mass cone will likely heat just fine at that speed  though and shut off when it gets enough heatwork.

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4 hours ago, PauloG said:

Screenshot_20201107-085548_Gallery.jpg

The cone is getting hot enough to bend!  When the cone bends this much the sitter does what it is suppose to, stop the kiln.  This part is working OK!  
The question is why does the cone get hot enough but the ware does not get hot.  
 

check the coils with the kiln empty and the lid open and watch what the coils look like as you raise the temperature control device.   My guess is the coils near the cone in the sitter is working OK, but the coils elsewhere are not.  

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5 hours ago, PauloG said:

So you are suggesting to heat it up slower, maybe one hour at each level if I understood your comment correctly?

Yep! The more filled your kiln is the slower you should go so everything heats evenly. 3 hours is awfully fast. Try six, it worked before. If I understand correctly you went from a six hour glaze firing to three. So go back to how you fired before and see if you have success. In other words three hour glaze fire would be too fast.

Edited by Bill Kielb
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Well PauloG here is my 2cents. I dont know if you followed my "Oh Neil!!!" post but I was having the same issue as you are I am firing to ^5 but from my read of your posts I had the same issue and was getting pretty much the same advise. Following their input and reading their response to you I decided to just stick a ^6 sitter and let it rip. I am using Amaco P C choice glazes so what could be the harm, none of my other 2 firings had reached cone 5.

 But someones response; to you I believe, that had referenced "balancing the kiln" kept ringing in my head. To me that meant the same temp. throughout the kiln so when I got to 1800 I turned down the upper chamber to med. and left the bottom on high. It slowed my rate of climb but the temp in the two chambers evened out and when I reached 2100 I turned both chambers to high to finish. I did go 10 hrs. but I am most certainly a novice so I will work on my timing.

 below are my cones from left to right 4 5 6 7. top was middle shelf then bottom witness cones. and then my ^6 sitter I kind of over fired but not by much!

Hope this helps. 

20201109_191713 (3).jpg

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36 minutes ago, postalpotter said:

I did go 10 hrs. but I am most certainly a novice so I will work on my timing.

 

Typical glaze firings for average home pottery kiln would be 7-9 hours (Ten hours is fine) with speeds that max out around 400 degrees per hour. Nice work!, slower definitely evens this out. I like your technique! You are becoming master of your kiln.

Edited by Bill Kielb
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I also had replaced the rod and followed all of the adjustment instructions from the manual.  I did notice the old rod was visibly bent, but seems it was somehow working better with the bent rod than the new one (I.e. firing for longer). Also, the sitter cones used to look way under fired even though the glaze was reaching proper temp and kiln was shutting off after about 5-6 hrs.  Now the sitter cones look right, but kiln shutting off after just 3.5 hours and everything under fired.  Any other parts in the tube that could be the cause of the problem?  I feel like it is such a mystery,  no idea what else to try/ replace.  Tried finding a repair tech but no luck, even  considering maybe need to get a new kiln but my model is 30A and all the new ones of similar size seem to need 50-60A which I don't have the wiring to support.

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On 11/7/2020 at 7:56 AM, PauloG said:

1 level every 1/2 hour up to 5.  Then used to go for about 3 hours after max  now shuts off after about 1/2 hour and everything coming out under fired.

Did you fire it the same as this?  If you haven't tried it already then I'ld slow down how quickly you turn the dials up. Do you have low/med/high settings on the dial or numbers? If it's the former then I'ld leave it on low for 1 hour, then medium for 4 hours then high until done. If it's the latter I'ld leave it at least an hour on each of the middle range settings, wouldn't turn it to the highest setting until the kiln has been on for at least 6 hours. Just to clarify, you are firing cone 06 glazes and this is the cone recommended by the glaze directions? Have you tried putting a glazed test piece in with your cone 04 bisque (on a waster cookie) to see how a hotter firing works?

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