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Middle Of Kiln Is Overfiring


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#1 missholly

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Posted 21 March 2014 - 07:37 AM

hellooo again....

 

i FINALLY got all of my elements replaced and now the middle of my kiln is overfiring.

just enough to crack the bottoms of things. is there a way to adjust anything? something? somewhere?

 

i tested a ^06 firing empty and the cones bent perfectly.

now im firing to ^3 and things are coming out slightly warped and the bottoms are cracking.

ive been packing the kiln as full as i can.

 

i havent tested the ^3 firing with the free standing cones yet to see how off it is.

 

any thoughts?


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#2 Tristan TDH

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Posted 21 March 2014 - 07:43 AM

How do you know it's over firing if you haven't checked with cones?
I would start by testing with cones, top, middle, and bottom. And go from there.
If you have an electronic controller, you can adjust the offsets to compensate for hot spots.

#3 Biglou13

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Posted 21 March 2014 - 07:48 AM

Warping and cracking isin't always temp related

Orton cones are your friend
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#4 bciskepottery

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Posted 21 March 2014 - 05:10 PM

Is your kiln manual or digital? One zone (thermocouple) or multiple (3)? I think some elements for single zone kilns are designed so the lower element fires hotter than middle and top, which tend to fire hotter. Could you have put the wrong element in the wrong place?

#5 missholly

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Posted 25 March 2014 - 04:41 PM

it is a gare 1822 with 5 switches.

it didnt matter where each of the 5 elements went, this is the info the manufacturer gave me.

 

i know its over firing because its doing exactly the same thing as when i fired in another kiln (that was overfiring)

 

so i just dropped another $30 on 3 boxes of cones, so once i test fire and see that it is overfiring, then what?

all i found on my own was that you can slide the triangle shaped kiln sitter cone to adjust for temp, but along with everything else, its a guess.


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#6 Mark C.

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Posted 25 March 2014 - 08:09 PM

The kiln setter has more adjustments than just the cone sliding side to side

You may be able to find them on the net

I know of two makes -Dawson and Skutt

Both have small adjustable sliding weight on swing shut off arm-you can adjust this-just get the instructions and follow .There also is a rod set up as well but you will not need this.

I have not messed with one in decades as I only bisque in one on a blue moon but kiln sitters are very adjustable .

Having a set of various cones is a must.

You should use large cones in the spy hole and small ones on the sitter-Thats how to see whats happening-once its tuned up it will be good to go.

Mark


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#7 jpc

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Posted 26 March 2014 - 10:40 AM

I have just posted a thread in the technical section you might be interested in. It is on temp. regulation. The center of the kiln has always been the hottest for me and now trying to really regulate it I have found I have to pay close attention to that section. I have been close to seriously over firing the work there. I believe there is a "scientific" reason for it.  Okay! Best of luck.



#8 missholly

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Posted 02 April 2014 - 01:20 PM

ok, so i test fired with cones 2, 3, 4 and a 3 in the sitter.

1 set on top, middle, bottom.

i put a few pieces of broken pottery around so it wasnt firing empty.

the only cone that bent was the 2 in the middle.

 

its obviously a huge difference in firing a kiln full or empty.

so do i have to sacrifice an entire load of work just to get an accurate test?


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#9 neilestrick

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Posted 02 April 2014 - 05:12 PM

It makes sense that the middle is firing hotter. The top and bottom are losing heat out the floor and lid, the middle is not. This is the problem with single zone kilns. To compensate, when you load the kiln, pack it tighter in the middle so there is more mass there to heat up. Use cones for the next several firings to see what's happening.


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#10 Rockhopper

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Posted 02 April 2014 - 05:30 PM

I'm still working on getting proper electrical hookup for my kiln, so can't speak from personal experience - but I've done a lot of reading in anticipation of troubleshooting such issues when I do get it plugged in ...

 

It seems that, even in an otherwise empty kiln, if your sitter is shutting off with a 3, but you're not even bending all of the 2's, your sitter may not be adjusted correctly. 

 

What did the '3' on your sitter look like when it shut off ?  Below is a pic from a Dawson kiln-sitter manual.  If your sitter cone looks more like the one on the left... it could suggest that the "sensing rod" is either bent, or out of adjustment - causing the sitter to trip prematurely. (A bent rod could also stick - causing it to trip late.)

 

You said it has 5 switches - are they all simple on/off switches, or do they have high/low options?  Doing a 'staggered start' (not turning all on at the same time) - or, if they're adjustable (high/med/low), not setting all to the same level - might help you balance the temps.

 

Attached File  sitter1.jpg   43.93KB   1 downloads

 

Also... any chance you accidentally put a 2 on the sitter for your test, instead of a 3 ?  (Sorry, my tech-support background makes me ask those 'is it plugged in' sort of questions.)



#11 Min

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Posted 03 April 2014 - 02:35 PM

My test kiln only has a sitter, I go one cone higher for the small sitter cone than I want the kiln to fire to, large cones read cooler than small sitter cones I have found. 

 

Cone pack in front of peep(s) every time, on top, bottom and middle shelves, until you figure out how your kiln fires, they are all different. 

 

Without a multiple zone controller it is very common to have hotter and cooler areas in the kiln. Couple things to help even it out. If the kiln has infinite switches you can turn the elements down by 1 setting on the hotter kiln area elements near the top of the temp you are firing to then soak for at least 15 minutes near your top temp, keep an eye on your witness cones.  Or else put a lot of mass in the hot part of the kiln, shelves closer together, keep the area near the top with less mass so it doesn't underfire. 



#12 perkolator

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Posted 03 April 2014 - 03:26 PM

like everyone's already stated, it sounds like you just need to make some adjustments to both your kiln's sitter as well as stacking method.

 

All kiln sitters should have an adjustment available to set when the sensing rod lets go of the weight that shuts it down.  I believe you typically have 2 places to adjust - one is the "claw" on the end of the sensing rod, the other is the "trigger plate" that's attached to the weight.  The proper tool for adjusting the trigger plate's height is one of these firing gauges, which is placed on the cone rests/sensing rod, and mimics the "correct" cone drop that's pictured in the image Rockhopper posted:

firinggauge.jpg

you can look up specific directions for adjusting a kiln sitter, it's really simple.

 

sometimes you will have to re-adjust the setting on the sitter after multiple firings (maybe a dozen?) - this is because your sensing rod and cone supports DO wear down/lose material slightly with each firing.  once you have everything properly set, both your sitter's cone and the cone on the shelf should be fairly close. this adjustment may require a few firings to dial in for your kiln.

good luck!



#13 missholly

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Posted 05 April 2014 - 01:58 PM

nielestrick,

it did the same thing before i changed out the elements. my kiln isnt that big, so not filling the top and bottom to capacity frustrates me.

i was hoping the new elements would change that.

when i do load the middle full and not so full on the top/bottom, its still guesswork. things still come out incorrectly.

 

rockhopper,

hi fellow ohioan. where are you located? im in zanesville, an hour east of columbus.

my 3 cone in the sitter was severely bent. i also replaced the sitter rod mechanism, so thats new. (im not sure what its called)

and the 5 switches dont have a high/low option. just on/off. and i do stagger. bottom switch, wait 1/2 hr, up one switch, wait 1/2 hr.

i tried firing it top/bottom switch, wait 1 hour, 2nd one up/2nd one down, wait 1 hour, then middle. that did nothing.

 

and yes, it was a cone 3 in the sitter. :)

 

perkolator,

i spoke with a tech at evenheat (where i got the elements) he was pretty confused and didnt know what to say for awhile.

he then suggested the trigger plate adjustment, which i did, but wasnt informed of that tool dealio. ill try that.

 

 

i appreciate everyones thoughts. thank you for your time!!


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#14 JBaymore

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Posted 05 April 2014 - 02:14 PM

A lot of variability comes from the WAY the cone is placed in the sitter.

 

The cone is a pyramidal form, that gets progressivenly thicker as one goes from the tip to the base.

 

If the cone is placed in the sitter so that the pressure arm is more toward the thicker part of the cone... that raises the "stopping point"...and if more toward the thinner part... it lowers that point.

 

So for consistency (and matching to witness cones on the shelves) you need to make sure to install that sitter cone very carefully each time you fire.  And the SAME way each time you fire.

 

This is why pyrometric BARS exist.... to eliminate that variable.

http://www.axner.com...etric-bars.aspx

 

From my point of view the old "Sitter" (and even new controllers) was simply there to prevent inadvertant overfiring of the kiln... not to use to substitute for the potter watching witness cones and manually shutting off the kiln at the correct point.  In fact the "sitter cone" was a couple of cones higher than the desired firing cone so that it dod not shut things off early.

 

best,

 

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


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#15 neilestrick

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Posted 05 April 2014 - 02:47 PM

Does your kiln have graded elements, as in the very top and very bottom elements run hotter? If not, maybe Evenheat can make some for you. You shouldn't have to keep the top and bottom sections so empty that you're wasting a lot of space. Just put small and short pots that you can pack really tight in the middle section, put big and tall pots in the bottom, and medium pots in the top. Having short pots with more shelves in the middle will create a lot of mass there. You may also want to keep from turning on the middle elements until the very end of the firing. Let the other 4 switches do most of the work. Maybe just turn the middle on for that last cone or two. This would, of course, require putting visual cones in the firing.

 

If cone 3 in the sitter was not bending cone 3 on the shelf, then you need to calibrate your sitter, or adjust how your cone is placed in the sitter. Also make sure you are putting the kiln into the sitter properly. The flat side of the cone should rest on the two bottom support bars, with the sensing rod resting on the top corner edge of the triangle. A good way to remember is to have the cone number that is stamped into the cone facing to the interior of the kiln.


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#16 Chantay

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Posted 05 April 2014 - 03:22 PM

I feel your pain.  I just unloaded a glaze firing.  The bottom shelf was cone 4 , middle two cone 5, and top cone 6.  I can handle cone 5 and 6, I have glazes that I like at each.  At cone 4 everything is under fired.  My kiln is digital but only has one probe, in the middle.  I'm hoping to stack things differently next firing and see if it will even out.  I don't know what else to do.

 

 


- chantay

#17 neilestrick

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Posted 05 April 2014 - 04:17 PM

There's a tendency to put the tallest pots at the top. Everyone does it because it's easier to load that way. I've always done it, too, but my kiln doesn't care. But your tallest pots should actually be at the bottom. Shelves can account for a large portion of the total mass in the kiln. Small pots also create more mass than large pots. So the fewer shelves and small pots you have in the bottom, the hotter it can can get down there. Also, never load directly on the floor of the kiln. Put a shelf down there on 1/2" or 1" posts (broken kiln shelf shards work well for this). It provides another layer of insulation, and it protects the floor.


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#18 Chantay

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Posted 06 April 2014 - 10:42 AM

 

There's a tendency to put the tallest pots at the top. Everyone does it because it's easier to load that way. I've always done it, too, but my kiln doesn't care. But your tallest pots should actually be at the bottom. Shelves can account for a large portion of the total mass in the kiln. Small pots also create more mass than large pots. So the fewer shelves and small pots you have in the bottom, the hotter it can can get down there. Also, never load directly on the floor of the kiln. Put a shelf down there on 1/2" or 1" posts (broken kiln shelf shards work well for this). It provides another layer of insulation, and it protects the floor.

Niel, was this directed toward me?  Thanks for the suggestion, I will give it a try this week.


- chantay

#19 missholly

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Posted 08 April 2014 - 03:00 PM

thanks so much for the advice, neil.

i will try again!!


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#20 missholly

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Posted 19 April 2014 - 01:45 PM

well, i got the tool today and it places the rod exactly where it was.

this means, it isnt going to fire incorrectly for the 9th time.

 

can i give up now?


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