Jump to content
Sign in to follow this  
AndreaB

Help - My Kiln Shelves Melted!

Recommended Posts

Some advice please......

 

My thermocouple blew on my last glaze, so we had it replaced. I ran a glaze yesterday and the results are disasterous.

 

My kiln does not have a programmer attached so I watch and set temperature and the other thingy manually.  I fired to 1200oC watching the temp gauge and manually stopped at 1200oc with a 30min soak. Shut down the kiln, temp guage 0, the other thingy (heat ramp) to 0, switched the power off at the electrical connection (3 phase) and then went to bed.

 

Went out at 5am to see the temp. Switched on at the plug and the temp indicated 500oC. I could not see the cones and assumed that the kiln had overfired a bit. 

 

When the kiln came down to 200oC I opened the peep hole and saw two pots welded and one off kilter. Never quessing that everything had melted.  3 out the 4 shelves have cracked into pieces. The top shelve has slumped down. The thermocouple is about 30cm higher than the ware on the top shelf.  I haven't been able to remove anything yet but it seems as if the elements on the side are ok, I don't know about the element on the bottom

 

Can anyone help me with the checking of the temp and how should I conduct the next fire.

 

:(  :(  :(  :(

 

Andrea

Share this post


Link to post
Share on other sites

To be clear, you cut off ALL the power to the kiln after what you thought was a firing to 1200 C with a thirty minutes soak.

 

What did you use to monitor the temp?

Did the load take a lot longer to fire than usual?

 

It would seem it over fired to a considerable degree ... The only way to know what it fired to would be witness cones inside.

 

I am not the kiln genius, but I would not fire again until you nailed down the problem ... Best wishes,

Share this post


Link to post
Share on other sites
Guest JBaymore

Have you fired THOSE exact shelves to 1200 C before?

 

When you replaced the thermocouple...... are you certain it was the correct type?  Analog meter or digital meter?

 

best,

 

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

Share this post


Link to post
Share on other sites

Were the posts directly one over the other for all shelves? Also, what do the bricks look like? Any change from what they looked like before this firing? Type K or S thermocouple previously and the new one also same type?

 

Sorry for the firing disaster, must have been horrible opening the kiln and seeing that.

 

edit: Also, the wire going from the thermocouple to the pyrometer, is it the same or did that get changed?

Share this post


Link to post
Share on other sites

What a disaster.  I'm very sorry.  I hope the bottom element is okay.

 

I was a little confused by your description.  It sounded as if you were firing by temperature rather than by using cones.  A thermocouple is good to have, but it doesn't tell you an important part of the information you need.  When all my kilns were manual, I had a thermocouple, but its only purpose was to let me know the kiln was going up on schedule, and to tell me when certain critical temps had passed (and it would be safe to turn the switches.)

 

I may have misunderstood you.  But, using cones to determine the endpoint of the firing is the only way to get repeatable results in a manual kiln.  Some folks will use a kiln sitter for this purpose, but I think it unwise to rely completely on sitters. I fire to 8, and I would put a Cone 10 bar in my sitter to avoid disastrous overfiring, and go strictly by the cones to determine when the firing had reached Cone 8.

Share this post


Link to post
Share on other sites

I've fired the kiln up till now using the temperature gauge. The kiln was at least 10 years old when I got it last year. I fire using the guage and do a manual shut off at 1200, switching off the temp and rheostat , setting both to zero. I also turn the 3 phase connector off and flip the trip switch to off.

 

Two firings ago the kiln was switched off using the same method as always. When I went in the next morning and switched on the temp gauge flipped over to the right past the 1300o mark, so I knew something was up. The load looked Ok but my hubby checked the thermocouple and found that it had blown. We called in a chap who has built kilns before and is the only person we know of in Jhb to have a look. He did the same check and agreed with hubby. He replaced it with one of the same resistance. Model K I believe

 

The first firing after that did not seem to mature and the cones were not bent in any way.

 

So this firing I used cones again and monitored manually. Increasing by 150o at a time until 1200 where I held for a 30 minute soak. After that I switched all off as usual. I saw that the cones had melted but didn't realise how badly.

 

Iv'e used the same shelves which have been fired to 1260o with no problem.

 

Unfortunately hubby had unloaded before I could take pics.

 

The bricks look quite friable and have a brownish stain. The ware was placed evenly apart and the posts were two at the back at the corners and one in the middle in the front.

The wiring attaching the thermocouple to the pyrometer was not checked as far as I know.

 

 

Share this post


Link to post
Share on other sites
Guest JBaymore

 I also turn the 3 phase connector off and flip the trip switch to off.

 

For the kiln to continue firing upward after you shut the MAINS supply to the kiln off, should be totally impossible if that installation is done correctly.  No mains power.... no flow of electricity.  No flow of electricity... no heat generated from element resistance.

 

Maybe I'm not understanding this correctly.

 

A type K thermocouple is starting to go pretty non-linear for its electrical output at 1200 C....and the thermocouple itself (not counting meter / system inaccuracy) is +/- about 8 C (17F)  at that top end anyway.  Also, if that is an analog meter (dial face type) it is possible that the meter/thermocouple system needs calibration with the new thermocouple....and that 1200 C reading is actually indicating low.  So you are actually over 1200 C..... and then add the soak factor as to heatwork.

 

And that is maybe getting near the use rating of those shelves.  (Note that heatwork on kiln refractories is also cumulative... so repeated firing takes its toll on kiln bricks, posts, and shelves.  Maybe it is "coincidence" that this finally caught up with the shelves in this firing.

 

best,

 

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

Share this post


Link to post
Share on other sites

"Model K I believe"

This is something you absolutely need to be 100% sure about. Some controllers have modes for different types of thermocouples.

Replacing a thermocouple is a very very easy place to introduce problems. They cannot go on backwards and must be made of the same lead wire. Each one could have the same resistance (an insignificant electrical aspect in thermocouples) but generate a different milli-voltage.

 

The temperature range of K is on the edge of what kilns fire to. Type R and S are what I would expect to see, especially if your kiln was rated to ∆10.

 

--

 

Test it with an ice bath and boiling water.

Share this post


Link to post
Share on other sites

If you did swap out a type R/S for a type K it would be reading 1300 at 400c so I don't think that has been the problem but bogus reading could be a factor.

 

Was the firing particularly longer than normal? Never seen a shelf do that before and I had a kiln fuse on for 12 hours extra at about 1150 to 1200.

 

Sorry to have seen this happen, I hope you can recover and the kiln has not been taken out.

Share this post


Link to post
Share on other sites

If the people who are saying no need for cones read this post they will get it! But you do have to check them on the way up!

What a blow Andrea.

Was the time taken for the firing a very similar length to your usual firing?

What do you mean thermocouple blew? Sheath broken? I fired for years with a broken sheath...not advising just saying.

Hope you get back on track soon.

Share this post


Link to post
Share on other sites

Andrea- very disappointing; sorry to see you lose a load.

 

Some observations: those look like corderite kiln shelves?? If so, they are rated to cone 6 (2230F) They can take some abuse, of over firing, but will eventually collapse. I know from previous discussions you are using stoneware clay: which can easily handle cone 10: and some can go as high as cone 12. My point being is that I do not see much distortion to the clay itself. The fluidity of the glaze is also an indicator: some run off is present, but not a total disaster. All things considered: I would say you hit somewhere in the cone 12 range. 1200C with a 30 min hold in that size kiln would easily jump another cone in heat work: would guess you hit the 1300C +/- range looking at the evidence. The shelves slumping looks more ominous than it really is: if indeed they are corderite.

My big problem is a technician putting a type K thermocouple in a "very" high firing kiln- big big mistake in my opinion. Type K are reliable up to cone 6, after that they can drift and misread. Secondly, pending on the controller: there are toggle switches to program the controller to the type of thermocouple used. .Not sure that is applicable in this case, not overly familiar with your controller. Anytime you replace a thermocouple, the kiln needs to be calibrated for accuracy by using witness cones before loading. Sorry for the mess, but not sure it is a disaster other than losing a full load.

Nerd

Share this post


Link to post
Share on other sites
Guest JBaymore

My big problem is a technician putting a type K thermocouple in a "very" high firing kiln- big big mistake in my opinion. Type K are reliable up to cone 6, after that they can drift and misread.

 

Nerd,

 

Type Ks are what most potters firing to cone 9-10 stoneware range are (unfortunately) using.  Been that way since god was a kid.  I've been on a campaign for years now to try get people to understand the limitations this imposes on control, at the minimum. 

 

ASTM use rating for K goes to 6 (if you fire slowly).  Just.

 

The issue is that people want "cheap".  Don't want to plunk down for platinum ;).  

 

And yes.. the shelves look like cordierite or mullite or something like that.  A clay marl type shelf. 

 

Sorry about the mess Andrea, but as has been said... it could have been far worse. 

 

I am still curious to find out how the kiln climbed after the electricity was supposedly shut off (in the previous firing).   Have you looked into that?

 

best,

 

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

Share this post


Link to post
Share on other sites

John:

I think "cost" needs to be revisited and calculated. A K type cost $50 and a type S cost $250.00. I just replaced my type S in my test kiln after nearly 1000 firings. How many times would a type K been replaced in that same period? How much did Andrea loss in one firing? How many under-firings or over-firings have occurred due to inaccurate readings from a Type K that is burning out due to firing past its intended limits repeatedly? I have the same feelings about APM elements vs Kathanal A-1 elements. My 6.5Cf crystalline kiln has over 60 cone 10 firings, over 100 cone 6 firings, and roughly 20 bisque firings and the elements are not even beginning to lay over. In my test kiln, I average around 400 firings at cone 6 per set of APM's. So are they really more expensive, or just higher up front costs?

Nerd

Share this post


Link to post
Share on other sites

John:

I think "cost" needs to be revisited and calculated. A K type cost $50 and a type S cost $250.00. I just replaced my type S in my test kiln after nearly 1000 firings. How many times would a type K been replaced in that same period? How much did Andrea loss in one firing? How many under-firings or over-firings have occurred due to inaccurate readings from a Type K that is burning out due to firing past its intended limits repeatedly? I have the same feelings about APM elements vs Kathanal A-1 elements. My 6.5Cf crystalline kiln has over 60 cone 10 firings, over 100 cone 6 firings, and roughly 20 bisque firings and the elements are not even beginning to lay over. In my test kiln, I average around 400 firings at cone 6 per set of APM's. So are they really more expensive, or just higher up front costs?

Nerd

 

Type K only cost $16, not $50. I can get a couple hundred firings from my thermocouples, so that's $80 for 1000 firings. Even if I only got 100, that's $160. And with three thermocouples, that's $480 compared to $750. That's almost the price of an entire set of elements. Yes, type S are more accurate, but type K are accurate enough for what I do, and what most people do at cone 6 and lower. If I was going to fire to cone 10 regularly, then yes I would invest in type S and probably APMs. If you do regular kiln maintenance and inspect your thermocouples regularly, then you can avoid the problems that occur as they wear out. Personally, I've never lost a load of pots due to thermocouple issues.

 

Yes, type S last longer. Yes, APM last longer. Yes, solid state relays last longer. But that's not the issue. If all these things were standard on kilns it would drive up the cost to the point that we wouldn't sell very many kilns, and the ceramics community would shrink considerably. It's hard enough for folks to afford a $3000 kiln. Add all the upgrades and the cost goes up 40% or more. APM elements are also very sensitive to debris. A shard or glaze glob on one can easily cause one to fry out. That level of sensitivity is not good for school situations, or even most hobbyists. Fry out on element prematurely and you've wiped out much of the savings. The equipment that we use now is a balance of cost and durability, and for most people it is an acceptable balance. If you want or need the very best, it's available.

Share this post


Link to post
Share on other sites
Guest JBaymore

So are they really more expensive, or just higher up front costs?

 

You are preaching to the choir here.  ;)   For most, it is all about the upfront costs. Your analysis of the real costs is darn accurate.   Maybe add in to that the cost considerations of inaccuracy causing other what might be considered "normal" firings to have work that becomes unsaleable or are seconds.  Or at the least, "less than desired". 

 

Some types of firings don't require all that much temperature accuracy... just cone end points.  Heck.... folks potters fired by eye or draw trials.  Other types of firings do.  Macro crystalline is one of the more "fussy" areas for sure.

 

For comparisons for some folks reading this who may not be familiar with this general info........

 

http://www.omega.com/techref/colorcodes.html

 

Note the "useful" range for the thermocouples and also the error factors in the chart.

 

It is not that everyone MUST have such accuracy.  It is just that it is important to understand that the fancy looking "high tech" digital readout attached to a kiln is only as accurate as it is designed to be.  The fancy looks can easily lead you astray into assumed precision.  Plus or minus 0.75% (Type K) at 2377 F (a fast cone 10) is plus or minus almost 18 degrees F.  So when the meter says  2377.... it could be anywhere from  2359 to 2395.  2359 is closer to cone 9 and 2395 is about cone 11.  So you have almost a three cone spread there.  And that is not taking into account the +/- accuracy of the meter itself, nor the impacts of the deterioration of the (typical) bare wire thermocouple over multiple firings.

 

best,

 

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

Share this post


Link to post
Share on other sites

 

Are there similar shelves that are rated to cone10? My shelves don't look any different and go to cone10

Joel:

Not sure I have an absolute answer to your question. I just remember reading an article about corderite shelving a couple of years back. In that article, and a few suppliers rate corderite to cone 6 use. That said, I have also seen some ads for corderite shelves up to cone 10. I know part of the rating is the thickness of the shelves: not sure above that. I do know I have to flip my corderite shelves every few firings because they begin to slightly cup. Yet again, I have thicker versions of corderite shelves I have fired up to cone 10 repeatedly back in the day. In the process of making my own custom tile setters, so my collection of shelves will be collecting dust soon enough.

 

Neil:

The $50 for type K was the retail price from Paragons website with block and shield, so I have to assume the $16 is just the thermocouple. I think up to cone 6 a type K would work just fine: but after that I would not recommend their use for cone 10 production situations.There is also the added cost of having them installed if you do not know how. I have wore out sets of APM elements, but I have never burned one out: accidents happen I suppose. The pottery biz is no different than any other biz: supply and demand dictate. I do think kiln sellers have a duty to recommend options according to use.If I were selling kilns and knew my customer was going to fire cone 10 primarily: I would certainly recommend that option.

Nerd

Share this post


Link to post
Share on other sites

Update;

Just finished a test glaze and found that the temperature gauge is out by 150oC. The cones were just like the orton pic.

 

So if I fired to 1200 then the kiln went to 1350oC. The technician replaced the thermocouple which had fused together and burnt out.

 

So lesson learnt - cones every time and watch when kiln gets near to cut off temp

Share this post


Link to post
Share on other sites

Andrea, were you firing to C04? on the test fire? C04 would touch its toes around 1060deg.C if you fired at 150degC/hr. It may even be different with an almost empty kiln.

AND don't carry the error at that temp thro to the final temp as the error may become different towards the top end of your glaze firing. Not expressing myself well, one of the scientific brains will explain this much better.

Share this post


Link to post
Share on other sites
Guest JBaymore

Update;

Just finished a test glaze and found that the temperature gauge is out by 150oC. The cones were just like the orton pic.

 

Is this an analog meter or a digital meter?  And....... how do you now know it is out by 150 C?

 

If it is out by 150 C (huge error factor)... then I am guessing it is an analog meter.  The ones that potters typically have/use are notoriously inaccurate.

 

best,

 

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

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.