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AndieF

First time firing my kiln - overfired?

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Hello all, I'm new to the forum and to making and firing pottery!

Yesterday I fired my kiln (MKO40 40 litres, electric, it's a German kiln) for the second time. The first firing was to bisque a pot and it went well, I was aiming for cone 04 and it bent almost perfectly. Yesterday, I did my first glaze firing. I was aiming for cone 6 and set the temperature to 1240°C with a 6 minute hold. I monitored the temperature regularly for the last 100°C leading up to 1240°C and it seems the rate of rise in temperature was 104°C/hr. Looking at Orton's chart I see that for a rise rate of 60°C/Hr a Cone 6 is 1220°C, and for a rate rise of 150°C/hr a cone 6 is 1241°C. As my rate of rise is pretty much in the middle of those two, I see I should perhaps have set the temperature to 1230°C. When I opened the kiln this morning I was surprised to see that all the cones in my pack (5,6,7 from left to right. I clumsily snapped off the cone 5's tip) were completely down with a bubbly surface. I'm not sure what temperature was reached but it must have been at least cone 8 which at the rate of rise of my kiln is, according to Orton's chart, about 1258°C. So my kiln is overfiring by at least 20°C. Should I fire my next pots with the temperature set to my target temperature minus 20°C?

I have a glass thermometer that goes up to 400°C, is it worth heating the kiln up to say around 350°C with the glass thermometer positioned very close to the thermocouple(type s) and see if there is an offset? Would that be a reliable test?

The glaze actually turned out really nice, but I am rather concerned that the kiln is a lot hotter than I thought!

Thank you

Andie

 

 

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Edited by AndieF

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You can conntrol this maybe by monitoring your cones by viewing through peep hole towards end of firing and noting temp where they go down..

Placing cone packs throughout kiln will give  bccbtter idea of performance also.

What temp rise did you enter?

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Thank you Babs. The controller is quite primitive, so it's not possible to enter a temp rise.  The temp rate of 104°C/hr was calculated from manually noting the temp every ten minutes leading up to max temp. The kiln doesn't have any peepholes either, perhaps I should drill a hole!

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Think you could have been closer to cone 9 the way the cone 7 is blistered. Any chance you have infinite switches on the kiln as well as the controller? Also wondering what cone your clay is rated to? Are you using one of those wide range claybodies that go from 6 - 10? Reason I'm asking is if your pots came out without any sign of overfiring the clay it makes me wonder if is is a broad range clay. 

51 minutes ago, AndieF said:

I have a glass thermometer that goes up to 400°C, is it worth heating the kiln up to say around 350°C with the glass thermometer positioned very close to the thermocouple(type s) and see if there is an offset? Would that be a reliable test?

Nope, not going hot enough. 

32 minutes ago, AndieF said:

perhaps I should drill a hole!

I would. Can buy or make a plug for it or attach a swinging style spyhole cover made from stoneware or stainless.

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Thank you Min. The clay is rated from 1200°C - 1260°C, as is the glaze I used. At the rate of temperature rise I had, I calculate that  a cone 9 would be about 1268°C, so slightly above the ratings for the clay and glaze. There's absolutely no blistering of the clay and the colour of the Glaze actually looks better then the manufacturer's photo with lots of small variations of colour. Would a lower temperature produce a more muted effect?

If there is an offset, wouldn't that be linear over the whole temperature range? In which case the glass thermometer might work?

Edited by AndieF

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Pottery Firing Time Temperature Chart Google Search Ceramics And Orton ConeThis chart should help you watch prepare for when to be nearing your target cone.

Athe the same time, I would recommend setting the cones in the clay wad so that they will fall perpendicular to the wad or opposite sides so that you can see them easier. I have been firing many years using these techniques and it really helps out. . . . unless of course I oversleep!:o

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Thank you Pres. Maybe I really should drill a hole so that I could observe the colour of the pots and the cones? Would it not damage the kiln if I drill through the stainless steel cladding? How big a hole would I need? Drilling the brick should be easy as they are very soft.

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So you have no holes through walls of kiln? How is it vented?

Won't hurt kiln unless you hit an element:-))))

Look at spyholez in other kilns to get the idea. Don' make too small. Can block  it with a completely dry plug of clay. Some odd ones made by some folk here.

And now you have to read up on how to position cones so you can see them.

Search the forum here.

Fun eh!

Oh protect eyes when peeping into kiln

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Don't forget to shade your eyes when looking into the kiln, you can burn your retinas just like looking at the sun. I don't know the shielding required, I use my free souvenir shades from the eclipse last year. 

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Thank you Babs and yappystudent for the advice, I have several welding masks I could use to look in the kiln. 

Babs, I spoke to the manufacturer of the kiln, and they say that the bricks are 'breathable' and there is no need to vent. 

Edited by AndieF

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20 hours ago, AndieF said:

I spoke to the manufacturer of the kiln, and they say that the bricks are 'breathable' and there is no need to vent. 

Just to clarify, your kiln is made from insulating fire bricks with a stainless jacket? IFB's are porous but not to the point of being able to vent a kiln. I really would drill out a spyhole. If you use just one spyhole place it about 1/2 way up the kiln, since your kiln is quite small one should be enough.  For the stainless use a sharp high speed steel or carbide holesaw, once you start drilling don't stop until you hit the brick. Drill a pilot hole from the inside out to place the spyhole. Spyholes can be reamed out to a taper to fit plugs.

On 7/1/2018 at 11:59 AM, AndieF said:

If there is an offset, wouldn't that be linear over the whole temperature range?

TC's can have drift. The higher the temp the greater the drift.

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Thank you Min for your advice. I'll look into getting a drill suitable for stainless steel - it's quite a hard material? This is the company that makes the kiln, http://www.marienfeld-dortmund.de/brennöfen.htm (Sorry, it's all in German) looking at the pics on their website there don't appear to be any spy-holes installed. What would be the consequences of the kiln not being sufficiently vented?

The thermocouple installed is a platinum-rhodium/platinum type S. They are supposed to be more accurate and suffer less drift than the Type K often installed by other manufacturers. 

Edited by AndieF

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You can vent the kiln by propping the lid up a couple cm's until it gets to around 750C to vent it. It allows a clean burn off of organic materials that are within the clay and glazes. Pinholes, blisters, bloating (with dark clays especially) can be avoided to a great part by having a clean bisque firing. The main advantage of having a spyhole is to allow you to see the cones to calibrate the firing while it is going on. Problem with looking at the cone packs after firing is obviously it's too late to go back and change the firing for the load of pots you just fired. That is great you have a type S thermocouple, yes they are more accurate than the K type. If you can get the firing temp to correlate with the cones and prop the lid you could avoid drilling a hole. I'ld suggest keeping a really good log book of your firings, include time, number of shelves used, approx mass (ie loose, or densely packed kiln), temp, etc. You will likely have to adapt your firing schedule as the elements wear. Yes stainless is a hard material, if you do decide to drill a spyhole once you start drilling don't stop the drill as stainless work hardens.

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It's been a while since I used cones on a regular basis, butI don't recall ever seeing cones bubble like that. This could be due to the lack of venting, or the cones got wet as some point. I woudn't trust their accuracy if they're behaving oddly.

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Thank you Neil, I fired the kiln again yesterday with a reduced target temperature of 1210°C, however the cones indicate a hot cone 6, as cone 7 is already starting to bend. Both cones show the same bubbled texture as my first firing. I'm not sure if the bubbling could be due to insufficient venting because the 04 cone I used for the bisque firing looked perfectly clean without any sign of bubbling, if venting was an issue with the cones, wouldn't the 04 cone show the same signs of bubbling?  Maybe the cones are not to be trusted. 

Min, I'll try leaving the lid propped until 750°C on my next firing just to be sure the kiln is well vented. Ultimately I'd like to drill a hole to be able to see the cones, although the prospect of doing that scares me a little bit! Thanks again!

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Edited by AndieF

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51 minutes ago, AndieF said:

Maybe the cones are not to be trusted. 

I've used cones for 35 years or more, and never seen them behave like that. And that includes firing without any form of venting (other than the natural crack where the lid meets the wall, and around the spyhole plug itself), something I still do today with one of my kilns, to no detriment at all to pots, kiln, or anything else that is discernible.

Orton themselves say that things like keeping cones in a damp environment, although not to be encouraged, doesn't actually affect them at all.

I would think that the appearance of those cones is sufficiently unusual that it would be worth contacting Orton, including the photograph, and asking their advice about what might be happening here. (And of course, once in receipt of an answer, coming back here to share!)

Edited by Sputty

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One of the pots that came out of that last firing described above  (controller set to 1210°C, cone 6 completely bent over and cone 7 half bent)  looked under-fired. There are lots of  pits and holes in the glaze where it looks like it hasn't melted properly, especially in the rim, also the colour doesn't look anything like it's supposed to. This is a glaze rated for between 1200°C -1260°C.

I refired the pot at a higher temperature similar to my first firing, controller set to 1240°C, all cones (5,6,7) went completely down - according to the cones it looks like at least a cone 8 or 9 firing again. Now the glaze looks like it's supposed to, the colour looks like the manufacturer's pic and the glaze surface looks smooth. 

Haven't heard back from Orton yet, but I really don't trust these cones. I notice now that the supplier of the cones has written on their website that in their tests the cones fall at a temp 20 -35°C lower than stated in the charts supplied by Orton. Most curious, but that seems to fit with my experience!  I'd like to get some more cones from a different supplier but haven't found anyone yet. 

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Orton explains that the rate of climb affects the final melting point of the cones - at 108F/hr, ^6 melts at 2194F, at 270F/hr, ^6 =2332F. These numbers are for large cones, not kilnsitter cones. I'm reading this from a small handout card I've had for 50 years.

I think that your Orton cone defect is probably caused by improper storage/handling at some prior point in their lives, possibly moisture. Orton cones are depended upon for their reliability (since 1896, says here) so they've probably been mishandled. 

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Thank you Rae Reich. The temperatures you gave are for cone 5.5 not cone 6. I am using large cones, not kiln sitter cones. I bought the cones recently from a German supplier, so unfortunately I'm not able to say how they may have been handled in the past. I wrote to Orton last week but haven't received a reply from them.  I believe you are most likely correct that the cones are not good anymore because the glaze results just don't match up with what the cones are saying. A cone 6 glaze was not properly melted when the cone showed it to be a hot cone 6 firing. The cones seem to be difficult to obtain here in Germany, but I've seen that another supplier has rings on sale which shrink after firing in a way proportional to the heat work imposed on them. Accurate measuring of the shrinkage of the ring and an accompanying chart enable the temperature reached to be determined.

 

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Sorry I wasn't clear. The manufacturer tells us that the same cone (Large^6) will soften and melt at a lower temperature when you are heating at a lower rate of climb (108F per hour) than at a faster rate of climb (270F per hour). I further muddied my point with a typo. Should be "melts at 2232F at 270F/hr"

so, firing faster will result in a different (Higher) melting point 

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Hi Rae, thank you. Yes, I'm aware of that, if you read my very first post at the top of the page, I discuss that very point, the rate of rise of the temperature of my kiln is pretty much in the middle of the two rates orton give on their chart in my possession, so I'm interpolating between those values to arrive at a target temperature.

The temps you gave are for cone 5.5 on the Orton chart I have. 

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Apologies, AndieF, for missing that part of your original post 

Funny, my old '70's chart was made when very few potters were working at ^6. I've got no ^5.5! Does this range have a greater sensitivity to degree?

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