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tomhumf

Just another electric to gas kiln conversion

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Turnbuckles are ace! Thanks for the suggestion, maybe I'll tighten them up a bit later on. 

Not sure about the chimney height, I guess I'll just give it a go.

I do have some ceramic fibre blanket, but not sure I'll be able to get it in the gap between chimney and kiln. 

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Hmm, I have a few more bricks if needed, will see how this goes first I suppose. I'm aiming for ^8 firings. 

The burner is from a company called Stedmark. I think they manufacture them and sell to pottery suppliers from what I can tell. Their website is rubbish but I rang them and it was quite a bit cheaper compared with pottery shops I know. 

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Stupid question alert! 

I've never used a thermocouple. I'm guessing they are intended to be pushed in all the way so the collar bit is flush... There is some plastic around the wires which I guess will melt if I do this. It's not a totally snug fit as the ceramic bits are slightly oval. 

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I've just done a little test firing. I was mainly wanting to try my burner out but ended up getting the kiln to about 600 degrees, with no pots in. 

Having never done a gas firing before I'm not sure, but it seems the kiln climbs in temp fairly fast - I know it will get a lot slower at higher temperatures. 

I struggled a bit to keep the kiln below 100 celcius, even with the lid propped a few inches. Is this standard practice when getting rid of water? I know my burner is quite oversized for the kiln.  I couldn't get my flame any lower without it blowing out. In the end I just about managed to keep it just below 100 degrees. 

I couldn't feel any heat coming out of the chimney join, even at the end of the firing, so I'm pleased and will leave the ceramic fibre off there for now. 

One thing I'm not sure of is how far my burner should be inside the kiln. The attached image shows where I started it, and I moved it in about an inch when I turned the gas up. 

I did a spreadsheet log of my temp rise but not sure how to post it here. Basically it took about 40 minutes to go from 200c to 600c , with the gas still very low. The flame was still mainly yellow throughout, which I think is normal at lower temperatures? 

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Edited by tomhumf

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Yes, it will climb quickly. Most burners cannot stay under 100C. That's what pilot burners are for. Your burner should not be inside the kiln at all. It should be outside the wall 1.5-2 inches. Otherwise you're going to block off the secondary air being drawn in through the port, as well as burn up your burner tip and flame sensor. With your wall being angled you'll just have to guesstimate the best spot for it.

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Thanks! I'll move the burner further away for sure then. I'm planning to fire raw glazed pots so I suppose I'll have to have the lid propped wide, and maybe look into some kind of pilot attachment...

I plan to try follow what I read Neil suggest in another post about the firing schedule.

I'll start firing with the lid off, then shut it and make sure no water coming from pots before increasing temp as fast as I dare without stuff blowing up. At cone 08 I plan to start reduction for 45 mins. From what I can tell the best way for me to do this is to cover the chimney with a brick gradually until flames are coming evenly out of the top and bottom spy holes? And to 'stall it out' I suppose adjust burner power to maintain temp...

After 45 mins I plan to go to neutral atmosphere, so uncover the chimney most of the way I suppose. Hopefully the temp will get high enough to bend cone 8.

Not sure if I'm going to be able to tell what's going on with the atmosphere really. Apart from reading reduction firing has long smokey flame, I don't know if you can tell difference in oxidation vs neutral without an oxygen probe? 

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If you are starting a  single fire cycle, do not start reduction until you have completed the normal bisque cycle in both time and temperature.  This probably means hold at cone # that you know your clay body will have completed the burnout of all combustibles.  The hold time depends both on temperature and the amount of ware and the ware thickness and porosity.  

Once you are sure the ware has completed the burnout, start reduction and fire as fast or slow as you want to.   

LT
 

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Well I unpacked the kiln this morning. 

The firing went fairly well I suppose. It did take 10 hours, but I'm sure I can do it faster next time. It seemed to fire fairly evenly. The bottom got colour first, but ^05 dropped on top shelf about 20 mins before the bottom shelf. I think ^5 was about the same, and I missed the ^8 dropping because I didn't expect it to get there so fast. 

The cone packs look pretty much the same, ^8 is really snakey so maybe I got ^9.

The only problem I think was I didn't get enough - or any reduction? All I did was close the chimney so it was about 1 inch left open. And left it for 45 mins around 1060C, then I opened the chimney fully up to finish firing. 

Perhaps I should have pumped the gas up more, or left the chimney partly closed for the final part of firing...

I didn't get any reds from glazes I thought might go red - although they are all trial recipes so it might just be the recipe. But the clay doesn't seem to be reduced - not that I really know, maybe someone else can tell from the picture? 

Thanks everyone for the amazing help on this project. I certainly couldn't have done it without you all!  

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Edited by tomhumf

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On 8/13/2018 at 1:11 AM, tomhumf said:

Thanks magnolia, I'm going to try a hold at around ^05 I think, and then reduce. I've just lit the kiln. Everyone cross fingers please! 

Try reducing earlier on your next batch, like cone 08. Might get better penetration.

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13 hours ago, High Bridge Pottery said:

Looks like it works a treat, makes me think the flow in my kiln must be all wrong.

I think I got very lucky considering I have no clue what I'm doing. The chimney cross section  in your kiln looks a lot smaller than mine - if it's still the same as the photo of the round opening. Also, couldn't you just prop your lid for your earlier stage firing? You should keep your thread going, it was an inspiration for me. 

8 hours ago, neilestrick said:

Try reducing earlier on your next batch, like cone 08. Might get better penetration.

I read this just as I was starting reduction at ^05 for the second time. This evening I've watched a load of YouTube videos of reduction firing. They all start around 800 Celcius, with hard reduction, then lighter reduction to end of firing. 

Like LT said though I need to burn off carbon before reduction though as I'm doing single firing. I used to bisque to ^05 with this clay in electric kiln as I had problems with pinholing - though I'm not sure the bisque temp was definitely the issue. 

So I'm a bit confused but I suppose I'll get some ^08 cones and try it out next time. And maybe do the rest of firing with the chimney a quarter closed or something. 

I've used a lot of propane so far. The tank was freezing up when I was finishing today. I ran hot water over it, and seems about 1/4 of a 19kg tank left after 2 and a half firings! 

I was very cautious first time though, and quite cautious today.

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I did go from the circle hole the same size as the burner port to making it a square hole so it is a little bigger.  Doesn't look like I posted a picture but there might be a post in there somewhere about it or I just never recorded that. I don't think that is an issue and more the flame path is just wrong in the kiln as that was one of the first things decided on. I have had the idea to rejig the flame path but never got round to it as the forced air burner gets me to cone10. 

 

The thread is still going but I just haven't had any time to fire the kiln, life is getting in the way of pottery atm. Glad it inspired you to go for the conversion, that's what it is there for, also as a way to record so I can look back at what I have done :lol:

 

I have some special fleck clay around here somewhere that is fired in electric kiln / oxidation so I will try and update this post with a picture to see how it compares to your look. I think you are probably light on the reduction as that is the same issue I have had with half the kiln looking reduced and half looking oxidised. Even after your body reduction I would try and climb in a little reduction instead of removing the brick from the chimney completely. 

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