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Gas kiln won't heat up above 650C/1200F


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#1 glinum

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Posted 03 April 2013 - 07:45 AM

Hello! I'm a beginner potter and still learning things. I was inspired by Simon Leach to make a similar gas kiln. I decided to go for this type of kiln because I thought it's simple to build and chip. I made it in a metal drum instead of Simon's welded wire.
The inner volume is about 115L (30gal) . It's covered with the 50mm (2in) ceramic fiber inside. I used a regular burner from a hardware store and 50L propane tank with reducer that people use in households here in Ukraine. Also I inserted there a thermocouple connected to some simple multitester with TEMP. Cones seem to be very hard to find here and expensive ($1 each, 400pieces min lot)

So I loaded some ware and started my first firing. In the first hour temp climbed to 230C/446F (valve 50% open). In next 4 hours (valve 100% open) the temperature climbed to 650C/1200F and stopped. I waited for another 1.5 hour ans then gave up and turned it off. I think I need to mention that I fired outside and it was a little windy but that didn't affect the burner. The air temp was about 12C/53F. I put the gas tank in a container with hot water so it was not freezing. The outer surface of a drum was very hot. I could touch it, but not hold my hand there.

Can someone suggest why this could possibly happen? Simon says he gets 1000C/1832F in 3 hours. So I must be missing something in my setup.
Here's my guesses:
1. The kiln volume is too big for a single burner.
2. The ceramic fiber layer must be thicker.
3. The drum must be placed in some insulation. Maybe put it in a box and cover with sand (leaving the front face open of course)?
4. The gas tank pressure is not enough (thou I don't really think this could be a case).

I attach some pics.

Another question is if it's ok to refire the same ware again to get to 1000C/1832F?

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#2 justanassembler

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Posted 03 April 2013 - 09:25 AM

Hello! I'm a beginner potter and still learning things. I was inspired by Simon Leach to make a similar gas kiln. I decided to go for this type of kiln because I thought it's simple to build and chip. I made it in a metal drum instead of Simon's welded wire.
The inner volume is about 115L (30gal) . It's covered with the 50mm (2in) ceramic fiber inside. I used a regular burner from a hardware store and 50L propane tank with reducer that people use in households here in Ukraine. Also I inserted there a thermocouple connected to some simple multitester with TEMP. Cones seem to be very hard to find here and expensive ($1 each, 400pieces min lot)

So I loaded some ware and started my first firing. In the first hour temp climbed to 230C/446F (valve 50% open). In next 4 hours (valve 100% open) the temperature climbed to 650C/1200F and stopped. I waited for another 1.5 hour ans then gave up and turned it off. I think I need to mention that I fired outside and it was a little windy but that didn't affect the burner. The air temp was about 12C/53F. I put the gas tank in a container with hot water so it was not freezing. The outer surface of a drum was very hot. I could touch it, but not hold my hand there.

Can someone suggest why this could possibly happen? Simon says he gets 1000C/1832F in 3 hours. So I must be missing something in my setup.
Here's my guesses:
1. The kiln volume is too big for a single burner.
2. The ceramic fiber layer must be thicker.
3. The drum must be placed in some insulation. Maybe put it in a box and cover with sand (leaving the front face open of course)?
4. The gas tank pressure is not enough (thou I don't really think this could be a case).

I attach some pics.

Another question is if it's ok to refire the same ware again to get to 1000C/1832F?


Its fine to refire your ware... It could be your burner is too small, or it could be that you are reducing too hard and causing the kiln to stall... another question--what is the shelf that your ware is resting on made of?

#3 Marcia Selsor

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Posted 03 April 2013 - 09:55 AM

I'd need to see your burner. You may also have it too far into the kiln. Is the top peep hole open?
What is the BTU rating on your burner? You may need a better flu system, better circulation or a better draught with a chimney..
Marcia

#4 TJR

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Posted 03 April 2013 - 10:10 AM

I agree with Marcia. Pull your burner completely out of the kiln. You should have some flames coming out your top spy. Also, you should be firing for a longer duration. Do you have any cones in there to indicate the temperature?
TJR.

#5 justanassembler

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Posted 03 April 2013 - 12:09 PM

I agree with Marcia. Pull your burner completely out of the kiln. You should have some flames coming out your top spy. Also, you should be firing for a longer duration. Do you have any cones in there to indicate the temperature?
TJR.


I didnt even look at the burner position--you are definitely starving it for air if you're firing with it in that position.

#6 High Bridge Pottery

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Posted 03 April 2013 - 12:32 PM

That looks so good, I want one Posted Image Hope you can sort out your temperature problems! (I have 0 knowledge to help you with Posted Image )

                                                                                                                 1384226_215924051918490_1181728069_n.jpg


#7 glinum

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Posted 03 April 2013 - 01:02 PM

Thank you for all replies!

The shelf is not actually a kiln shelf. They sell carborundum shelves for $100 and this one was $9 for the same size, So I bought it. They actually use these to insulate heating stoves.

It is made of refractory fiber mullite-silica composition, which is produced by melting in electric furnace, aluminum and silicon oxides with subsequent formation of fibers by blowing. To make the material stronger clay added to the fibers.

This text was originally in russian, I hope I translated it correctly.
The problem with this plate is that it has very low thermal conductivity, it just doesn't heat up on top if you drive flame on the bottom. So I had to drill many holes in it for better air circulation. It seems that the heat is distributed quite evenly in the ware because there are no color spots on the pieces like I had in a wood kiln.

Here's a diagram I made quickly to show how it works (in theory).
Posted Image

it could be that you are reducing too hard

Sorry, I don't really understand what that means...

Marcia, here are some pics of the burner


Posted Image Posted Image Posted Image Posted Image http://imageshack.us...22/1000134b.jpg

It's just some simple burner from ware store they use for roofing etc. The BTU rating - have no idea...
It seems Simon Leach uses similar burner but with some side holes. And I just noticed in his video that the burner is all the way out of kiln while I stupidly inserted it inside...
So could it be a problem that it lacked oxigen?

The top hole was open, but when the temperature stoped climbing I thought maybe it looses to much heat so I closed it a little with some pieces of brick.

Can you explaine a little about a better flu system and better draught with a chimney.


TJR
, yeah now I see that the burner had to be completely out of the kiln, that was the problem... I don't have any cones as I said earlier just because they don't sell it here. I determine the temperature with a thermocouple.

justanassembler, I agree about the burner. I just didn't pay attention how people do it. Next time I'll put it otside the kiln and point the flame in the hole. Do you think I have to route some side holes in the burner?


And I still need to understand if this single burner is enough for heating up 30gal kiln? Or maybe I need to cut some volume and make it smaller?


I got some off topic question - if I fire red and white clay in one load will there be any color blends? I tried it once in a wood kiln and white clay pieces came out darker, but maybe that was just because of ash or something?

#8 justanassembler

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Posted 03 April 2013 - 01:06 PM

I doubt you need to add more holes to your burner, the air intakes on the burner were put there by the manufacturer to match the oriface size, so that should all be OK. In order to determine if your burner is adequate, you do need to know the BTU rating, the manufacturer probably lists this somewhere. once you have that, this clay times article is a good reference.

#9 INYA

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Posted 03 April 2013 - 01:23 PM

I have very very similar kiln (it is vertical, not horizontal and without the drum- constructed only from wires, actualy fence material- wire forming little squares. Mb burner is at the bottom and the hole is on top)


I had the same problem but I was stupid enough to reduce its size (was very sorry later)- the point is that the burner has to be outside and there has to be another hole ( I do not see it from your pics ) which has to be big enough.
You will need to introduce LOT of air, in reduction your temp will not rise normally.

* I just saw you do not understand reduction- it is firing without enough oxigen, the flames that go out of the kiln are blueish, not orange. It is one of big advantages of gas kilns but they are more popular at the end of firing, not in the middle- you can get many interesting colors, glaze efects etc. we are talking reducing versus oxidizing athmosfere in the kiln.

So I reduced the size and nothing happened ;( I even bought bigger burner and that did not help either
and then I doubled the size of the upper hole I got to 1250 Celsium easily (if I wanted I could get there in about 4 hours
I am sure you have the same problem = not enough oxigen. Consider making your exit hole bigger- but plan some kind of lid in case you will need reducing athmosfere in the future
hope this helps a bit

greetings from slovenia Posted Image
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#10 INYA

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Posted 03 April 2013 - 01:46 PM

I watched Simon`s videoa again- my kiln is much bigger (two or three times bigger- I fired with 3 shelves)
anyway I hardly use it now, I switched to electric- mainly due to its BIG dependance on high temperature in the air. I could fire succesfully only from April to Oktober and not after 9 pm (it was just not warm enough)
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#11 glinum

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Posted 03 April 2013 - 02:11 PM

Wow, INYA, so much helpful information in your posts! :-) Thank you!
Now I'm eager to start firing again with this tweaks.
I do understand reduction, I just didn't understand what justanassembler meant by "you are reducing too hard" and what that pratactically means, what I was doing wrong for "reducing too hard". Now I know it was because I inserted the burner in the kiln so it wasn't able to suck enough air and also the top hole was too small.

the point is that the burner has to be outside and there has to be another hole ( I do not see it from your pics ) which has to be big enough

Do you mean the top hole or some extra hole? I have the top hole, check fire8.jpg in my first post. I put some rocks there just because I didn't know it must be bigger and not smaller. Now I know I have to make it bigger. Do I have to make the bottom hole bigger too?

I could fire succesfully only from April to Oktober and not after 9 pm (it was just not warm enough)

Was it becase the gas tank freezes or because your kiln surface loses too much heat?

#12 neilestrick

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Posted 03 April 2013 - 02:26 PM

We call that shape of kiln a Pushmi-pullyu. You can make little mini wood kilns of the same design, out of bricks and kiln shelves. The problem with yours is it's not pulling enough air through. Because you have a venturi burner, you're relying on the draw of the kiln to be strong enough to introduce enough air into the combustion. A chimney is necessary with a venturi burner. How to get one built on your design will be a bit of a challenge, but a small fiber and steel mesh tube could do the job just fine.

****
The Pushmi-pullyu
The pushmi-pullyu (pronounced "push-meā€”pull-you") is a "gazelle-unicorn cross" which has two heads (one of each) at opposite ends of its body. When it tries to move, both heads try to go in opposite directions. Dr. Dolittle meets it on his voyage to Africa to save monkeys (See: The Story of Doctor Dolittle). In the 1967 film, the pushmi-pullyu was instead portrayed as a double-headed llama. The Eddie Murphy film has a brief scene where it is walking in the background while Dr. Dolittle talks to the tiger in the cage. This is in keeping with the fact that Murphy's movie version was only loosely based on the books.[4]


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Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

neil@neilestrickgallery.com

#13 Marcia Selsor

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Posted 03 April 2013 - 05:45 PM

The diagram of the draught shows the flow going up the back wall. Make sure the shelf is far enough away from the wall to allow that flow.
Inya offer good experience with this type of kiln. With the burner out of the kiln it will pull much more air into the kiln.

Marcia

#14 INYA

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Posted 04 April 2013 - 02:58 AM

I will take a picture of my kiln later today or tomorow... and I will measure it so you can see
My upper hole is way bigger! and my burner hole also. But I will send you measurments and you can adapt them to your size.

I am not sure what was the problem in colder season- the tank was almost freezing, it just did not reach high temp- as I remember I could not get it above 1050 C.



Hey thank Neil for explanation, never heard about The Pushmi-pullyu!
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#15 glinum

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Posted 04 April 2013 - 04:19 AM

neilestrick, chimney?? Oh no.... I hope I'll make it work without it. This is a low-end project and chimney will ruin the whole concept :lol: :D
But in case I decide to make one, what would be the height of it?

Marcia, the shelf is 40x60cm and 10cm away from the back wall, do you think that's enough?

INYA
, pictures would be great! Thanks

#16 Ben

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Posted 04 April 2013 - 08:26 AM

I'm guessing that you have enough insulation. If the metal skin is not glowing red while at 1200F you have good insulation. I'm going to propose that the burner isn't making enough btu OR the kiln can't get enough draft for proper combustion. Lots of info above on this topic.

You could possibly do a simple test to get an estimate on the burner BTU. use a metal container and a set volume of water and time how long it takes to get the burner to boil the water. Water takes a specific amount of btus to change temp so you could calculate the heat needed to raise from your starting water temp up to start of boiling. Of course there will be a certain percentage of heat lost to air around the pot but this test will give some kind of estimate about the burner btus.
Ben

#17 potterbeth

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Posted 04 April 2013 - 09:54 AM

"I do understand reduction, I just didn't understand what justanassembler meant by "you are reducing too hard" and what that pratactically means, what I was doing wrong for "reducing too hard". Now I know it was because I inserted the burner in the kiln so it wasn't able to suck enough air and also the top hole was too small."

You might want to adjust one thing at a time to determine which change is actually making a difference.

You could start by relocating the burner. If the kiln stalls (stops increasing in temperature), then start working with the flue, etc. Blocking the flue a little bit might help increase temperature, while closing it off too much could create heavy reduction which also stalls the kiln.

Remember, every gas-fired kiln has its own personality. It just takes some time working with it to get it right. But, oh what a joy when you do!

#18 INYA

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Posted 04 April 2013 - 01:32 PM

ok here are pictures...
measurements:
65x65 cm x 50 cm height + the lid

insulation 16 cm wall, upper hole 10 x 10 cm, the same hole for the burner

It really did quite a good job, I was firing with two shelves and about the same element for measuring temp you have
BUT I got tired of sitting by the side every time... and now it is all alone in the garage
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#19 Marcia Selsor

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Posted 06 April 2013 - 11:08 AM

neilestrick, chimney?? Oh no.... I hope I'll make it work without it. This is a low-end project and chimney will ruin the whole concept Posted Image Posted Image
But in case I decide to make one, what would be the height of it?

Marcia, the shelf is 40x60cm and 10cm away from the back wall, do you think that's enough?

INYA
, pictures would be great! Thanks


see if you get a pull of a draft with a lit newspaper torch by your burner port. This is important since you are using a natural draf6 and no forced air.
Marcia

#20 glinum

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Posted 04 May 2013 - 12:34 PM

Today I fired it again and couldn't even reach my previous result of 650C, now it climbed to 600C only. :-(
I am so disappointed...
Anyway, this is what I did - made both holes bigger (especially the top one) and pull the burner out oh the hole.
In 5 hours it climbed to 520C/968F just fine and then stalled... 2 hours more and only +20C. Then I decided to cut the top hole even bigger and pulled the burner even more out. It then raised to 600C and stalled. I tried everything - adjusted the burner to different positions, I tried to blow some air with a room fan, I even found some oven chimney pipe and inserted it into the top hole, but that actually dropped the temperature to 550C.
I ran around for almost 12 hours and nothing helped, so I just turned the burner off and feel very bad now. Don't know what to do... Maybe I have to switch to vertical design?
Or maybe I have to switch to woodworking...

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