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#1 Rapid Dog

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Posted 27 July 2010 - 01:25 PM

Hello everyone. First post here.

I've returned to potting after a 20 year hiatus strictly for the therapy of throwing.

I built a downdraft gas kiln of used softbrick that measures 3'wide X 2'deep X 4'tall. Flat roof.
I know it's not the required cube but I was limited to what alterations I could do to the used frame I bought.

That said, it has two air/gas burners that are 3"X12" pipe with Stiktite burner tips and squirrelcage blowers.
I am firing on propane.

The bagwalls are soft arch bricks stacked (2 bricks) atop each other leaving 1" gaps in between the bricks.
The flame alley is 6" wide X the kiln depth.
The roof is flat and made of soft brick on it's side.

My first shelf is at the top of the bagwall.
The structure is one brick wide (4") with a coating of some hi tech stuff on the interior (source through other forum research).
The door is one brick deep with a 1" fiber later on the interior.

The flue is appx. 6" X 9" with a floor cut of 9"X9".
The chimney is built up to the top of the kiln with softbrick with a 9"X9" opening to the top of the kiln.
From the top there is a 9" diameter stack that reaches an additional 6 feet, lined with fiber.

Whew! Do I still have an audience? I think that about covers it.

First test firing I wasn't able to reach cone 10, rather maybe cone 9 max.
Resigned to that I've decided to go with cone 5 clays and glazes, which are new to me.

My first cone 5 firing worked out like this.
Preheated for 2 hours, plugs out flue open.
Gradual gas and airflap adjustment to 1000 degrees took about 3 hours.
From there it I kicked evrything up a good notch ramped up quite quickly, close to 1500 degrees in about 45 minute.
Total time to reach cone5/cone 6 was 9 hours including preheat and soak.

All good, correct?
However I'm finding that there is about a cone difference between top and bottom as I expected.
To get the top shelf to cone 5 I need to fire the bottom to cone 6.

Cones at the front center (door) were spot on. Top both sides cone 5 half down, but bottom cone 5 completely down.
Rear center/top glazes appeared not quite mature, so I suspect the rear is not getting the heat.
The bottom and bottom center are defintely getting high heat.

I tried closing the flue, opening the flue, adjusting gas and air.
The digital pyrometer place mid kiln at the rear read max 2115F.
I did find that the temp would rise at the peak about 25 degrees if I actually turn the individual gas valves down to 1/3rd and added more air.

So, outside of the poor interior design, what's the theory on trying to get more heat to the top?
Could it be the flue size? Chimney length?
I've thought about lining it with fiber but not sure this would help the even-ness.
I've considered pulling the roof and taking the walls down 3 bricks to make it more square.
That's admittedlymore work than I'd like to do.
It has been some time since building my first kiln in the late 70's and what was second nature is now a fading memory.:blink:

Good news is all the pots made it, most glazes matured (some better than others!)

But I have a feeling this recycled kiln can still be tweeked a bit. Just can't remember how...:)

#2 Potterstu

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Posted 27 July 2010 - 03:28 PM

Hello everyone. First post here.

I've returned to potting after a 20 year hiatus strictly for the therapy of throwing.

I built a downdraft gas kiln of used softbrick that measures 3'wide X 2'deep X 4'tall. Flat roof.
I know it's not the required cube but I was limited to what alterations I could do to the used frame I bought.

That said, it has two air/gas burners that are 3"X12" pipe with Stiktite burner tips and squirrelcage blowers.
I am firing on propane.

The bagwalls are soft arch bricks stacked (2 bricks) atop each other leaving 1" gaps in between the bricks.
The flame alley is 6" wide X the kiln depth.
The roof is flat and made of soft brick on it's side.

My first shelf is at the top of the bagwall.
The structure is one brick wide (4") with a coating of some hi tech stuff on the interior (source through other forum research).
The door is one brick deep with a 1" fiber later on the interior.

The flue is appx. 6" X 9" with a floor cut of 9"X9".
The chimney is built up to the top of the kiln with softbrick with a 9"X9" opening to the top of the kiln.
From the top there is a 9" diameter stack that reaches an additional 6 feet, lined with fiber.

Whew! Do I still have an audience? I think that about covers it.

First test firing I wasn't able to reach cone 10, rather maybe cone 9 max.
Resigned to that I've decided to go with cone 5 clays and glazes, which are new to me.

My first cone 5 firing worked out like this.
Preheated for 2 hours, plugs out flue open.
Gradual gas and airflap adjustment to 1000 degrees took about 3 hours.
From there it I kicked evrything up a good notch ramped up quite quickly, close to 1500 degrees in about 45 minute.
Total time to reach cone5/cone 6 was 9 hours including preheat and soak.

All good, correct?
However I'm finding that there is about a cone difference between top and bottom as I expected.
To get the top shelf to cone 5 I need to fire the bottom to cone 6.

Cones at the front center (door) were spot on. Top both sides cone 5 half down, but bottom cone 5 completely down.
Rear center/top glazes appeared not quite mature, so I suspect the rear is not getting the heat.
The bottom and bottom center are defintely getting high heat.

I tried closing the flue, opening the flue, adjusting gas and air.
The digital pyrometer place mid kiln at the rear read max 2115F.
I did find that the temp would rise at the peak about 25 degrees if I actually turn the individual gas valves down to 1/3rd and added more air.

So, outside of the poor interior design, what's the theory on trying to get more heat to the top?
Could it be the flue size? Chimney length?
I've thought about lining it with fiber but not sure this would help the even-ness.
I've considered pulling the roof and taking the walls down 3 bricks to make it more square.
That's admittedlymore work than I'd like to do.
It has been some time since building my first kiln in the late 70's and what was second nature is now a fading memory.Posted Image

Good news is all the pots made it, most glazes matured (some better than others!)

But I have a feeling this recycled kiln can still be tweeked a bit. Just can't remember how...Posted Image



#3 Potterstu

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Posted 27 July 2010 - 03:40 PM

Your kiln sounds like a great success but you should be able to to get C/10 without too much "tweeking". To get hotter at the top add three to five inches in height to the bag wall. Try hard bricks for your bag wall instead of soft. If you are using propane why not eliminate the blowers and goose up the pressure for more heat? You'll have the same volumn of fuel but higher pressure should also increase temp rise.

Good luck and enjoy,

Potterstu

#4 Seasoned Warrior

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Posted 27 July 2010 - 04:31 PM

An arched roof would create better circulation within the kiln while firing, also there is a mathematical relationship between the walls and the curve of the roof. There are a number of good references available that go inot this quite extensively. Good luck.



#5 Marcia Selsor

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Posted 27 July 2010 - 08:10 PM

Dear Rapid Dog,
I think the damper should be about 3/4 to 1/2 shut for most of the firing. Are you using target bricks? i.e. wedges cut at 45 degrees to bounce the flame up at the end of the bag walls. I would add another course or two to the height of the bag wall as well. I used similar burners with propane and squirrel cage blowers. Try using a hard neutral to oxidizing flame when climbing in temperature. You may have to play with the balance and the flame lengths . Propane varies around the country. Use ^09 and 04 for watching for body reduction and 5,6,7 for the glaze cones. Make a little dish at the end of your cone pack to catch the melt from ^09 and 04. Damper settings may vary with altitude. How did you decide on the flu size and exit hole to the chimney?


Make sure the roof of the flat top remains tight under tension. You could tighten a brace on two sides using car valve springs and fender washers screwed onto a threaded rod bracing angle iron on the other two sides of the roof. If it loosens from expansion from heat during the firing , it could fail when it cools. Nils Lou has a design a flat top that you may want to check out.


To reduce, damper in to maybe 1/8 or less open and cut the air down by using a disc screwed over the intake of the squirrel cage blower until you have a hard bluish flame out of the top peephole and a licking flame out of the bottom peep hole. Have the tension tight enough so you can adjust the air without it slipping. Check for reduction flame by holding a dry stick in front of the peepholes in the flame. If it doesn't ignite, you are reducing. Repeat this when ^5 is down and 6 is going. Reduce for about an hour or 45 minutes. No need for lots of billowing smoke. To come out of reduction , resume the climbing settings (after you have attained 1000 degrees.) Your initial settings seem fine for the early stages.. Clear the kiln when your are finished for about five minutes (more air in oxidation atmosphere). Then plug it up and seal it to cool. The thin walls and insulation will cool rapidly and could cause dunting. Most soft brick gas kilns are two layers thick. Firing down may help. Also throwing little stick into the kiln as it cools helps with copper reds.

I think you could use better insulation..maybe another later of 1" all the way around. Check ebay for ceramic fiber. I also think you are wise and green to go to ^5-^6. I fired that temperature from 1980 til 2000 at the university. I am attaching my shop glazes from there. Four or five of them are included in Michael Bailey's Oriental Glazes book...they are the only ^6 glazes in the book. In my studies of pyrotechnic information back in 1980, it looked like the consumption of gas fuel to get from ^6 to ^10 is as much as getting to ^6 indicating it takes twice the fuel to go all the way to ^10. Many may disagree with that and maybe it is a little off, but it is a still significant amount of fuel.
John Britt has been developing ^6 reduction glazes and has published them in CM. Diana Pancioli has also done much work in this area as has Sharon Russell from the Baltimore area..


Good luck.
Marcia

Marcia Selsor Cone 6 Reduction Glazes

Semi Matt Black ^6 Reduction

Manganese Dioxide. 125 2.5

Whiting 587.5 11.75

F-4 feldspar (soda) 2725 54.5

E.P. Kaolin 325 6.5

Silica 1250 25

Iron Oxide 500 10

5000 100.25


Shino ^6 Reduction

Gerstley Borate 4.9

Soda Ash 2.9

Neph. Syenite 54.5

Spodumene 22.8

Ball Clay 14.9




Copper Red ^6 Reduction

Neph Syn 2766.5 54

Gerstley Borate 630.5 12

Whiting 535.5 10

Silica 1067.50 20

Tin Oxide 75 1.5

Copper Carbonate 20 0 .3

Red Iron Oxide 20 0 .3

5115 97 .1




CLEAR ^6 Reduction

Whiting 18.5

Neph. Syn 25.8

EPKaolin 18.8

Silica 31.1

Gerstley Borate 4.6



Celedon^6 Reduction

Whiting 18.5

Neph. Syn 25.8

EPKaolin 18.8

Silica 31.1

Gerstley Borate 4.6

Red Iron Oxide




SemiMatt ^6 Reduction

Magnesium Carbonate 1.3

Whiting 13.1

Zinc Oxide 1.2

Neph. Syenite 66.4

Spodumene 5.7

Silica 12.3

100.0




Marci’s Matt ^6 Reduction <- this glaze is very versatile and works well in oxidation with a variation of the colorants.

EPKaolin 1150 23

Dolomite 1000 25

Neph Syenite 1900 38

Silica 900 18

Whiting 200 4

Gerstley Borate 500 10

Variables:

Blue 1% Cobalt Carb

LightGreen1.5%Nickel Carb + 1.5%RedIron Oxide

Gray 2.5% Rutile +1.5% Nickel Carb

Warm Blue 1% 1% Cobalt Carb+ 5% Manganse Di

Tan 6% Manganese Di. +2.5 Red Iron Ox +1%

Rutile OR try using only 5% Rutile as a colorant




Nelson’s Base ^6 Reduction

Custer Feldspar 64 1200

Whiting 18 450

Ball Clay 9 225

Talc 5 125

96

Black= Red Iron Oxide 8%

Chocolate = Red Iron Ox 4% +Chrome Ox. 2%

Teal=Cobalt carb 1% +Chrome 1%




White Liner ^6 Reduction

Ger. Borate 20

Neph. Syenite 30

Kaolin 13.3

Whiting 9.4

Talc 17.2

Silica 10

Zircopax 10




ADD :Pumpkin 5% Red Iron + 3% Rutile

Gold 3% Rutile

Lt. Blue 0.5% Cobalt Carb




Ohata Red ^6 Reduction

Bone Ash 420 12.9

Dolomite 240 7.4

Gerstley Borate 120 3.7

Lithium Carbonate 120 3.7

Custer Feldspar 1560 48.1

Kaolin 180 5.6

Silica 360 11.2

Red Iron Oxide 240 7.4





#6 Rapid Dog

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Posted 28 July 2010 - 10:25 AM

Thanks for the great input folks.

Raising the bagwall a brick is on the list. I'll look for some hardbrick as well.
I need some for the bottom shelves anyway. I made the mistake of using soft brick which I've never done and they actually sagged (compressed) at the rear near the burners.

So you think 1" fiber would be worth a try? The reason I didn't do that to begin with is I was told that fiber reflects rather than absorbs the heat.
Therefore the soft brick wouldn't absorb the heat and get hot enough.
Wasn't sure how to decipher that...

The roof is made with the standard old school config of tierod and channel iron with valve springs so that it can expand.
I'll not that I have a second layer of softbrick laying atop the roof on their sides.

When I did the first cone 10 attempt, you are right, the fuel consumption was close to 12% trying for 12 hours to get to ^10 from the start time.
This first ^5 firing was about 4-6% which calcs to about 12 gallons.
I have a 250 gal tank.

That being the case, the suggestion of higher pressure might not be doable as my tank is appx 75' away using 3/4" pipe.
As far as the flue goes, I went by suggestions and everything I could try and calculate including the old Kiln Book.
I'm not a math genius for sure. From what I could tel, a chimney of about 10' from the flue would suffice.
I'm open to suggestions there for sure.
After the first ^10 try I opened thhe mouth by cutting about 2-3" in the wall and tapering the wall opening and floor slot .

I'm not sure what 'firing down' means. Does that mean a ramping down with the burners going?

Thanks Marcia for the glaze recipes, they sound nice. I would like to attempt a nice hot reduction next time. I'm not real impressed with the Laguna glazes as far as aesthetics even though they fired fine. Nothing ran at all.
Next stop will be mixing some up...B)

#7 Marcia Selsor

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Posted 28 July 2010 - 12:58 PM

Thanks for the great input folks.

Raising the bagwall a brick is on the list. I'll look for some hardbrick as well.
I need some for the bottom shelves anyway. I made the mistake of using soft brick which I've never done and they actually sagged (compressed) at the rear near the burners.

So you think 1" fiber would be worth a try? The reason I didn't do that to begin with is I was told that fiber reflects rather than absorbs the heat.
Therefore the soft brick wouldn't absorb the heat and get hot enough.
Wasn't sure how to decipher that...

The roof is made with the standard old school config of tierod and channel iron with valve springs so that it can expand.
I'll not that I have a second layer of softbrick laying atop the roof on their sides.

When I did the first cone 10 attempt, you are right, the fuel consumption was close to 12% trying for 12 hours to get to ^10 from the start time.
This first ^5 firing was about 4-6% which calcs to about 12 gallons.
I have a 250 gal tank.

That being the case, the suggestion of higher pressure might not be doable as my tank is appx 75' away using 3/4" pipe.
As far as the flue goes, I went by suggestions and everything I could try and calculate including the old Kiln Book.
I'm not a math genius for sure. From what I could tel, a chimney of about 10' from the flue would suffice.
I'm open to suggestions there for sure.
After the first ^10 try I opened thhe mouth by cutting about 2-3" in the wall and tapering the wall opening and floor slot .

I'm not sure what 'firing down' means. Does that mean a ramping down with the burners going?

Thanks Marcia for the glaze recipes, they sound nice. I would like to attempt a nice hot reduction next time. I'm not real impressed with the Laguna glazes as far as aesthetics even though they fired fine. Nothing ran at all.
Next stop will be mixing some up...B)

I was suggesting that you might re-light the burner a few times to keep the kiln from a rapid drop in temperature in you were losing heat too quickly that it caused dunting.
Also you could put some fiber on the top of the second layer of bricks on the roof in case you haven't already done that. Keep us posted on your progress.



#8 carlaherren

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Posted 28 July 2010 - 06:38 PM


Hello everyone. First post here.

I've returned to potting after a 20 year hiatus strictly for the therapy of throwing.

I built a downdraft gas kiln of used softbrick that measures 3'wide X 2'deep X 4'tall. Flat roof.
I know it's not the required cube but I was limited to what alterations I could do to the used frame I bought.

That said, it has two air/gas burners that are 3"X12" pipe with Stiktite burner tips and squirrelcage blowers.
I am firing on propane.

The bagwalls are soft arch bricks stacked (2 bricks) atop each other leaving 1" gaps in between the bricks.
The flame alley is 6" wide X the kiln depth.
The roof is flat and made of soft brick on it's side.

My first shelf is at the top of the bagwall.
The structure is one brick wide (4") with a coating of some hi tech stuff on the interior (source through other forum research).
The door is one brick deep with a 1" fiber later on the interior.

The flue is appx. 6" X 9" with a floor cut of 9"X9".
The chimney is built up to the top of the kiln with softbrick with a 9"X9" opening to the top of the kiln.
From the top there is a 9" diameter stack that reaches an additional 6 feet, lined with fiber.

Whew! Do I still have an audience? I think that about covers it.

First test firing I wasn't able to reach cone 10, rather maybe cone 9 max.
Resigned to that I've decided to go with cone 5 clays and glazes, which are new to me.

My first cone 5 firing worked out like this.
Preheated for 2 hours, plugs out flue open.
Gradual gas and airflap adjustment to 1000 degrees took about 3 hours.
From there it I kicked evrything up a good notch ramped up quite quickly, close to 1500 degrees in about 45 minute.
Total time to reach cone5/cone 6 was 9 hours including preheat and soak.

All good, correct?
However I'm finding that there is about a cone difference between top and bottom as I expected.
To get the top shelf to cone 5 I need to fire the bottom to cone 6.

Cones at the front center (door) were spot on. Top both sides cone 5 half down, but bottom cone 5 completely down.
Rear center/top glazes appeared not quite mature, so I suspect the rear is not getting the heat.
The bottom and bottom center are defintely getting high heat.

I tried closing the flue, opening the flue, adjusting gas and air.
The digital pyrometer place mid kiln at the rear read max 2115F.
I did find that the temp would rise at the peak about 25 degrees if I actually turn the individual gas valves down to 1/3rd and added more air.

So, outside of the poor interior design, what's the theory on trying to get more heat to the top?
Could it be the flue size? Chimney length?
I've thought about lining it with fiber but not sure this would help the even-ness.
I've considered pulling the roof and taking the walls down 3 bricks to make it more square.
That's admittedlymore work than I'd like to do.
It has been some time since building my first kiln in the late 70's and what was second nature is now a fading memory.Posted Image

Good news is all the pots made it, most glazes matured (some better than others!)

But I have a feeling this recycled kiln can still be tweeked a bit. Just can't remember how...Posted Image



#9 carlaherren

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Posted 28 July 2010 - 06:39 PM

I suggest you use an angled (45 degrees) brick at the opposite side of the gas burner to deflect the heat up.

#10 Rapid Dog

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Posted 29 July 2010 - 11:43 AM

Yes, I have 45 degree target bricks at the end of the burner alley.

Any opinions on lining the inside with fiber?

#11 Marcia Selsor

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Posted 29 July 2010 - 01:41 PM

Yes, I have 45 degree target bricks at the end of the burner alley.

Any opinions on lining the inside with fiber?

Lining the inside would help but Feriz , inventor of ITC once sent me some compress fiber squares to be sprayed with ITC.. If you can find 8# fiber use that.
Also need a good adhering system. I lined an old brick West Coast Kiln years ago for Raku.
It is a tough call whether to put the fiber inside or out. Sqme would want slower heat up and cool down therefore using the bricks to that advantage.
Another thought would be , as you said earlier, heat reflects off the fiber..sort of.
The other problem with fiber inside the kiln is that it could start dropping gifts into your pots if it starts to break down after several firings and rubbing as you load.
I had a fiber door on a ^6 kiln and sprayed it with Rigidizer to prevent flaking. That was 15 years or so ago. It held up well. The outside was insblok fiber board with hot face ratings at 1900F. So there were 2 inches of fiber then the insblok. Check with local refractory companies and see what is available. They may give you some up to date technical printed information like heat tranfser properties etc. One thing to consider is fiber is nasty to breathe...bad, bad , bad. I made a kiln out of the bio degradable type of fiber and did not like it's life span. It seemed to fall apart. That could possibly be from the very high humidity down here on tropical Texas.

Maybe you should just spray ITC inside your kiln. It reflects heat too.
Whatever you do, look into high tech refractories and insulation.
Marcia

#12 Rapid Dog

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Posted 30 July 2010 - 09:29 AM

Thanks Marcia,

Yes, ITC is what I coated the inside of the kiln with. I also put a 1/2" layer of blanket inside the door with ridgidizer. If I can find some cheap blanket I may try insulating the outside. but like you say, bad stuff to breath. My kiln is outdoors though which helps some.

I'm ordering some hardbrick up today.

The only real question I have now is my flue size.
That said, I'm pretty sure it's sufficient.

#13 Rapid Dog

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Posted 16 August 2010 - 11:42 AM

Just to follow up, I did some recommended mods to the bagwall.
My ceramic tech buddy told me the bagwall was too many bricks not allowing heat to circulate.
I had 4 bricks laying down on the long edge satcked two high leaving about 1-2" between each.
He suggested I use 4 bricks on end leaving more like 4" space in between.

I've also bought some 2800 brick on ebay for my shelf base.
This weekend I installed a 1/2" Superwool blanket on the outside walls sealed with galvanized sheet metal.
Here's hopin' for better firing and even heat distribution! :)

Here's the arrangement:

Attached Files



#14 Marcia Selsor

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Posted 16 August 2010 - 02:09 PM

There are always 2 or more opinions on everything involving clay. I have working with solid badwalls with the theory that it forces the flame UP and around. If you have it open UNDER the first course of shelves, won't the heat/ flame go out the flu without circulating around the pots?
Marcia

#15 the Potter's Daughter

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Posted 16 August 2010 - 07:49 PM

I don't mean to ride Rapid Dog's coat tails, but since we're on the subject... I think that I have almost come to the end of my electric kiln. I am planning to build a gas kiln in the next year or two, but really know little at all about it. I am wondering if anyone (Marcia?) has advice on some intruductory information for me out there? I wan't to really take advantage of the hibernating I am forced into here in Canada this winter, and do some hard core research on this. Thanks in advance for your time.
April

#16 bciskepottery

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Posted 16 August 2010 - 10:29 PM

I don't mean to ride Rapid Dog's coat tails, but since we're on the subject... I think that I have almost come to the end of my electric kiln. I am planning to build a gas kiln in the next year or two, but really know little at all about it. I am wondering if anyone (Marcia?) has advice on some intruductory information for me out there? I wan't to really take advantage of the hibernating I am forced into here in Canada this winter, and do some hard core research on this. Thanks in advance for your time.
April


Check out "21st Century Kilns" by Mel Jacobsen and Friends. Web site is www.21stcenturykilns.com. His book comes with a DVD that shows how different folks have built their own kilns -- gas/propane and wood fired. Lots of good advice/experiences, plus plenty of drawings and schematics.

#17 Rapid Dog

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Posted 07 September 2010 - 02:07 PM

Tried the bagwall design above for a bisque, but still fired hotter on the bottom.
So, made a solid wall two bricks on edge the height of the bottom shelves (9"), with a target brike 12" about 2/3rds away from the burners each side.
Glad to say it helped bring the heat up on top earlier.
The bottom is still hotter but nothing I can't correct with some soaking time.

Adding 1/2" of superwool to the outside helped quite a bit in the fuel department.
Cone 6 firing in a 24 cubic foot /2 burner propane kiln is 8 1/2 hours including candling and soak. B)

#18 Seasoned Warrior

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Posted 07 September 2010 - 03:11 PM

I don't mean to ride Rapid Dog's coat tails, but since we're on the subject... I think that I have almost come to the end of my electric kiln. I am planning to build a gas kiln in the next year or two, but really know little at all about it. I am wondering if anyone (Marcia?) has advice on some intruductory information for me out there? I wan't to really take advantage of the hibernating I am forced into here in Canada this winter, and do some hard core research on this. Thanks in advance for your time.
April


In my opinion the best data on the subject of building and designing kilns is Olsens tome on Kilns ("The Kiln Book" by Frederick Olsen) available from ceramicartsdaily.org/bookstore. The information is available elsewhere also and I'd recommend a Google search on the Web, you'll find plenty of information but Olsen is still my favorite. A website that has a lot of burner information is Ward's burner site at wardburner.com. Sounds like the Potter's Daughter just might be a potter herselfPosted Image

Regards,
Charles

#19 Rapid Dog

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Posted 09 September 2010 - 10:33 AM

I'm considering going to four side venturi propane burners and tossing the old sticktite/blower burners.
Any suggestions on wnat model number to get?
24cf kiln
dimensions: 24Dx48Hx26W

#20 Deb Evans

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Posted 10 September 2010 - 11:46 PM

Rapiddog
i've been firing and building kilns since the 70's.
Past 20 yrs - firing w/ propane/cone 9 and 6 in the adks .
took some getting use to from natural gas.
it's like going from manhatten to n dakota, it works, just different.

been firing a 35 cu 4 venturi burner system ,downdraft( 2/side) and love it.

from this experianc/e....
some suggestions, comments and questions........

bagwall - solid rows of hard brink- good idea
probably know this but is your bottom shelf level above flue? I've seen it all.
how many "floors" are you firing? - tight or loose stacking?
your kilnseems small for flue - try putting a post or 1/2 h brick in middle, something that simple can cut the draw and improve circulation.

target brick is sq or slopped?
the firing chamber can be slopped up ward by refractory slurry or sandy raku clay and brick mixture .
remeber you are directing the fire.
play around w/ bagwall and target bricks - so flame slides up wards to the roof and circulates then goes to the front and finds the exit under your first shelf.
i agree w/ ms - knock down the draw of squarrel cage ( prob 50%or so), if burners are for nat gas need to redesign for propane (bigger oraface).
pesonal experiance - mix your own glazes - laguna glazes can suck.

you should be able to even out if not, go venturi -
,
iward burners will sell you the right size venturi plus lots of info on kiln design.




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