Homebuilt Gas Kiln - Need Advice

[Amiram]

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.

 

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...

 

 

May I see the plans for building your kiln?

amiram@khen.net

[Deb Evans]

easiest way to get even reduction, play w/ air mix and damper in or out , make sure you end the firing at night. then you can make sure you have a small orange flame coming out of top and bottom peeps - want even and just 3-4 in , also you can see if you've got too much back pressure and frying the burners. love firing at night! enjoy..............

[Rex Johnson]

Reduction was easy with the blower flaps and the flue, though a bit uneven top and bottom, more out the top peep holes.

 

I contacted Ward and he did mention that he couldn't recommed burners (unless I called to discuss) that my kile was off a cone top to bottom because it's too tall. This is what I guess I knew and suspected all along.

I've been considering suspending the roof and cutting down the walls to make more of a cube.

This will be quite a challenge because I'd have to cut and re-weld the frame and the door would also have to be cut and rehinged.

Ugh...

[Rex Johnson]

Reduction was easy with the blower flaps and the flue, though a bit uneven top and bottom, more out the top peep holes.

 

I contacted Ward and he did mention that he couldn't recommed burners (unless I called to discuss) that my kile was off a cone top to bottom because it's too tall. This is what I guess I knew and suspected all along.

I've been considering suspending the roof and cutting down the walls to make more of a cube.

This will be quite a challenge because I'd have to cut and re-weld the frame and the door would also have to be cut and rehinged.

Ugh...

[Pres]

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

 

 

 

 

 

Just a little clarification here. I hope you don't mind if I add these glazes to my glaze compendium? I may not ever use them, but whenever I come across a new glaze, they go in the book. I notice that some are batch recipes, and others 100% type. You did not have an amount for the iron oxide in the celedon recipe-could you state that?

[Marcia Selsor]

April,

Check out Mel's 21st Century Kiln book. It is full of good kilns.

My raku kiln is in there!

Marcia