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tomhumf

Getting reduction at top of gas kiln

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I've struggled to fire my little ( about 3 cubit ft?) Gas kiln for just over a year. I'm finally starting to get results I like. 

One problem was what temp and time to reduce at. I've found that reducing hard between 940C and 1000C for one hour is giving me the best results for my glazes. I reduce by increasing gas and blocking the chimney to about 1" opening.

I used to then open the chimney fully to complete the firing. I've finally figured out this is why (I think) I ended up with a lot on pinholing on quite a few pots out of each kiln. I think there was a large temp difference between different locations in the kiln.

For the last few firings I have reduced hard as before, but closed the chimney to half open to complete the firing. This gives a flame from the chimney but not from the peeps like during hard reduction. Firing this way means I get hardly any pinholing - just one pot usually on the bottom shelf right bear the burner entry. 

The main problem with the last few firings is the reduction on the top couple of shelves is not very good. 

Is this a common problem? And are there any tricks to get good reduction at the top as well as the bottom?

Thanks :)

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2 hours ago, tomhumf said:

I've struggled to fire my little ( about 3 cubit ft?) Gas kiln for just over a year. I'm finally starting to get results I like. 

One problem was what temp and time to reduce at. I've found that reducing hard between 940C and 1000C for one hour is giving me the best results for my glazes. I reduce by increasing gas and blocking the chimney to about 1" opening.

I used to then open the chimney fully to complete the firing. I've finally figured out this is why (I think) I ended up with a lot on pinholing on quite a few pots out of each kiln. I think there was a large temp difference between different locations in the kiln.

For the last few firings I have reduced hard as before, but closed the chimney to half open to complete the firing. This gives a flame from the chimney but not from the peeps like during hard reduction. Firing this way means I get hardly any pinholing - just one pot usually on the bottom shelf right bear the burner entry. 

The main problem with the last few firings is the reduction on the top couple of shelves is not very good. 

Is this a common problem? And are there any tricks to get good reduction at the top as well as the bottom?

Thanks :)

I agree with Mark. Hard to know from the description. Anytime flame is not coming out of the peeps, air and therefore oxygen is going in. Kilns always allow primary -and - secondary air in during operation. If too much air (oxygen) creeps in before mixing with the reducing environment you can locally fire in oxidation.  I need to know more about your firing, cone, schedule, reduction tim, gas pressure  etc...

here is a reduction basics video we created which has helped  those new to it.  It might be too simple, we are working to make something better in the next several months and a bit more comprehensive. 

 

 

Edited by Bill Kielb

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Thanks, I cover most of the design in the thread I link to in first post. I'll add a video below it covers most of the details in first 2 minutes. 

Basically it's one burner, not much area around the shelves for flames - they are all same size as the bottom one in video and stacked to the top. 

Not sure about pressures, my regulator is set to 1 bar I think and I adjust the needle valve on burner as I'm firing. I'm firing to cone 7 which usually takes about 7 hours. It fires fairly evenly top to bottom but slightly hotter at bottom on recent firings. 

The video was first ever firing. I'm now doing a bisque firing in about 5 hours total. 

 

Edited by tomhumf

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6 hours ago, tomhumf said:

Not sure about pressures, my regulator is set to 1 bar I think and I adjust the needle valve on burner as I'm firing. I'm firing to cone 7 which usually takes about 7 hours. It fires fairly evenly top to bottom but slightly hotter at bottom on recent firings. 

I watched the video and still have no idea of your reduction methodology. My questions would center around  what is your planned reduction schedule? Are you doing a body reduction, how strong and for how long? Since you are going to cone 7/ 8 I don’t see any reason to come out of reduction or at least slight reduction  through completion.
 

In your next video it would be helpful to see the reducing flame and see the port plug flames.  Without a gauge downstream from the metering valve it would be helpful to have a sense of pressure adjustments and some way to say how much gas is being used..  Half shelves and offsets are probably a great idea although  later in the firing no matter how turbulent you believe it is, most of the heating is by infrared radiation so allowing wares to see each other contributes to more uniform heating. Lots of interesting things to explore for sure. I would also suggest you mix up some celosia red and start firing with test tiles in various locations to give you an indication of reduction strength and consistency throughout.

Edited by Bill Kielb

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Sorry that video was just meant to show the kiln design. I've uploaded one from my firing since I started this thread. The clip is during my body reduction phase.

I fire in oxidation until body reduction phase, when I close chimney to about and inch and increase gas. I start this at around 940 °C and reduce like this for and hour, usually climbing to about 1000°C .

Then I try to fire neutral / light reduction to cone 7. In the last firing for this stage I opened the chimney a few more inches and reduced gas until the flames were just not coming out of peeps, but still a chimney flame.

I'm trying to follow advice I've read by Neilestrick although I've just realised I should probably start body reduction sooner according to him. - "

Get up to body reduction temps- cone 012 to 08- as quickly as you can while not cracking any pots and maintaining evenness. No sense wasting gas with a really slow climb if everything has been bisqued. Put the kiln into reduction at cone 012-08, and stall out the climb. Hold temperature with the reduction for 45 minutes. If you're trying to carbon trap a shino glaze, then make this a heavy smoky reduction. Then put it into a neutral atmosphere and let it climb to cone 10. Stalling it out in reduction will ensure that you get good reduction throughout the kiln, and climbing in a neutral atmosphere will give you an efficient use of gas."

The reduction was better but this time but the top fired a fair bit cooler ( about a cone) than the bottom.  I seemed to have the gas up very high during reduction this time. Perhaps I should close the chimney more to allow less gas to create the same sized peephole flames...

I'm a bit confused about what effect this would have.

 

Not sure what you mean by  "allowing wares to see each other "

I've googled celosia red but just get loads of plant stuff. Is it a glaze or something? 

Thanks

And yes I should have put my gloves on - good job I've got chefs fingers. 

 

 

Edited by tomhumf

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4 hours ago, tomhumf said:

Sorry that video was just meant to show the kiln design. I've uploaded one from my firing since I started this thread. The clip is during my body reduction phase.

I fire in oxidation until body reduction phase, when I close chimney to about and inch and increase gas. I start this at around 940 °C and reduce like this for and hour, usually climbing to about 1000°C .

Then I try to fire neutral / light reduction to cone 7. In the last firing for this stage I opened the chimney a few more inches and reduced gas until the flames were just not coming out of peeps, but still a chimney flame.

I'm trying to follow advice I've read by Neilestrick although I've just realised I should probably start body reduction sooner according to him. - "

Get up to body reduction temps- cone 012 to 08- as quickly as you can while not cracking any pots and maintaining evenness. No sense wasting gas with a really slow climb if everything has been bisqued. Put the kiln into reduction at cone 012-08, and stall out the climb. Hold temperature with the reduction for 45 minutes. If you're trying to carbon trap a shino glaze, then make this a heavy smoky reduction. Then put it into a neutral atmosphere and let it climb to cone 10. Stalling it out in reduction will ensure that you get good reduction throughout the kiln, and climbing in a neutral atmosphere will give you an efficient use of gas."

The reduction was better but this time but the top fired a fair bit cooler ( about a cone) than the bottom.  I seemed to have the gas up very high during reduction this time. Perhaps I should close the chimney more to allow less gas to create the same sized peephole flames...

I'm a bit confused about what effect this would have.

 

Not sure what you mean by  "allowing wares to see each other "

I've googled celosia red but just get loads of plant stuff. Is it a glaze or something? 

Thanks

And yes I should have put my gloves on - good job I've got chefs fingers. 

 

 

Here's my best thoughts:

  • This video looks good
  • Early reduction or  body reduction (Claybody reduction), go in about 1500 F (810C). This is a temperature reasonably above the preignition of gas, below this risks accumulating uncombusted gas and is more dangerous with respect to explosion.
  • Keep your gas pressure up, flames coming from the ports are your indication the kiln is in reduction and reasonably pressurized to be evenly in reduction.
  • Soot hampers reduction, push your kiln to soot and back off till it just disappears. This is heavy reduction for your  kiln.  Your wares will be covered with very fine char which will aid in reduction. Soot shorts it out. You will also trap all the carbon  you want, no soot needed only wastes more planet resources.
  • Judging a neutral atmosphere by eye is extremely difficult, keep the kiln in slight reduction to avoid going into oxidation in select parts of your kiln
  • Celosia red is just a copper red glaze that is a bit picky to develop the red so it ends up to be a nice visual indicator of effective reduction. These glazes, (lots on Glazy.org) all feature copper in 0.25 to 0.5 with about 3X more tin. Tin is pretty essential.
  • Offsetting half shelves allows for more uniform heating, not necessarily by convection. Later in any kiln firing, roughly 80% of the heating is done by radiation, then conduction through the shelves and lastly by convection. At this temperature air simply does not contain a bunch of heat and the atmosphere is super thin. Staggering shelves allows the wares to look at each other and transfer their energy. Staggering provides better line of sight from top to bottom .
  • Stalling is not necessary, slow down to 50 - 100 F  (10 - 35C) per hour likely plenty good. Many popular early schedules have been revised to eliminate the stall. Never go backwards either. 
  • Slow down to 8-9 Hours to even things out, five hours would be tough and quick to be even.

Suggested schedule for bisque fired stuff: Slow to 200F (93 C)  degrees, dry everything very slowly then ramp up reasonably. 400 degrees per hour (200C) is typical for most ordinary automated glaze fires so 400 - 600F (200-315 C) per hour till reduction is likely fine. The slower you go the more even this will be top to bottom. Maintain a medium reduction through to end of firing until you can repeat successfully, then begin experimenting with one thing at a time for effect with your glaze combinations.

Lots of reduction copper reds on Glazy.org

Some pics below.  Also have some carbon trapping pics, no soot involved. 

IMG_1686.JPG

IMG_1681.JPG.786b358785a621b21b2d5a82a91a843b.JPG

IMG_1680.JPG

IMG_1684.JPG

IMG_1679.thumb.JPG.41ba8fedfa48c866f3394f71b5012289.JPG

Edited by Bill Kielb

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On 12/19/2019 at 8:02 AM, tomhumf said:

Bill, I just wanted to thank you for your very generous reply. I will go through all that in detail when planning my next firing. Need some time to process everything I think. Will let you know how I get on. 

Many thanks! 

Best of luck and enjoy exploring.  The stuff in the pictures above should be something you can easily turn out time after time with a basic schedule. From there let us know what cool stuff you learn to make.

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