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One Burner 10 Cubic Ft Gas Kiln?


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Thanks Bill yes that's what I was thinking you meant.  After some thought I've decided to go the welding route. I will use pipe like Fred suggested welded on to the angle iron.  I think the

Doesn't make sense to me, round shelves won't fit inside as well, less room to jam stuff in, more empty space.  Plus kiln shelves aren't so expensive, might as well build the kiln around standard size

You need to know the specific BTU rating of the burner. Saying it will handle up to 10 cubic feet can mean a lot of things. Does that mean 10 cubic feet of total interior volume, or 10 cubic feet of s

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6 hours ago, tomhumf said:

This type, natural draught I think? 

This comes up often so we will give this a go-
There are basically two types of burners,  powered and atmospheric. Powered has a blower to inject primary air into the flame, Atmospheric relies on a pressure drop across some form of Venturi to suck primary air into the the stream of gas. Both usually provide approximately a 10:1 mix of air and gas which yields approximately 3% excess 02  or more for safety. Generally about 50% of the air is provided as primary and about 50% is provided around  the outside of the burner penetration as secondary air. Some kilns have auxiliary secondary air ports as well usually located low on the kiln.

Types of draft, generally  there are three:

  • Forced draft,  likely a power burner pushing gas and air into the firebox
  • Induced draft, often induced by sucking the precise amount of combustion byproducts out of the firebox by a high temp fan
  • Natural  draft which relies on chimney height and the buoyancy of heated air to suck the combustion byproducts out

Kilns- Tunnel kilns can be a combination of forced and induced actually but regular hobby kilns are generally natural draft relying on the chimney temperature difference and height to remove the right amount of combustion byproducts regardless of whether there is a power burner or an atmospheric one. This is generally why chimney hight is the primary tuning method combined with the area or internal size for a given amount of required suction at a certain maximum btu.

The last bit of confusion is often with hoods to capture the heat, often installed on updraft kilns but certainly can be used on downdraft. Generally they are designed to suck up all the combustion products plus a bunch of dilution air and usually for gas are designed to run about 400f degrees and often use simple Bvent for flue piping.
 

Ok, think I got all that correct. So in other words if your burner doesn’t have a blower on it, it is atmospheric and likely some form of Venturi.

Edited by Bill Kielb
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Thanks for the previous messages about burners. I think I may get another but I had a niggle about the chimney diameter. I decided to spend a couple of hours reducing the internal size to 6" X 6". Tests are below in degrees C. 

The first test with open damper was an improvement with this size. With the damper at 1" though it wasn't quite as good as the 9 X 6 chimney, no matter how I adjusted the chimney height. 

So I guess I'll have to change it back although the tests aren't probably super accurate. I may do a bisque firing first to see how it copes at higher temperatures. 

Can anyone give a ballpark figure you'd expect of amount of propane for a reduction glaze firing in the kiln? It's around 9 cubic ft, fired to cone 7, around 12 hours and used around 40kg of propane I think. 

Seems a lot but maybe I'm just expecting too much from it. 


Inner layer of door bricked
Start temp 100C

Tests 9 X 6 internal chimney
0.5 bar , 
Damper fully open
After 5 mins 238

Damper 1" open
After 5 mins 397


6 X 6 internal chimney
0.5 bar
Damper fully open 
After 5 mins 264

Damper 4" open
After 5 mins 311

Damper 1" open
After 5 mins 394

Damper 2" open
After 5 mins 374

Added 2 course on top
1" damper 
2:30 280
After 5 mins 360

Took 2 course off
After 5 mins 385

Narrowed top
After 5 mins 391

1 course off
After 5 mins 393
 

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21 hours ago, tomhumf said:

So I guess I'll have to change it back although the tests aren't probably super accurate. I may do a bisque firing first to see how it copes at higher temperatures. 

In my experience it’s really tough to work through this data. My suggestion, measure the draft and work from there. We had  a similar discussion about even temperature  distribution In the kiln  and I would still suggest using actual  thermocouples data to help with speeding up that learning process.  Draft measurements to me are the easy way to figure this out for sure, guessing on early data I find difficult.
With respect to how much gas, it’s going to be tough depending upon the level of reduction and speed you fire at. Reduction levels vary a bunch and when we fire to cone ten we actually finish in about eight hours so speed is very different than yours.

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  • 4 months later...

I just wanted to update this thread with my latest results. I have been using the kiln mostly just for bisque firings since I last wrote - these have been going fine. The previous glaze firings didn't turn out very well - didn't really have successful reduction. 

I've read a bit more and found this article really helpful 

http://ceramicsfieldguide.org/pdf/val-cushing-handouts/special-extras/Cushing-HighFireProcess.pdf

Using that as a guide I started getting great results from my other tiny kiln. Basically oxidised to start,  reducing hard @900C for around an hour, then firing in a neutral atmosphere until the cones drop. I had previously misunderstood the neutral atmosphere to be just when there aren't flames from the chimney.

Now I check it with the bottom spy hole and a lighted paper on the outside, if the flame is sucked in the kiln is oxidised, the flame just stays burning up if the atmosphere is neutral, or will be pushed away from kiln in reduction.

To check this on the 10 cubit ft kiln I've made a spy hole right near the bottom on the opposite side from the burner port.

I put in 15 glaze tests (same glaze and clay on all) one at the front, middle and back of each 5 shelves. ( the top shelf was only half length so I put the end tests on props at the same height. The photo has the tests with back of shelves on the right, and arrange top to bottom as in kiln.

I put the cones at the front, middle and back on the bottom, middle and top shelves.

In the reduction phase I found I needed to tape some of the air inlet holes on my burner up to get a yellow reducing flame from the kiln, I also closed up the burner port as much as possible with slices of kiln brick. I'd be interested to know if people think these tests are over- reduced? I think I could make the reduction phase 45 mins next time probably.

The kiln was climbing pretty well in neutral atmosphere, I could probably get to cone 10 if I want. The cones are a bit uneven but I guess a slightly longer firing might even them out more. Overall I'm really happy with this test and the few pots I put in. I will probably try another small firing before loading it fully with orders.

One thing I was worried about while firing was the flames were blue from the main spy hole during the neutral phase, most things I've read says the flames should be green / yellow. It seems this wasn't a problem though as long as the pressure at the bottom spy hole wasn't sucking flames in.

This is my not very accurate firing log in time / temp C.

Glaze, cone 7

1 kiln on, damper 3"
2 50
3.17 200
4 313
4.45 450
6.15 650 damper 2"
7 777
Went to Aldi and gas lost pressure so changed tanks
8 700
9 893 damper 1" , gas 1½ bar, top 4 holes burner taped and one bottom . Burner port doors closed as possible

10 909

Tank froze, Switch tanks.
Take all tape off burner. Damper 1½" gas 1 bar, just no flame from bottom peep. light blue flame from main spy. Open burner port door. Small flame from chimney. 

10.50 1000
11.16 1065
11.36 1090
11.59 1119
12.20 1136
12.42 1165
1.03 1186
1.22 1203
1.30 1215 kiln off
 

1608825810117.jpg

1608829974798.jpg

Edited by tomhumf
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I like to scatter some copper red samples for testing, they are tough to keep reduced and tell how well you are doing quickly.  If you are satisfied with that method then this is your method. I personally think you should move into reduction with the damper and stay away from taping the primary air holes.

I also favor having a slightly positive reducing environment by verifying there is some flame at the lower spy hole I have found it to be very simple  and a decent way to minimize dead spots (reduction) The picture I posted several posts above shows a very nicely positive pressure kiln in reduction. There was a very specific reason for posting it as most folks just starting generally do not have a good concept without the picture.

I also favor early or body reduction as a big benefit to your glazes starting about 1600 F (above the the auto ignition temp of nat gas, let’s be safe)   so +900c is fine. Maintaining neutral is tough as you can lose reduction locally so going all the way to neutral has risks in my view and I never go all the way back.

I think learning to use your damper and the small movements it takes to control the reduction as well as the proper amount of gas pressure to keep a nice healthy reducing environment are key to learn. Once ya get a method for that, reduction is easy.

Edited by Bill Kielb
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Thanks Bill, I did try initially to reduce with just the damper. It seemed to not work very well, i.e loosing temp or blue flames from the spy holes. I'm right to think blue flames would give bad reduction? Maybe I just needed to crank the gas more or something. 

I did have positive pressure during the hour reduction - yellow flame from bottom peep. 

I can see you point about keeping above neutral for the later stage. I was a bit worried about having enough gas in this case so wanted to make sure I completed the firing. 

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Only thing I can say is very, very, very very tiny adjustments with the damper are often required  (1/32” as an example) to go into reduction as well as come out of it. Consider this: 1)  taping your burner primary air reduces the output of your burner meaning if it was capable of providing 100k btu, when you tape it it will only be capable of providing 90 k or something to that effect. 2) There is a perfect damper position at a given temp / gas input that will allow the most output of your burner and the least amount of wasted heat out of the kiln.

Consider this also, whatever you can do with your tape and blocking off secondary air can be done with greater precision using  the  damper and proper back pressure. It’s likely just very sensitive and new to you at this point.

So for any given gas pressure there is a perfect damper position, anything more is wasting energy and anything less is starting to reduce. By taping your burner you are saying that your burner is too large for the kiln to begin with which is obviously unlikely because you can turn the output up and down with the gas pressure.

I think at least you have a way to fire for now and eventually you will figure out what works best for you. In the meanwhile enjoy experimenting.

Most of the time when I attend a first reduction firing I watch what is being done then show this very feature often just nudging the damper ever so slightly and waiting  several minutes while the kiln starts going into reduction. The single observation or comment I get is folks exclaim, they never would have believed in the sensitivity of the damper or never have they gotten such nice flames out the spy holes or never would they have set the gas pressure as high, etc....

You will figure it out over time. It’s often a very sensitive balance and as the kiln goes up in temp slowly opening the damper ever so slightly is required to avoid being too deep in reduction and having insufficient power to finish. We probably ought to make a video of just how sensitive this can be ............ hmmm, ideas here .....

Edited by Bill Kielb
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I live and die with reduction in my Porcelain pottery world. I start light reduction around 1800 -1900F and keep it there for the rest of fire. About 2100F I dial up a bit more reduction using my oxy meter and keep that setting until cone 11 gets down 1/2 way. Then turn it off and seal it up.

When I did all stoneware I did a body reduction  (for the clay body color) and a light reduction up the rest of way until the cones went down (cone 10)

I never use my venturi air control  flaps-I am talking about natural draft (not forced air)I have left my air flaps untouced for 4 decades

I only use the damper to control reduction .

One can do it by other means but for me less is better. My results have confirmed this for 40 plus years.

I am firing low pressure natural gas with a tall chimmney to create the draft, I also fire my updaft (no chimney ) the same way -always controlling the damper for reduction.

Edited by Mark C.
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5 hours ago, Mark C. said:

One can do it by other means but for me less is better. My results have confirmed this for 40 plus years.

Once you get the hang of it then firing with a tested schedule is a thing. Body reduction for porcelain, not as necessary, Body reduction really improves how glazes behave so that would be the primary purpose. The damper always influences back pressure so primary and secondary air are affected while optimizing the draft of the kiln. Unfortunately we are talking inches of pressure influenced by moving a damper that uncovers / covers an area which means square inches which means exponential response to damper moves to influence very tiny pressures. Once folks get a feel for this I think it becomes very easy to explore real schedules and their glazes and clay. Prior to that guessing usually leads to confusion and undependable results. I’ve watched folks fire for decades with random results only to be shown the simplicity and ask why in the world didn’t someone show them this earlier. You likely don’t consider reduction much of a challenge anymore. 

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  • 1 month later...

I did take your previous replies on board however -

I've been continuing to fire reduction with tape on burner ports, as I can't get rich yellow flames just with closing the damper. I don't know if it's my kiln design or what...but closing damper enough for yellow flames means temp starts to drop - I can't increase gas anymore at this point. 

I'm actually getting a little too heavy reduction though overall by reducing hard at 900C for one hour, with 4 of the burner air holes taped.

I'm wondering the best option for lighter reduction. 

If I reduce without the tape on burners, just using the damper and the flames from peeps are blue will I still get some reduction?

I'm thinking of reducing tape to 2 holes instead of 4 next firing - it will be quite a full load so don't want to risk having it all oxidised.

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Tape less. If you can close your damper and make the temp drop, you can close it to pressurize the kiln slightly and go into reduction, yellow flames and all. It can take super small adjustments of the damper though. As little as 1/64 of an inch movement depending on how your kiln drafts.   Even for power burners where the primary air is blocked partially depend on the correct damper closure to pressurize the kiln and help ensure reduction.

Closing the damper limits the secondary air coming in as well as the primary air. Secondary air is actually a large portion of where your flame gets its oxygen in normal use. Without the proper use of the damper you are likely to get areas of the kiln to oxidize randomly as air wafts in or differences in reduction levels. The damper method is fairly precise and spyport flames are easy to see to help tune. Time tested for over a hundred years.

earliest time for reduction is about 900c for safety!

Edited by Bill Kielb
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