Downdraft gas kiln conversion

[soarbc]

A year ago I posted an obscure blog article of two very simple gas downdraft conversions that have worked incredibly well. I have had some very nice thank-yous and I thought I'd make it available to a wider audience. It is an evolution thanks to many other people and is an inexpensive build process accessible to most. The kilns are easy to fire and get excellent reduction. 

Here is a link: http://www.sebastianmarkblog.com/2018/07/gas-kiln-conversion-downdraft.html

 

Cheers,

Boris 

[Min]

@soarbc, thanks for sharing your kiln conversion . I like how clear you made your instructions on how to do this.

[C.Banks]

25 minutes ago, Min said:

@soarbc, thanks for sharing your kiln conversion . I like how clear you made your instructions on how to do this.

agreed!

clear and comprehensive

The use of kiln shelves and burner ports in the floor makes a lot of good sense.

I expect more than a few of us with dreams of converting old kilns will look to this design.

[neilestrick]

That's a great design. The biggest problem with these conversion kilns is that people want to load them up like they would an electric kiln, which doesn't work. Like you said, it has to breathe. Yours is a clean, simple solution.

[Sorcery]

Boris, 

I'm the guy that emailed you about the kiln a couple days ago.

Here is my chimney. Dry fitted and ready for mortar!

Hope to get er done this week!

 

Sorce

[oldlady]

how will the old element grooves affect the firing?

[soarbc]

Hi Sorce, I will copy my comments here and I must say I am crossing my fingers and hoping for the best.

You did an amazingly good job cutting and fitting those bricks, very nice. As the flue area is essentially the same, you shouldn't get restrictions there. 

But I do have one concern. The shelf is not a solid 1/2" to 5/8" kiln shelf, it is hollow with air holes. This might act as a bit of an insulator between the kiln area and the flue. The kiln works without a chimney because the flue wall gets red hot heating the flue gases and helping the draft suction. If the your kiln stalls (assuming your using a MR-750 burner, etc), it'll probably because of this reason and at least it won't be terribly expensive to buy some solid shelves to cut and fit into your ready-made slots.

 

Or, you may find that the shelf has enough heat flow through it to keep the draft up. I hope this is the case.

 

And to 'Oldlady' concerning the grooves. The groves do not seem to affect the gas flow in any way.  Walford Campbell has been firing the 23x27 kiln (converted electric) for quite a while now and it works very well. 

 

Cheers,

Boris

[Bill Kielb]

8 hours ago, Sorcery said:

Boris, 

I'm the guy that emailed you about the kiln a couple days ago.

Here is my chimney. Dry fitted and ready for mortar!

Hope to get er done this week!

 

Sorce

Nice work! You can always tune your chimney based on the draft and how much pressure you get out of the lower spyhole and the level reduction you can maintain. Taller chimney induces more draft, Albeit a removable one.  The level  of reduction and total power you can produce during heavy reduction will be very dependent on the amount of primary air your burners can provide and the size of the secondary air inlets as well. In recent testing with the MR series we found the primary air shutters to be amazingly sensitive during heavy reduction. To the tune of half turn sensitivity. (Never would have guessed)
After this experience we theorize that this may be the best way to fine tune the primary air. An O2 probe helps but tuning by rate of temp rise is very doable with patience and a pyrometer.  The tuning results are dramatic and are  easily observable. The thought is to optimize these air shutters probably in the 1900-2000f range during reduction. This provides the maximum power possible at the top end. This is a nice thing to optimize to reduce stalling. Most kilns slow down to 50-100 degrees per hour (60 degrees per hour is 1 degree climb per minute) at best at .7 (heavy) reduction level at the very top end. If you can get that, in my opinion it is equivalent or better than most I have seen.

[Sorcery]

29 minutes ago, Bill Kielb said:

.7

Forgive my ignorance, is this an oxygen probe number?

Thanks for your information.

I spoke with @soarbc about the hollow shelf, he asked me to mention that this is untested, his design uses a solid shelf.

Says Max Operating temp for this shelf is 2336F but/and says cone 10. Everywhere I read 2336F is barely ten, if not nine. I forget.

Looking for users of these shelves.

Getting shelves from Malcolm up in Minnesota for this kiln, thinking about getting an extra for this flue wall.

I'll try to document everything as best I can.  I think it can encourage folks to make this convert with the materials they can source.

I've made some tests using this Sairset mortar, some hold, some don't. Hoping to get these bricks laid during a humid spell to best slow the drying.

Gotta fire a birthday shotglass for my oldest by the 12th! Looks like we'll be drinking in! 

 

Sorce

[neilestrick]

If that's a Corelite shelf it'll be fine.

[Bill Kielb]

49 minutes ago, Sorcery said:

Forgive my ignorance, is this an oxygen probe number?

Thanks for your information.

I spoke with @soarbc about the hollow shelf, he asked me to mention that this is untested, his design uses a solid shelf.

Says Max Operating temp for this shelf is 2336F but/and says cone 10. Everywhere I read 2336F is barely ten, if not nine. I forget.

Looking for users of these shelves.

Getting shelves from Malcolm up in Minnesota for this kiln, thinking about getting an extra for this flue wall.

I'll try to document everything as best I can.  I think it can encourage folks to make this convert with the materials they can source.

I've made some tests using this Sairset mortar, some hold, some don't. Hoping to get these bricks laid during a humid spell to best slow the drying.

Gotta fire a birthday shotglass for my oldest by the 12th! Looks like we'll be drinking in! 

 

Sorce

Yes, 0.7 is a typical “potters” heavy reduction number from the typical 02 probe. It’s really a measured voltage or output from the probe that potters have translated into a low, medium, high reduction range.

An example below. Note the readings strength are relative to temperature as well. As inexact as it might seem it’s very repeatable and definitely  more consistent than firing by eye.

As far as the shelf working, I believe they are rated cone ten but something to double check. Although, not sure how much physical load will be on it so failure is likely pretty low risk. Since it slides into position it likely has decent room for thermal growth. As far as it being an insulator, I would speculate it will work fine as after red heat most heating in the kiln is done by radiation so not a bunch of air in the kiln and even what is left has molecules spread so far apart they just don’t contain much heat with respect to convection. Radiation is king from 2000 degrees on up. As far as less mass, I am all for it, as no matter how many times I fire heavy shelves they look no different. They still look like shelves and  just consume more energy.

[Sorcery]

25 minutes ago, Bill Kielb said:

Yes, 0.7 is

That's pretty slick!

How do you connect the 02 reader to the computer?

I have been hesitant to explore 02 probes for fear of needing it forever, though I am quite interested in the further efficiency it must bring.

Sorce

[Bill Kielb]

54 minutes ago, Sorcery said:

That's pretty slick!

How do you connect the 02 reader to the computer?

I have been hesitant to explore 02 probes for fear of needing it forever, though I am quite interested in the further efficiency it must bring.

Sorce

It’s all PLC stuff ( Industry) so it’s just calibrated to the voltage output of the probe. The monitor we created is for teaching and probably for serious experienced potters. As far as which parameters to monitor, obviously oxygen is a big one but firing rate is huge, recording the firing data precisely leads folks to repeat or learn more. So the touch panel monitor is just a  very fancy tool. An extremely productive one at speeding up the learning curve though and helps folks learn in a more uniform way. Lots of sometimes good and not so good info around reduction firing or gas kilns in general. Often just because it’s hard for people to observe things in real time so they pick what seems to work for them most of the time.
Now of all that the O2 probe is super handy but can be read with a simple voltmeter and chart in hand. Most economical ones I have seen are the oxyprobe. I’ve purchased several from Axner now and really don’t need the meter that comes with it so they give me some minor deduct which makes it a little more affordable. The tool has proved so handy that at one studio they had a fixed probe for reduction and we made a removable probe for soda firing all hooked to one monitor.

heres a nice cross section of carbon trapping and well reduced stuff. Do this every time, all the time.

[Sorcery]

8 hours ago, Bill Kielb said:

heres a nice cross section of carbon trapping and well reduced stuff. Do this every time, all the time.

 

Beauties.

I mostly make bonsai pots, so frost proof is necessary, and I fear carbon coring a bit.

I have read a lot about gas kilns and have found some of the backwards truths.

Been around boilers and chimneys my whole a life so I have a pretty solid foundation of knowledge to build on.

I greatly appreciate your help!

As screwing this up simply isn't in the cards!

 

Sorce

[Sorcery]

Flue fits.

Brick is still a bit damp after drying covered under tarps all day yesterday, weathered the storm last night with just a quarter size puddle on my direct kiln cold cover.

 

Sorce

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[Mark C.]

That coralite shelve should work fine in this application .

I have made one of these kilns out of a larger oval kin a few years ago as a salt kiln-used a few times before its dead-It was a workshop I did on Molokai in 2018

It may still be working -I just have zero info on it now.

4 burner oval-propane.The salt is hard on stuff.

 

[Sorcery]

Is using softbrick for posts a bad idea?

I looked up the lowest rated Mpa of a firebrick I could find which was 145lbs per square inch. Cold.  

With 120lb of shelf and if with wares it's double, I'll be at 240, and ok if they don't melt.

Trying to go to Cone 6 tomorrow but wanna hit cone 10 later. 

Experience?

Thanks!

 

Sorce

 

 

[Mark C.]

Are you talking about cone 10 or cone 6 as many here fire to??.

since the shelves are old silicon carbide my guess is gas fire cone 10.

Soft bricks compress over time.Hard bricks not so much. Soft bricks crumble hard bricks not so much

I think its great idea as you will learn from my alma mater the School of Hard Knocks.(I got a PHD from there)  Here is where learning is best as mistakes take that testing curve to another level. Big hint--- They call them soft  bricks for reason.

You can do that on the Top  layer but the bottom is asking for issues. May work for a spell. let us know the outcome .

They did that with a tower in Italy once back in the day I think .

[CactusPots]

I have a set of hard bricks that are cut long ways for my bottom posts.  I think they are called "soaps".  Very stable for a 4' stack.

[neilestrick]

Do not use soft brick for shelf supports. I've heard a lot of horror stories over the years about them crumbling or shearing off during a firing. Proper shelf posts are inexpensive.

[Sorcery]

I assume not small posts yes?

Thanks!

Sorce

[neilestrick]

2 hours ago, Sorcery said:

I assume not small posts yes?

Thanks!

Sorce

For stability, the 1.5" wide posts are best.

[Sorcery]

 

10 hours ago, neilestrick said:

For stability, the 1.5" wide posts are best.

Thanks!

I went and got proper posts and restacked. Good fer it. Better cone packs. Better gas movement!

Fire tomorrow!

Sorce

[Mark C.]

 

 

More than you want to know on posts/stilts

Soaps are bricks cut in two the long ways -9 inches tall. They can be wet diamond sawed down to any other height (I use them in salt kiln in many lengths) as thet are super tough in all sizes.

I also have those  9 inch soaps in recrystalized siilicon carbide (same material as Advancer shelves )_I bout a slew of them at a speciality place out of state I have mentiond here before who buys up deals on materials at steel mills and ceramic industrial places that either go under or sell out overstock. I paid 2$ each for those soaps. They are supewer dense and heavy and do not work well in salt as salt is atracted to silcon carbide-they hold heat a longer time that all my other refractories due to densitie.They do not cut well at all as they are to dense. diamond saw is not enough to cut them withour killing the blade.

I like the 2 inch triangle posts  that Thorley made for decades but laguna bought out the manufacture  some time ago and a few years ago they say they broke the equipment to extrude that form-since then its 1.5 inch squares. I have enough of those triangles to load two kilns still. The reason I like them is they fit around pots better than a square form. With the 2 inch they are better support in my car kiln as well-where often the stack is over 5 feet tall.The car rolls so stability is a key factor as well.Over 40 years of firing many of my stilts have gotten small from all the weight and heat. I  wet diamond saw them down to next size when they get motre than 1/4 inch smaller.

I also in the 90s had special stilts made 5.1/4 inch for kiln loads of slip wares for another business I was partner in.Thorley did this for us.

I really love the 2 inch triangle size. They seem more stable as well. I snap them up wherever I can find them even bought some on e-bay once.

The small posts are for small hobby kilns-I have a few of them I use if needed on stop shelve to cram more wares into/around but they are prone to collapse on bottom of stack at cone 11. Those  are 1 inch series

Most posts are either 2 inch or 1.5 inch series.Smaller kilns with short stack can use the 1 inch series fine. Low fire slip  bunny making was king at one time and these posts where the rage back then.Now with cone 6 electrics 1.5 is the norm.

Us old school high fire dinosaurs seem to prefer all 2 inch series  posts as all my friends use them as well.

oh ya I have seen a few soft brick post disasters  in my time but none where from me-I knew better long ago.I do rest cone pads on various thickneses of soft bricks to line up with my spy plugs.Never had one collapse yet-but theres still time as one lifetime is not enough to learn it all with ceramics.

[Sorcery]

 

Sorce