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Kiln Conversion Updraft Downdraft Chimney?


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#21 High Bridge Pottery

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Posted 06 October 2015 - 11:12 AM

Well I was going to make the burner port and exit flue the same size, like 1/2 inch bigger than burner. Exit flue goes to 3x3" chimney  so 9" square.

 

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#22 neilestrick

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Posted 06 October 2015 - 11:31 AM

Yes, burner port and exit flue the same size. You may want to go bigger with them, though, so you've got 1/2" all the way around the burner. Those holes should be roughly the same size as the 3x3 chimney.


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#23 High Bridge Pottery

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Posted 06 October 2015 - 11:39 AM

I think now it is about half the area of the chimney. The hole is 2.5" diameter so 4.9square. Is that too small? I thought it would be ok going from exit flue to a bigger size. Burner head is 1 3/4 inch diameter


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#24 neilestrick

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Posted 06 October 2015 - 12:30 PM

You want to make sure there's enough room for secondary air to be drawn in through the burner port. Don't crowd the burner. It's worth a try as is. You can always open them up later. In fact when you do a first light up of the burner you'll be able to see if the port is big enough or if the flame is hitting the wall around the port. Make sure you've cut away the metal around the port or it's going to get really hot.


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#25 curt

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Posted 06 October 2015 - 08:42 PM

 

Is there a reason you would fire gas oxidation instead of electric?

 

 

There are a couple of reasons but I am very interested in what others think.  

 

The main reason is that where I am at kiln elements are expensive and their installation is particularly expensive.  Since they will only last for so many firings (while the rest of your kiln will last for ages) one wants to make them last as long as possible.  The easiest way to do this this is by not using them to heat up kilns when you don't have to. 

 

Electric elements with an electronic controller gives very good control, ramp rates, holds, etc. and you often really want/need this when glaze firing.   However, the thinking is that when bisque firing you don't need this kind of tight control, so you could use gas instead, thereby saving your electric elements and extending their life.  

 

Also, I believe that bisque firing with fuel, and particularly the extra ventilation that having a flue implies, facilitates better organic burnout than electric firing decreasing the likelihood of bloating, etc.  For an old school clay recycler and native clay user like myself, that is important as I have seen the problems first hand many times!  (in fact even in brand new clay we have had bloating problems here lately!) 

 

Finally, back to the glaze firing stage, I think that fully matured clay looks different (and IMHO better) when fired in a fuel kiln as opposed to electric, even under oxidising conditions.

 

Thoughts?



#26 neilestrick

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Posted 07 October 2015 - 09:26 AM

The main reason is that where I am at kiln elements are expensive and their installation is particularly expensive.  Since they will only last for so many firings (while the rest of your kiln will last for ages) one wants to make them last as long as possible.  The easiest way to do this this is by not using them to heat up kilns when you don't have to. 

Around here, gas costs more that electricity, so firing the electric kiln can be cheaper, even with the price of elements. Not to mention the fact that a proper gas kiln will cost several times the price of an electric kiln, especially with venting issues. My 21 cubic foot electric kiln was $8,000 including all electrical and venting costs. My 24 cubic foot gas kiln was $18,000 total, and I built the kiln and burner system myself.

 

Electric elements with an electronic controller gives very good control, ramp rates, holds, etc. and you often really want/need this when glaze firing.   However, the thinking is that when bisque firing you don't need this kind of tight control, so you could use gas instead, thereby saving your electric elements and extending their life.  

Many gas kilns don't operate well at low temperatures, meaning they ramp up very quickly- risk of explosions. But even with a gas kiln that can handle it, as I said before it's cheaper here to fire electric around here. I think it comes down to costs. The digital electric kiln also gives you the option of preheating for a specific amount of time, and delayed starts when needed, both of which can come in handy when bisque firing.

 

Also, I believe that bisque firing with fuel, and particularly the extra ventilation that having a flue implies, facilitates better organic burnout than electric firing decreasing the likelihood of bloating, etc.  For an old school clay recycler and native clay user like myself, that is important as I have seen the problems first hand many times!  (in fact even in brand new clay we have had bloating problems here lately!) 

Issues of poor burnout can be handled quite easily in an electric kiln by using a vent and/or slowing down the firing a bit at the high end, or simply firing a cone or two hotter. Having fired bisque in both for many years, I don't feel that one is better than the other.

 

Finally, back to the glaze firing stage, I think that fully matured clay looks different (and IMHO better) when fired in a fuel kiln as opposed to electric, even under oxidising conditions.

If the gas kiln is fired under truly oxidizing conditions, there should be no real difference in appearance between electric and gas. The problem is that it can be difficult to fire a gas kiln in oxidation and keep it even. Every gas kiln I've ever fired wanted to go into reduction. So if you're seeing a difference, there's probably some small level of reduction at play, or it's also possible that flame contact is having an effect. As for reduction vs. oxidation, with porcelain or white stoneware clays it doesn't matter, because there's no iron in the clay to be reduced. With iron-bearing stoneware, many clays look better reduced, but you can't compare the same clay body in both types of firing. A body that fires dark in reduction will be much lighter and (IMO) less attractive in oxidation. But a body that fires dark in oxidation will be too dark and/or brittle or even melt in reduction. There are few stoneware clays that will look good in both types of firing, but there are clay bodies that are formulated for oxidation that are just as dark and toasty as the reduced bodies.

 

I think that whichever type of firing you do, there are ways to get the effects you want. I fired gas, wood and salt for 16 years before switching to electric, and I don't miss firing with fuel one bit. The ease of firing electric is awesome. It gives me more time to do the other things I enjoy in life, and more time with my family. If you like firing with fuel, great. If you like electric, great. Do what you choose. Neither is better or worse, just different. And not really that different.


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#27 High Bridge Pottery

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Posted 07 October 2015 - 01:19 PM

I would have thought bisque is the reason to go electric, easy to control the slow ramp needed for not blowing up work. I am never planning to bisque in this kiln, use my electric for all bisque and glaze reduction gas or oxidation electric.

 

Think Neil covered it.


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#28 curt

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Posted 07 October 2015 - 11:35 PM

Hi Neil,

 

Thanks for your comments, and apologies High Bridge for slightly hijacking your thread, but I do think these issues are pertinent to what you are doing and for others contemplating this kind of project.

 

I guess was thinking about all this in the context of converting an old kiln like High Bridge is doing rather than buying a new one.  You can often pick up old top loaders for just a few hundred dollars, and sometimes free if they need a lot of work.  In that scenario I guess the expenses are more about initial conversion costs and then ongoing running costs.

 

I take your point that the cost of setting up venting for fuel powered kilns should not be forgotten/underestimated - that flue has to lead somewhere!.  However, since a proper electric kiln setup also has a cowling, extractor fans, hole in the roof, external piping, etc. and ideally a kiln venting system of some kind, I am not sure there is that much difference between electric and gas in terms of this initial expense?  Am betting you have a better feel for this.

 

To replicate the kind of automated control commonplace in electric kilns in a gas kiln is very expensive I think.  There are PID controlled solenoids available for gas systems, but these seem prohibitively expensive.  If one was just converting an old top loader this would seem to be ridiculous overkill, so Highbridge I am betting your converted gas kiln will be manual all the way?

 

As far as ongoing running cost, if we set aside the issue of cost of electric power vs gas (which will differ for everyone) the significant downside to electric all else equal still seems to be that you have to replace the elements after every X firings ( x= 30? 50? 100?), and that will be a few hundred dollars each time that you would not be spending for a gas kiln.  This extra cost for electric has got to be worth quite a few firings?  Or put another way, a KwH of electric power had better be a lot cheaper than gas in order to pay for periodic element replacement.  Or am I missing something?

 

Finally, I would still be interested to know if one can have a kiln that does both?  That Is, a single kiln equipped with both electric elements and gas burners so that it could be fired using either power source.  Has anyone ever seen this kind of setup?  Highbridge I cant tell if that is what you are planning?



#29 bciskepottery

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Posted 08 October 2015 - 05:51 AM

Finally, I would still be interested to know if one can have a kiln that does both?  That Is, a single kiln equipped with both electric elements and gas burners so that it could be fired using either power source.  Has anyone ever seen this kind of setup?


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#30 High Bridge Pottery

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Posted 08 October 2015 - 06:39 AM

Not what I am planning with this one. I do have the elements from the kiln but where I am firing won't have the power to wire it up. One of the elements would run of a standard plug socket but then there is 9 missing. Don't think I would get much doing that.

 

This guy has done a mostly electric kiln and gets reduction with Bunsen burners. I bought a small kiln to test with a Bunsen burner and electric but never got round to it as I can't test in my studio building safely.

 

Some ITC coating is a good idea talked about in Bciske's post. I think it is also worth firing reduction then oxidation to 'repair' 'build up' the oxidation layer again.

 


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#31 neilestrick

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Posted 08 October 2015 - 08:12 AM

I take your point that the cost of setting up venting for fuel powered kilns should not be forgotten/underestimated - that flue has to lead somewhere!.  However, since a proper electric kiln setup also has a cowling, extractor fans, hole in the roof, external piping, etc. and ideally a kiln venting system of some kind, I am not sure there is that much difference between electric and gas in terms of this initial expense?  Am betting you have a better feel for this.

The BTUs coming out of the flue of a gas kiln are much higher than what comes through an electric kiln vent, so the simple downdraft vent systems used on electric kilns will not work. The vent system on my 21 cubic foot electric was under $1,000 including having an HVAC guy do all the work on the roof penetration. Normally in a home it's as easy as drilling a 4" hole through the wall, which brings the cost down to $400 or less. The hood and vent on my friend's 24 cubic foot gas kiln was $6,000, and that was a natural draft system, no fan.

 

As far as ongoing running cost, if we set aside the issue of cost of electric power vs gas (which will differ for everyone) the significant downside to electric all else equal still seems to be that you have to replace the elements after every X firings ( x= 30? 50? 100?), and that will be a few hundred dollars each time that you would not be spending for a gas kiln.  This extra cost for electric has got to be worth quite a few firings?  Or put another way, a KwH of electric power had better be a lot cheaper than gas in order to pay for periodic element replacement.  Or am I missing something?

Firings in my 24 cubic foot gas kiln cost me $75+ per load with natural gas, and that was 8 years ago when gas was cheaper. Last year my friend's kiln cost him over $100 per firing on propane. My 21 cubic foot electric costs me around $35 per firing in electricity, plus about $5 per firing in element and thermocouple replacement costs.


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#32 Mark C.

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Posted 08 October 2015 - 09:54 AM

I think which fuel is more a oxidation vs reduction deal. 

 

Electrics are easier to buy and install and fire-hence mass appeal-they do oxidation well

Gas kilns require more cost to install  buy and fire-I do not know anyone firing a gas kiln for oxidation wares-its most always reduction.

 

Many who make a living with oxidation wares trend to wares that mimic reduction I have noticed over the last decade.


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#33 neilestrick

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Posted 08 October 2015 - 12:34 PM

Mid-range oxidation has come a long way in the past 10-15 years, and gained a lot of respect. Prior to that is was considered the realm of hobby potters, or was 'settled' on by folks who couldn't access a gas kiln. Colleges and universities worked in gas, and therefore it was considered 'better'. Many of our historical references were also high fire, like the Japanese and Chinese glazes that have become staples like shino, tenmoku, celadon, etc. The only people that were really doing any sort of glaze work in mid-range oxidation were the commercial glaze companies, because in general, hobby potters were not doing their own glaze formulation and mixing. Then along came the Mastering Cone 6 Glazes book, which gave hobbyists the knowledge and tools to start developing their own glazes. Combined with the increased strictness of zoning rules and building codes, the cost of gas, concern for the environment, and the development of digital electric kilns, more and more and more people have started looking at mid-range oxidation firings as a legitimate way to make pots.

 

We now have some really wonderful work being fired in electric kilns that is virtually indistinguishable from gas reduction work, or that is very good without looking like reduction fired work. I think the idea that mid range potters are always trying to mimic high fired reduction work is inaccurate (this is the 'up on my soapbox' section of this post). Personally, I'm just trying to make good pots with beautiful glazes, be they reduction-looking or not. I think we need to get away from this idea that pots have to look a certain way and get to the idea that pots simply need to be good, regardless of how they're fired. Do we add granular manganese to clay bodies so they look like reduced clay bodies? Yes, and no. They do look like that, but it's because it adds a richness to the glazes, not necessarily because we yearn to fire a gas kiln. It's kind of a semantics argument, but you get my idea. How about instead of saying it looks like reduction fired, we simply say that speckles look good, regardless of how they are achieved. If someone slow cools their gas kiln, do we say they're trying to mimic cone 6 electric firings, where slow cooling is very common? Many of the old school fuel-burning folks still have a snobbery about high fire reduction, but they need to get with the program, IMHO. The vast majority of kids graduating from college and grad school will not have the ability to build or buy or set up a gas kiln. Electric kilns are less expensive, easier to install, and don't violate most building codes. I'm not saying it's a better way to fire, but simply that for most people it's the most realistic option for firing.  A friend of mine who teaches at a university has recently switched the emphasis of his program to mid range electric firing for that very reason. He will still teach them to fire the gas kilns, but he wants his students to have the knowledge to do whatever method of firing is available to them when they graduate.

 

15 years ago I never would have accepted cone 6 oxidation as an option, because I was taught that serious potters burned fuel. I wasted tens of thousands of dollars and countless hours of my time when I opened my business due to believing that mantra. Now I say do what you want to do with the resources you have. Ig that's wood or gas or oil or electric, so be it. Just do it well.


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#34 Mark C.

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Posted 09 October 2015 - 12:48 AM

The funny thing is I rarely see cone 6 wares at my shows-I might even assume its more east coast than west coast. Maybe one in 15 at best. But also I do not see much in the younger potters coming up the ranks at least at shows .I only know of two younger potters trying to make a go of it. Thats saying something as I know a lot of potters all over the west.

So for me personally at age 62 who has spent a lifetime working with glazes and kilns that I make and suppling a product that my customer base of over 40 years likes a lot and always wants more of I'd be crazy to relearn this to lower a few dollars on the natural gas bill (propane has always cost more) . Natural Gas is cheap anyway.I'm a natural draft guy thru and thru until the end.

Also for potters like me who move we find we cannot get set up again due to permit changes-I have known 3 such cases all moved out of state and could no longer get gas hookups. So they either went propane illegal or switch to electrics-all went to propane just for the record .

I do a few huge shows and there are 1-3 potters doing midrange out of 20.

I'm a reduction man until I'm dead as I have found my medium and its been a great living and lifestyle . My pots sell well because they are functional and very bright in colors and reasonable for daily use. They are not art pots-just made for everyday use.

If I choose electrics back in day my back would be toast from bending offer loading for 40 years _I built my 1st car kiln in 79 and have had another since then-its war better on the back and just working with 10 tons a year is hard enough on ones back. Car kilns for production potters make sense-keeps the back straight -There are more factors in this than one gives credit-walking around your load on a car kiln trumps any electric other than the top hat lift off that L& L makes. One does not think about this until you are down the road I'm glad I did think about it in the 70s. Wish the van thing hit me then-as that was another body break thru moment.

Mark 


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#35 Chilly

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Posted 11 October 2015 - 03:25 AM

 

Finally, I would still be interested to know if one can have a kiln that does both?  That Is, a single kiln equipped with both electric elements and gas burners so that it could be fired using either power source.  Has anyone ever seen this kind of setup?  Highbridge I cant tell if that is what you are planning?

 

Nils Lou in "The Art of Firing, AC Black" talks about putting a small bunsen burner under an electric kiln to get reduction.  He doesn't light the gas until the temp is 650C/1202F.


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#36 High Bridge Pottery

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Posted 14 October 2015 - 06:21 PM

Bought some 10mm rope to seal off the lid and chimney. Rated to about 1400c max. Pinned it in with kanthal A1 wire that Was bought ages ago. Hole in for the burner and found a broken shelf that should give more space. Should have looked at hole placement as it was right where the metal stand sat!!! Had to cut the top metal off two legs so I can slide it back a few inch.

Put a light inside and all set up looked pretty air tight. Smallest bit of light in some places.

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#37 neilestrick

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Posted 15 October 2015 - 08:10 AM

You can cut those shelves easily with a masonry blade on a  circular saw. If you cut all of your shelves with a flat side that just clears the burner port by an inch or two I think you'll be in great shape.


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#38 High Bridge Pottery

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Posted 15 October 2015 - 09:12 AM

Don't have one of those things :( I was hoping just having the bottom shelf like so would be good and the next shelf it would probably catch the edge.

 

Now I am thinking I don't have a peep hole. There is one in the top but that's not very useful. Don't really want to cut another hole in the kiln. Might go without and see how I can do. Can always try and peep in the top without burning my face off.


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#39 neilestrick

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Posted 15 October 2015 - 02:34 PM

You ca pick up a basic 7 1/4" circular saw here in the states for $35, and an abrasive masonry blade is only a couple bucks. Might be worth it if you end up with poor heat circulation.


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#40 High Bridge Pottery

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Posted 16 October 2015 - 01:43 PM

Ok, I will try without the cutting for the first firing and see how that goes. Really short on any kind of money in the bank :(

 

Hopefully get a firing going the end of next week to see if it works :D Got to make some pots to fill it with first.


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