Jump to content


Photo
- - - - -

Uneven Temperature In Small Gas Updraft Kiln


  • Please log in to reply
19 replies to this topic

#1 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 11 February 2017 - 06:12 PM

Ive been doing pottery now for a few years  and recently bought an old small ‘test’ up draft kiln with a capacity of 2 cubic  feet and a single gas burner coming in from the side at the bottom ( an Australian Port-O-Kiln ‘Chameleon’ made about 1975)

 

The problem I am finding  is that of uneven temperature, something which I know is not uncommon in updraft kilns.

 

Bricks are used as a floor but the rest is ceramic fibre.Two shelves can fit in to accommodate the height of tall mugs or two cereal bowls stacked rim to rim.A third top shelf  is used as a baffle  to help draw through the heat.

 

The shelves have been placed at the same height (zig-zagging from back to front – again for heat flow  )for three bisque and one glaze firing .The bottom shelf  is sited just above the burner port opening. the second shelf up is approximately in the middle.

I have a pyrometer monitoring the kiln located in the upper part of the kiln with the only spyhole also located in the upper part  too.

 Reaching the projected  target of ^10 (for the  glaze firing)  proved very difficult and concerned at my previous bisque firings which showed overbisqueing in the lower half of the kiln I stopped at 1214  Celsius.

 

Cones on the two useable  shelves showed ^9  starting to fall  on the middle shelf and ^12 starting to fall  on the bottom shelf ! -so approximately a 40 to  50 degree Celsius temperature disparity.

Between the last Bisque and the latest Glaze firing  I had  reduced the size of the shelves  along the sides and  under the bottom shelf placed a small fire brick  with  it’s corner angled to split the flame. These measures were again designed to even out temperatures.

If the shelves are reduced further this would obvious decrease the amount of ware I can fire in this very small kiln.

Any suggestions to even out temperature ?



#2 MatthewV

MatthewV

    Alaskan

  • Members
  • PipPipPip
  • 562 posts
  • LocationAlaska

Posted 12 February 2017 - 02:47 PM

Work with the "problem" instead of trying to solve it.

 

One level fires hotter and some glazes will look great! The other level fires cooler and some glazes will also work great there. Once you have figured out what is possible and working, you will be happy.


Make More Mistakes


#3 neilestrick

neilestrick

    Neil Estrick

  • Members
  • PipPipPip
  • 5,047 posts
  • LocationGrayslake, IL

Posted 12 February 2017 - 06:05 PM

I respectfully disagree. There's no reason your kiln can't fire evenly. Post some pictures of the kiln, including some with the shelves in place, and we'll help figure it out. Tiny gas kilns can be tricky.


Neil Estrick
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

[email protected]


#4 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 12 February 2017 - 07:59 PM

Ive got 2 photos at hand -  one showing a bisque - the other the only glaze firing so far .The arrangement of the shelves are more or less the same as you can see . I have removed the top  'baffle' shelf in the glaze firing pic - ill send a few more pics a.s.a.p.

Attached Files



#5 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 12 February 2017 - 08:25 PM

more pics from me 

Attached Files



#6 neilestrick

neilestrick

    Neil Estrick

  • Members
  • PipPipPip
  • 5,047 posts
  • LocationGrayslake, IL

Posted 12 February 2017 - 10:17 PM

The shelves are too big. There's not enough open space for air flow. You need the shelves to be a couple of inches from the walls all the way around. I would also put in a target brick about 2/3 of the way between the burner port and the front wall to break up the flame


Neil Estrick
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

[email protected]


#7 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 13 February 2017 - 03:12 AM

Thanks Neil – I was hoping you had another solution as the shelves are very small already !

The old manual which I hope you can read  from the attachment  recommends  the shelves should off set from each other so the heat /flame can circulate – the bottom shelf away from the door but flush against the back wall – reversed for the second shelf and top baffle shelf again as the bottom shelf.

 

Are you suggesting that space should be made  all the way around and  no shelf should be butted against the back wall or the case of the second shelf, the door?

Would this apply to the top baffle shelf too ?

 

Attached Files



#8 neilestrick

neilestrick

    Neil Estrick

  • Members
  • PipPipPip
  • 5,047 posts
  • LocationGrayslake, IL

Posted 13 February 2017 - 09:25 AM

I don't think that staggering method provides sufficient air flow. You're trapping the heat down low. There's not enough draw in that little kiln to pull the air through to the top effectively. You may be able to get away with the current shelf at the bottom, but the middle shelf will probably need to be smaller. The air can't snake around all the shelves.


Neil Estrick
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

[email protected]


#9 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 13 February 2017 - 02:45 PM

It is possible that an extension to the flue might help draw the heat up too ? 



#10 neilestrick

neilestrick

    Neil Estrick

  • Members
  • PipPipPip
  • 5,047 posts
  • LocationGrayslake, IL

Posted 13 February 2017 - 09:09 PM

That is possible. What type of burner are you using- venturi or powered?


Neil Estrick
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

[email protected]


#11 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 13 February 2017 - 09:58 PM

The burner is (i think)  part of the original 1970s  kiln equipment - so not venturi more like a sleeve that slides over an aperture.

I have taken on board your good advice but im trying to have my cake and eat it at the same time  by scalloping the edges of the shelf.

This might be enough(???)and would allow me  a bit more shelf space .see attachment.

As ever with modifications the proof of the pudding is in the eating - excuse my food metaphors ,i must be getting hungry ! 

Attached Files



#12 neilestrick

neilestrick

    Neil Estrick

  • Members
  • PipPipPip
  • 5,047 posts
  • LocationGrayslake, IL

Posted 14 February 2017 - 09:20 AM

It's worth a try. However you don't want to put pots above the openings, so I'm not sure if you're really gaining much by doing that.


Neil Estrick
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

[email protected]


#13 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 14 February 2017 - 02:56 PM

Thanks Neil

I was thinking  that i needed to securely support the props holding up the top baffle shelf .

As mentioned the kiln shelves  are very small - i had even thought of  create a ledge to support the non loading top baffle shelf and substitute it for ceramic fiber board so I could remove the props with more space for ware.

The other obvious solution would be to get a bigger kiln - but  as ive just started firing for myself  it seemed a good idea to start small to get a handle on the process..

The new  pyrometer and other equipment cost way more than the kiln itself infact and these could be used if /when  I upscale .

Any other suggests grateful received



#14 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 14 February 2017 - 02:57 PM

 sorry - suggestions



#15 neilestrick

neilestrick

    Neil Estrick

  • Members
  • PipPipPip
  • 5,047 posts
  • LocationGrayslake, IL

Posted 14 February 2017 - 03:12 PM

Just scoot the posts in as needed to support the smaller shelves. I think you just need to accept that this kiln is not made to hold very much. :) It's probably meant to be more of a test kiln than an actual production kiln. Fire it up and see what happens. If it's still not getting there, then make the shelves a little smaller. Also check that you're not pumping too much gas into the kiln. Fire with the least amount of gas needed to get the job done. If you're pumping too much in then you can screw up the pressure in the kiln and affect the air flow and evenness.


Neil Estrick
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC
www.neilestrickgallery.com

[email protected]


#16 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 15 February 2017 - 04:40 AM

Thanks Neil

 ill see how it goes – maybe extend the flue too – not sure at the moment  -

A firing schedule was supplied  with the original booklet – ive been following that – The  maximum p.s.i. (as recommended ) is 3.5 at the end  of the glaze firing.

Ill let you know what happens via the forum when i get the opportunity to fire up again. 



#17 puny earthling

puny earthling

    Member

  • Members
  • PipPip
  • 10 posts

Posted 02 March 2017 - 05:21 AM

just a brief update for anyone interested - my last glaze firing experienced similar problems to the last

.I decided to increase the  duration of reduction as a way of drawing out the flame through the kiln.This had a marginally beneficial effect .The temperature rise/speed near the end of the firing was up by 20C relative to the same time of the last glaze firing.

However i pulled the plug on the firing after no increase in temperature  could be affected  at 1212C.The visible ^9 on the mid shelf  had started to drop at this point.After cooling , an inspection of the  bottom shelf showed  ^12 had started to drop also.This again indicated about a 40-50C difference.

I am planning to put in a flue  directly above the kiln.

This might again increase the draw within the kiln.This was recommended by a friend with experience in blacksmithing.

The kiln up and till this point has had a 'tin' roof shelter sited about 90cm/3 feet above it (pitched at about 10degrees) with a couple of backing walls and a lot of heat is being trapped within this shelter.This might be causing 'circulation'/pressure problems.

At least unwanted gases and smells will be driven off more  efficiently

Of course, reduction in shelf size is still a possibility but im resisting the temptation to throw many 'solutions'  at the problem at the same time if it is going to be difficult to unpick what actually works -assuming that there is going to be a single solution .

Life and pottery is a learning curve ...



#18 perkolator

perkolator

    Advanced Member

  • Members
  • PipPipPip
  • 454 posts
  • LocationCalifornia

Posted 02 March 2017 - 04:45 PM

I can't really tell from the pics, but it looks like your burner comes in horizontal, not vertical from the floor.  Where is your kiln's flue/exit/damper?  

 

Just like for those who make their own raku kilns from trash cans - many are finding the need to add an extended flue in order to get sufficient velocity/flow inside the kiln.  I can't explain exactly what happens here, but by venting more it somehow allows more efficient use of the heat energy produced.  So like your blacksmithing friend suggested - try extending the flue - perhaps some dry-stacked fire brick will work temporarily for testing purposes.

 

Take a look at this page, it will hopefully help explain: http://www.kilns.com...iringsystem.htm



#19 High Bridge Pottery

High Bridge Pottery

    Joel Edmondson

  • Members
  • PipPipPip
  • 1,884 posts
  • LocationNewcastle Upon Tyne. England

Posted 02 March 2017 - 05:43 PM

I can't seem to find the kiln book but I am sure when I was researching my 3cf~ gas kiln you need to size the length of chimney 2x the height plus 2/3 of the length of kiln to pull in enough oxygen with venturi burners. At least I remember thinking it being 3x the size of my kiln was too much and went with forced air burners instead.

 

I will try and find the exact equation and update.


youtube-logo-50x50.png facebook-logo-50.png instahack.png?1

 


#20 curt

curt

    Advanced Member

  • Members
  • PipPipPip
  • 313 posts
  • LocationWestern Australia

Posted 02 March 2017 - 09:24 PM

Just curious, have you tried firing it with no kiln shelves in it at all?

Maybe just as an experiment put in one large pot or sculpture which pretty much fills up the whole kiln, or a lot of it anyway, with no shelves at all. Put one set of cones at the very bottom of the kiln, and another set of cones on a little piece of kiln shelf on top of a long kiln prop, getting the cones as close as you can to the roof of the kiln without touching.

This experiment gets the kiln shelves and their spacing out of the story completely, and just see how evenly the kiln fires then, and whether or not you can get to temperature in a reasonable time. Maybe you have tried something like this already?

If that experiment works fine then maybe reintroduce kiln shelves one at a time in the next firings starting at the bottom, and see how the evenness evolves.

Just a thought, based mainly on the premise that their may be other issues with your kiln than just shelf placement which are causing issues.




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users