That Cold Top Shelf

[Rex Johnson]

My two kilns fire hotter on the bottom, per their design it's to be expected and I adjust to their inconsistencies.

That being said I'm often getting glazes that haven't matured up top, even at a cone over target.

 

Kiln 1 is a home built, about 24cu ft, two air assisted burners, down draft. Old skool.

Since this one isn't the perfect cube I expect an uneven firing.

 

Kiln 2 is the Olympic up draft with 4 venturi's at the bottom, notorious for being hotter on the bottom, tempermental and finicky to fire in general.

 

Now here's the thing. I did a firing this weekend with cone 5-10 glazes of varying ilk. Mostly Malcolm's Shino and a Temoku that have a wide range.

Most pots matured well enough, and especially nice depending on the location down under.

I fired to cone 7 to make sure I'd get everything nice and melted, including a 20 minute soak. 2265F.

 

But... the top shelf, and this has happened before, where all dull and not matured. Mostly flat slab pieces.

They had a good 4" space below the lid.

I'm thinking maybe it might be the draft of the lid's flue, OR, that maybe there should be an additional shelf covering the top shelf to retain heat .

 

This has happened before, and the glazes look like they are sandpapery, like they've been over oxidized if that's possible.

 

Your thoughts...

 

Pre-firing...

 

[glazenerd]

 

OR, that maybe there should be an additional shelf covering the top shelf to retain heat .

been doing that for years when I fire crystalline glaze.

 

Have also roped the lid to minimize heat loss.

 

Nerd

[GiselleNo5]

If it was me, I would definitely give the top shelf thing a try. But is it possible that the top coils are going out? 

 

The kiln we use fires up to two cones hotter on the bottom than on the top shelf. (Yes, new coils, all new everything). I have found that certain glazes don't complete unless they're on the bottom, while others bubble and pit because the glazes are over firing. This year has been all about getting to know my glazes. So I carefully plan my kiln loads to avoid putting certain glazes in certain areas of the kiln, even if that means wasting kiln space. 

[neilestrick]

If it was me, I would definitely give the top shelf thing a try. But is it possible that the top coils are going out? 

 

The kiln we use fires up to two cones hotter on the bottom than on the top shelf. (Yes, new coils, all new everything). I have found that certain glazes don't complete unless they're on the bottom, while others bubble and pit because the glazes are over firing. This year has been all about getting to know my glazes. So I carefully plan my kiln loads to avoid putting certain glazes in certain areas of the kiln, even if that means wasting kiln space. 

 

These are gas kilns, no coils.

[Rex Johnson]

It's a propane kiln.

Next time I'm going to try the top shelf cover. Just wanted some feedback on this.

Just seems like the top shelf is always lacking heat, especially near the flue.

Pots may have matured in some areas then thee will be this dry cold area on the same piece.

[neilestrick]

A gas kiln does not need to be a cube in order to fire perfectly. Not even close. Whoever came up with that idea is dead wrong. In a downdraft kiln, if the top is running cool, raise the bag wall a little bit. Even half a brick will make a difference.

 

The top shelf should make a big difference in the little round kiln.

[GiselleNo5]

 

If it was me, I would definitely give the top shelf thing a try. But is it possible that the top coils are going out? 

 

The kiln we use fires up to two cones hotter on the bottom than on the top shelf. (Yes, new coils, all new everything). I have found that certain glazes don't complete unless they're on the bottom, while others bubble and pit because the glazes are over firing. This year has been all about getting to know my glazes. So I carefully plan my kiln loads to avoid putting certain glazes in certain areas of the kiln, even if that means wasting kiln space. 

 

These are gas kilns, no coils.

 

 

It's a propane kiln.

Next time I'm going to try the top shelf cover. Just wanted some feedback on this.

Just seems like the top shelf is always lacking heat, especially near the flue.

Pots may have matured in some areas then thee will be this dry cold area on the same piece.

 

Oh, wow, I missed that.     Sorry, I'm operating on very little sleep today. 

[MatthewV]

You might take out a piece or two so the top layer gets extra space and air flow.

[Rex Johnson]

I've experimented with burner targeting in the Olympic. This time I changed it to have less restricted targeting aiming the flames towards the center. Then I staggered the shelves at each level at different heights to help draw the flame up. Also  mdified my simple kiln shelf flue by raising it off the top with other shelf pieces and centering the flue adjustment. Seemed to make the temp adjustment more controllable.

 

Thanks Neil, good input on both.

 

[Mark C.]

Top shelve over top work will help you-will not cure but will help raise the temp. and mature those glazes.

[Marcia Selsor]

I'd stack the middle tighter with taller pieces on the top shelf....and all of the above suggestions.

Marcia

[LinR]

I have a 7 cu. ft. updraft Olympic with 5 burners.  The instructions with the kiln said to place a 1/2 shelf above the top shelf with 1 1//2 inches to the bottom of the flue hole. I also still have a cold top shelf.  Do you put a guide cone at the top to see what temp you actually get?  When I couldn't get the top to get up to temp I formulated glazes to mature at the temp that the kiln will reach.

I have seen this particular kiln called "a hot toy"!  I'm too old to get a different one so will struggle on to work around this problem

[Magnolia Mud Research]

There have extensive discussions on Clayart over the years on how to get an Olympic kiln or (similar conversion kilns) to fire consistently even.  It is doable but not with the usual 'tricks' applied to down draft gas kilns.  The basic strategy is to keep the hot gases in the kiln longer and enhance mixing.  Having a sort of 'chimney' for the exit gases helps.  Also the peep holes closed to prevent cold air intrusion helps. 

Here is a link to the Clayart archives since December 2013 with the search term 'Olympic kiln' It will get you started:

http://lists.clayartworld.com/mailman/search?query=Olympic+kiln&submit=Search%21&idxname=Clayart&max=100&result=normal&sort=score

to search for posts prior to December 2013 go to :
http://archives.clayartarchives.com/  

 

LT

[Mark C.]

That top shelve looks like it one piece-I would use this to cover the top just under the lid and use 1/2 shelves below and spilt the layers up. I have been around a few of these and the loading configure is very key to evening it out as well. I like the tight center and lose top as Marcia suggested. Having a small stack will help as well. Keep the gas/heat in the chamber a as long as you can with the stacking and make it work for you.

That said I fire my 12 cubic updraft almost every week and just stack it with glazes that can take the bottom heat and the top coolness and am very happy with it .

[Rex Johnson]

Thanks for the Clayart links.

 

I like what this fellow Tony says:

 

"The olympic gas kiln- updraft is a great kiln in my opinion.

It is simple, well made, and delivers really nice results. I use it for cone 10 reduction.

 

The trick is to place the first  shelf about 2-1/2 inches off the floor
(split round shelf) with a one inch gap in the middle- forces the flame inward under that shelf.

Then above that shelf stack 1/2 the way up using 18 x 18 square shelves ( gives combustion room).

Midway, another round shelf split with the one inch gap, and above that more square shelves.

At the top, another round shelf split with no gap about three inches from the top of the kiln.

 

To fire, slide open your top damper shelf all the way up to 1650 degrees F.

At that point close down the damper about halfway. You need to
use a digital pyrometer to shut enough that the temp is climbing but not so
much you are stalling the kiln.

Fire in this manner up to cone 10. I usually reduce again at 1850 degrees F, crash cooling to that point.  

 

You will find with this arrangement that when you open the damper more, the top temp rises
faster than the bottom.

Closing the damper to the right point makes the bottom climb faster. I know there are a lot of negative opinions on this
kiln, but i really find it very easy and fast to fire and i really get nice reduction with it."

[LinR]

Hi,

This is very specific and more than any information that I have ever seen about these kilns.  Certainly worth a try.

Thanks for the advice.  Lin

[neilestrick]

That explanation makes a lot of sense. Gas kilns need a fair amount of space for air flow- much more than a typical round kiln would allow for. Using the smaller shelves should definitely help.

[Mark C.]

Thanks for the Clayart links.

 

I like what this fellow Tony says:

 

"The olympic gas kiln- updraft is a great kiln in my opinion.

It is simple, well made, and delivers really nice results. I use it for cone 10 reduction.

 

The trick is to place the first  shelf about 2-1/2 inches off the floor

(split round shelf) with a one inch gap in the middle- forces the flame inward under that shelf.

Then above that shelf stack 1/2 the way up using 18 x 18 square shelves ( gives combustion room).

Midway, another round shelf split with the one inch gap, and above that more square shelves.

At the top, another round shelf split with no gap about three inches from the top of the kiln.

 

To fire, slide open your top damper shelf all the way up to 1650 degrees F.

At that point close down the damper about halfway. You need to

use a digital pyrometer to shut enough that the temp is climbing but not so

much you are stalling the kiln.

Fire in this manner up to cone 10. I usually reduce again at 1850 degrees F, crash cooling to that point.  

 

You will find with this arrangement that when you open the damper more, the top temp rises

faster than the bottom.

Closing the damper to the right point makes the bottom climb faster. I know there are a lot of negative opinions on this

kiln, but i really find it very easy and fast to fire and i really get nice reduction with it."

This should help with the 10,ooo questions on how to make a trash can kiln work  that come in on a regular basis-finally some cures-get smaller shelves.

[Rex Johnson]

Updater, I did do another shino (and other) glaze firings at around cone 7+ using a staggered shelf set up and a top 'flue shelf' as discussed with some good results. Seemed to fire evenly albeit I only had one shelf of tall pieces.

Next season I'll focus on using the square shelves and a flue shelf.

Very glad to have the feeling I've got some control of this beast now.

Thanks all!

 

[LeeU]

Wow---nice piece!