Uneven Firing...........i Need Help!

[soursop]

Hello all,

 

I am new here, and this is to be my first post. First off, I'm very glad there is a resource like this out there for all of us who are still working towards a better understanding of our processes and their results.

 

Ok, so here is my dilemma; I have a 10 cu ft. circular kiln- the maker is unknown(probably homemade), they are stackable "rings" of soft brick. I have 3 burners on the bottom and flat lid with an approximately 4"x4" flue on the top. I fire using propane gas. No matter what I try to do, be it a slow firing (12-14 hours) or a fast firing (8-10 hours), I still cannot get my kiln to fire evenly. The bottom has cone 11 completly down when cone 8 is melted on top. As a result, I have worked with my handicap and developed items that look good either slightly over fired or slightly underfired, but now I am producing more work of greater value and I would really like to stop working with a handicap and fix the problem. Any advice would be greatly appreciated. Thanks much.

[Ben]

Do you use a damper on top of the lid ? You may want to try it. Use something light like soft bricks.

 

AND

 

Try using a kiln shelf piece or bricks as a sort of baffle inside the kiln and below the flue in the lid.

Stilt your baffle on regular kiln posts.

 

You will have to experiment with placement.

 

Let us know how it goes,

Ben

[atanzey]

I bought my first kiln and started firing it last winter (octogon, gas, Olympic 1827). I had the same problem in reverse; I could bisque at the bottom and glaze at the top. I did a lot of archive searcing on clayart.com and eventually found Bill Schran, who teaches ceramic and has a great site:

 

http://www.creativecreekartisans.com/creativecreek_upkiln.htm

 

Check out his great information, and see if you can use any of the principles. His kiln is similar in size to yours, I think. I also exchanged a couple of e-mails with him and he was very kind and helpful.

 

I changed burners, eventually, and now I do fight overheating at the bottom. The main thing I do to control it is to control the bottom shelf separation (keep good records!) - to get the fire to move through, and not overheat the bottom. I don't have any luck changing the heating rate top-to-bottom with a top baffle, if I put it in reduction, they both start dropping at the same rate. This might be a function of the small size of my kiln, because he seems to indicate that this works for him. But an interior baffle inside below your flue also sounds like a good plan to retain the hot gasses.

[soursop]

Thanks,

 

Both bits of advice are helpful. Ben, I do use a damper but not for the whole firing, it has primarily been used to help induce a reduced atmosphere when I desired to do it; I will try it as a very logical answer. After some more research, I have confirmed that most flue holes are too big........something I didn't know before today, so damper used throughout the firing will begin with my next firing.

 

Great link atanzey. I came across the same one some time this afternoon in a bought of needing answers now, and he does have great info. Very straightforward for those of us who are perhaps better at the design/craftsmanship end of things than the technical.

 

Much apprectiation to both of you.

[soursop]

Just wanted to give an update on my kiln and firing situation.

 

I have had this kiln for years, and bought it from a sculptor before me, so I assumed the kiln was built right........ wrong. What was happening was two of the burner flames were hitting the bottom shelf by about 2/3 and one burner was hitting the bottom shelf completely, and thats all the burners it has....3. So, what I have learned was happening, was my bottom shelf was acting as a gigantic baffle; holding all the head down on the bottom shelf and not allowing enough to reach the top of the kiln.

 

First thing I tried; what was recommended to me here, as well as what was recommended by a good friend. First I started using a damper again for the whole firing......I quickly realized why I had not been using it before, because instead of a 2 cone difference from top to bottom, it was more like 4 cone difference. Second, I played with my gas pressure. Trying to fire the kiln with lower pressure to see if it would even out better.....again, it only made it worse.

 

So I knew what I had to do; I rebuilt the floor of the kiln; plugged the old burner holes and drilled new ones against the walls, and away from under the shelves. I did a bisque fire to test the new design. I fired slowly, but at one point there was as much as 500 degree difference between top and bottom!, only now my problem was in reverse; top was coming on red heat, and the bottom shelf still had some black soot on pieces (I use propane, and black soot is a result of the dirty gas when candling). By the end of the firing by tweeking it this way and that with the damper and oxygen ports on the back of the burners, I was able to even it out better, but still totally unacceptable......about 175 degree difference from top to bottom, and again, it was this new problem; top was hot and bottom was cold.

 

Now, I am going to try taking a stab at something in the middle; I have moved my kiln shelves out slightly so that they baffle the flames perhaps 1/5 to 1/4 of the incoming flame and want to see if this will help even out the temperature in the kiln.

 

I just wanted to post to let people know what I have been dealing with on my kiln, and hopefully be able to help someone else. In the end, I know my damper movements and gas pressure will play the role of perfecting it, but this slight baffle seems to be what might be needed. I'll post again when I know how this new set up will work.

 

Thanks again to those of you who offered advise.

[soursop]

So I readjusted my shelves as I stated in my last post (bottom shelves only) and got a much better firing. There was still a difference from top to bottom, but only 2 cone difference, which I know might have many of you saying "well now you're back where you started only upside down" , and you're right, but now I feel I have more control over the fire. Before, the damper did little to nothing to help me even out the temp., now I can tell it is making a big difference. I think the bottom shelves are good where they're at. I'm doing another bisque tomorrow to see if I can hone the damper controls in order to even out the firing better, but I feel this is a really good adjustment I've made and feel no more baffling is necessary.......... damper and gas pressure from here, and I'll post how it goes

[soursop]

I just wanted to make a final note on how this worked out for me for anyone interested. The slight baffle of the bottom shelves worked perfectly in combination with properly managing the damper, gas pressure and O2 ports. This second firing of my redesigned kiln was a total success, and by that I mean how even the firing was- it was nearly completely even- maybe 1/4 to 1/2 cone difference between top and bottom. My kiln climbed nicely at 300-350 degrees per hour up to 1500 and then at about 250-225 per hour till I reached temp. My damper was left at 1" open for the first 4 hours, and then at about 2"open after that until the end. I turned my gas up twice- that's it. once after 2 hours of candling and once more two hours after that when I opened the damper to 2". Besides that, it climbed very nicely on it's own until it reached temp.

[LukeM]

I’m having a similar issue with mine, reaching almost cone 11 on the bottom and 8 on the top shelf!  It’s a Skutt 1227 converted to propane with a design from Simon Leach but I think my bottom shelf is too big.  I was trying to maximize space but it has caused issues with efficiency and seems to be what I expected!  I’m going to try and notch out a section for the flame to pass through more easily and hopefully this resolved my issue!