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Kiln problems

Hey I have built a miniature kiln. I have done multiple styles. I have done a rocket style. I have done pit fire style and I have  tried different variations of the same style. 
But I am unable to get the heat up high enough I have 100 fire bricks with the width of 2.5 inch and 9 x 4”. I also have some concrete bricks too, that I used at the top of the chimney.
I am aware that because I am not sealing it off with mortar that I am losing some heat but I’m scared to seal it off with mortar until I’m confident on the design. I know I could use mud but I really want to be confident with the design before I have to go clean off the bricks again. if anyone has some advice on what style I should do with the amount of bricks I have that would be great. I have local clay that I’m using that has a really low sintering point. I think it’s even lower than terra-cotta due to all of the impurities in it. I know that because I managed to get one good fire and that was when I put the wood underneath the iron grill that I used as a kiln shelf and it was just a simple up draft kiln with a grate underneath the wood so I could get some air flow. The only problem with that design was that some of the fire was coming out from the wood feeding area and not shooting up the updraft. That’s why in the current version I'm using I made the dimension smaller so that I could have a longer updraft and so I could have a longer space to put the wood and it is working great very similar to a rocket stove. There is a strong roar and any water that’s inside the wood you can see and hear it sizzling at the opening (I know I shouldn’t use wet wood) and when you look in you can see all the flames burning blue and the draft is fantastic. I’m just confused why it’s not getting hot enough for the pottery.  I even let the pots sit at the bottom and the coals surround them at the very end and then leave them in there all surrounded by orange Burning coals with a fan on them. But when they come out and I put them in water they semi  disintegrate and crack with a strong grip. I enjoyed the fire clouds that the wood coals produce. I suppose part of the reason could be that I’m setting the pottery on the bottom of the kiln without a shelf holding it up, so potentially none of the coals are getting underneath the clay. But still with my thermocouple, I was only getting a reading of 1150 f with it stuck into the coal bed. I cannot get a kiln nor attend local kiln shops. Look forward for any comments and advice.  Thanks 



Edited by Iesa
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A few points-you can make a mortar that comes off easy from fire clay ,vermiculite and sand and only as little water as you need. I suggest reading up on this a bit. That can seal up the cracks and will scrape off easy when not needed.It will also not fuze to the bricks

You need a taller chimney to create a better draw/draft-only use dry wood


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Could also keep the heat in the kiln longer by using a bit of brick, damper on the chimney top, slide over a bit, play with that to see if it alters temp reached and block all holes stacking and chimney at end of your firing.

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It is super important to draft just right amount for complete combustion else lots of heat goes up the chimney. A Damper is essential and understanding how to set the damper to get maximum efficiency from your kiln with just the right draft for what is burning in it is part of operating the kiln. As the temperature rises in the kiln there will be more draft because of the temperature rise often requiring adjustment of the damper to again minimize losses. Kilns heat mainly by radiation, not convection. You are not able to generate enough hot air to supply the necessary energy to fire the kiln to temperature using just convection. There ain’t enough heat and mass in the air traveling through the kiln to achieve high temperatures.

So minimal draft to keep the fire burning as efficient as possible (making lots of hot coals):and reducing radiation losses (and some convective) right up the chimney as much as practical without smothering the combustion.  How high the chimney and what size flue is a design requirement based on kiln size and in your case bed of coals developed to offset shell losses. Too big, too small, too high not really Goldilocks. Just right will be the one that best matches your kiln operating characteristics. Best way to conclude - testing and operation.  Do you run out of damper (wide open - meaning more size / more height or  or can you leave it fairly closed through the firing, meaning smaller opening maybe less height.

About Wet wood - simply cost energy to convert to steam and reduces the heating. Don’t fire wet wood if you can avoid it.

Edited by Bill Kielb
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On 6/22/2024 at 7:48 AM, Mark C. said:

mortar that comes off easy from fire clay ,vermiculite and sand and only as little water as you need.

This is brilliant. @Iesa you mentioned mortar and I wanted to say don’t do it, but there are a lot of places cold air could come into your kiln and it seems a little under insulated. This mix will be easy to clean off the bricks. The taller the chimney the greater the draft, more air, fuel will burn hotter. Wood must be dry as a bone. Air should come up through the fire, not just over the top of it. Hard brick takes a while to heat up, have more wood on hand than you think you need. Set the day aside. Good luck! 

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