This is the first kiln my husband and I built. The model we followed to the letter is of Joe Finch, as he explained in his book, "Kiln Construction: A Brick by brick Approach".
We have fired this kiln 3 times so far, and every single time it has gotten stuck at 550-600 degree celsius.
About the Kiln:
The Gas Kiln has an inner dimension of 1 cubic meter. The wall is made of regular insulated firebrick + 4" ceramic blanket +outside layer of firebrick again for more insulation. It is a downdraft kiln with two burner ports on either side. Our chimney is about 7 feet tall and the exit flue is equal to the area of the burner ports.
We initially did two firings with small burners (model S20) with a Btu of 30,000-83,000 per hour. And the next model (S80) we took for our third and last firing with a Btu of 120,000-336,000 per hour. I have attached a picture which has the list of burner models so you can see all their specs.
In India for commerical firings, we get 19kg LPG cylinders, and we are using two of those simultaneously for our gas firing. To keep the pressure from falling too low, I douse the cylinders in room temperature water when they start to ice up in the bottom.
For all the three firings we have done, the temperature always slowed down at 560 degrees celsius, and then stuck around 580 degrees celsius. The first time we kept firing for about 3-4 hours, but there was no progress, and the last time we stayed at the temperature for 1 hour before calling quits. I have added the firing schedule below for the first and third firing so you can see how the temperature had risen. All the numbers mentioned are in celsius.
First firing (Sunny day, Third firing (Sunny day,
not much wind) less wind)
1.30 pm 82.1 11.47am 215
1.51 pm 191 12.05pm 364
3.32 pm 279 12.09pm 400
4.42 pm 400.5 12.28pm 533
5.36pm 504 12.51pm 588.6
6.47pm 551 1.46pm 562
As you can see, we took less time to reach 550 degrees in the third firing with our bigger burners. But all three times, the temperature got stuck at around 580.
I have added pictures of our burners position as well, and we made sure they were outside of the kiln to pull in the air and burn the oxygen.
Based on these figures, do you think:
- Our chimney is too small in height?
- Burners are not enough?
-Some other factor that is stopping the temperature from rising?
Grateful for any advice and suggestions.