The Egg kiln is a small clay kiln fired with charcoal, or more precisely, it’s built with a type of homemade fireclay. It can be fired in something about 3 hours, and reach about 1000 degrees Celsius, though 800 – 900 degrees is easier to reach. Instead of relying on a pipe to make a draft, it uses a small electrical fan under the kiln to make a steady airstream.
Read about the glazes here: /lumen-rubrum_glazes-750-850/
The Egg kiln in pictures: /the-egg-kiln-in-pictures-low-fired-pottery/
How to build the Egg kiln: /coil-building-the-clay-kiln/
Experiences and notes from the egg kiln testing:
A Firing Diary:
Testfire 1 – Burning for the first time:
Temperature breakdown:
900 degrees Celsius, the max temperature for this test, so I stopped here, this time, I fired for 3 hours, and closed down the kiln to slowly cool down. This was measured with pottery equipment.
Not a good-looking kiln, eh? I agree, it’s quite clumsy, and made by different fireclay receipts. It’s a test kiln, though; its first goal is to survive 20 firings without breaking apart, and so far, so good.
Preheating with tealights doesn’t work; 7 tealights did not lift the temperature above 30-40 degrees. Fired slowly with thin sticks til 150 degrees Celsius, it’s not easy to make the temperature climb slowly, when the fire chamber is directly under the pottery. After reaching 150 degrees, I turn on the fan and start firing with charcoal; throw a few big lumps in the kiln, and add more charcoal every 10 – 15 minutes. 750 degrees, now the fire starts dancing out of the pipe. Stuff charcoal every 10-15 minutes; let there be no empty areas in the fire chamber. Charcoal burns from the surface in, so 3 small lumps burn hotter than one big chunk.
The kiln has developed small cracks on its surface.
Testfire 2 – Just perfect:
Temperature breakdown:
Lumen Rubrum fired pottery (750-850 degrees Celsius), with a maximum peak of 900 degrees. The flames sticking out of the pipe are maybe 30 cm long and hard to see. Measured visually.
18.00 I start to carefully heat up the kiln.
20.30 The maximum temperature is reached, and the kiln was closed. Burntime 2,5 hours.
I don’t use a meter or cones (this is after all primitive firing), så this is what my temperature estimations are based on:
The pottery is light-red glowing, not orange, there are dancing orange flames 20-30 cm out of the chimney, no trace of blue flames or sparks. Based on this, I estimate the maximum temperature for the pottery to be 750 – 800 degrees Celsius, with an absolute maximum of 850 degrees.
Testfire 3 – heavy rain makes it a failure:
Temperature breakdown:
The kiln did not reach maximum temperature. The pottery reaches dark red, somewhere between 600-700 C°.
Heavy rain forced me to give up the fire. At this temperature, raindrops vaporize in the second they are hitting the (quite thin) kiln wall. The rain is really not doing any damage when the kiln is hot. But the heavy rain made it hard to keep the charcoal dry.
Testfire 4 – all too hot:
Last time it started to rain long before the weather forecast estimated, this time I took no chance, and I wrapped the kiln in aluminium foil, but aluminium foil not only protects the kiln from rain; it closes every crack from leaking cold air and effectively reflects the heat back to the kiln. This increased the temperature.
In two hours was all too high for my glazes. It’s just hard to slowly increase the temperature using an electric fan. Aluminium foil has a thin layer of plastic, but it was mostly intact after firing. I estimated the temperature to 1000 – 1050°C (but I think now that the estimate was all too high. Reaching 950°C is hard work in this kiln).
Temperature breakdown:
“I don’t use a meter, but I have a temperature pistol measuring up to 800 degrees Celsius, and the kiln was hotter than the device could measure. I fired with charcoal, and the flames sticking out of the pipe were up to 50 cm long, the flames were visually strong and mainly orange, but also with white and a few light blue flames. The pottery in the kiln had passed red, and was now glowing orange”.
Testfire 5 – all too hot, again
Maximum temperature:
The pottery goes red, to orange, to what I would describe as on its way to white. But my temperature gun only says 880 degrees Celsius (are the measurements correct?). In the fire chamber, the charcoal looks even more intensely white. The flames don’t burn so lively out of the pipe as other firings, maybe I stacked the kiln differently this time? Reducing the passage to the pipe? With forced air, I really don’t rely on draft. The flames out of the pipe are 30 cm high, visually strong, no longer red but I would say yellow, with both white and blue flames in it. There are some sparks continuously coming out of the pipe. Not many, but they are always there, like 5 – 10 sparks all the time.
Asking ChatGPT; “cheap IR guns are notoriously unreliable above ~800–900 °C, and they are not calibrated for glowing pottery, so 880 °C is almost certainly an undercount. Pottery going from yellow to white, and yellow flames out of the pipe with sparks, tells a story about 1050 °C” (no, I just learned sparks are common at low temperature depending on the quality of the sharecoal, a short kiln and air-in driven by an electrical fan adds to this story). It’s hard to say how hot it was going.
The camera notoriously records the wrong colors of fire.
Now I start to get a grip on things! I started the fire with thin sticks til the temperature reached 100 – 150 degrees, and from there I continued with small chunks of charcoal and more sticks for an hour, the pottery was then about 300 – 400 degrees, and it was time to start the fan to speed up the temperature.
I could not hear any cracking this time, and managed to heat the pottery slowly til it passed 150 degrees. This is how I will burn from now on! When I open the kiln, I see the first plate has taken some beating, protecting the other objects, something to remember.
Testfire 6 – Failed reaching 1050C
This time I used a meter, and 830 degree Celsius was the maximum temperature. I learn it’s absolutely not uncommon with sparks out of the pipe at 7-800 degrees C. This time it didn’t burn intensely; it doesn’t burn as hot as it did before. I have rebuilt the top of the kiln; maybe the flue opening are too narrow? Stacked too tight? I know that could make a difference.
Could it be that I have never burned as hot as I estimated?
But my glazes boiled off, so… OK, I need to start using a meter.
Testfire 7 – Hard work reaching 960C.
Temperature breakdown:
“Reaching 800 degrees is simple: charcoal and a fan fix this. Everything over 830 is hard work in this kiln. This time, I measured 960, and the last 100 degrees was a fight.
Sparks like in the picture? It’s the type and quality of charcoal, more than it describes the temperature.
Ok, I have to reconsider my manual temperature estimates. I adjusted the fan perfectly and wrapped the kiln in aluminium foil. The temperature meter shows me that the length of the flame has nothing to do with the temperature inside the kiln. Since the kiln has thin walls and limited isolation, the temperature quickly drops 100 degrees in a few minutes when things are not perfect.
Firewood briquettes are not as good as charcoal, but I used them for the first 400 degrees. I did not see ash from the briquettes fill up the fire-chamber and limit the airflow and important space for fuel, making it a less effective kiln.
I learned one thing, though: When the fire inside your house-woodstove is reduced to embers, you can use these embers to slowly heat your pottery kiln. This is a safer way to lift the pottery over 150 °C, without using flames.
A Firing Diary – woodfireceramic.com