We Learned Something! (Temp. Test #2)
Okay, so on August 30, 2014 we tried another temperature test. This time we used a digital pyrometer and stuck the probe into the oven, with the cable wrapped up in ceramic fibre so it wouldn't melt.
We also used the cones again, except this time we covered them with a flower pot (like a saggar) so we could surround the pot with lots of hot coals and not worry about the cones getting broken. The thinking was that maybe maybe we would reach cone 014 if the fire/heat was a bit more protected from the cool air moving in, etc.
In just over an hour we reached 500° (4:10pm), but around that point the temperature readings started fluctuating wildly, jumping up and down in a flash by 60° or more, and then settling for a little while before jumping again.
We moved the pyrometer away from the side and towards the middle of the fire and hit 700° by 4:16 pm. But suddenly, eight minutes later, the reading dropped about 400° in one jump, and then stayed around 320° for the next half hour.
We moved the probe back to the side to see if that would have an effect, because it was clear that the fire was still going strong and that there was some reading error going on. We moved it back to the middle and lifted the tip out of the coals a bit, and hit 612° at 5:10.
And then again in one swoop everything dropped to 335°. It had now been about two hours of firing and the temperature reading wasn't really moving, so we let the fire die down to see what would happen. The pyrometer reading started dropping too (meaning the device wasn't broken).
At 288°, we closed the door and then the temperature started to climb about 1° every 4 seconds, reaching a max of 346°. Then it started dropping slowly again, and when I finally pulled the pyrometer out of the oven it took a bit of time but eventually read the correct outdoor temperature of 26 degrees.
So... foiled again! The only time the pyrometer climbed steadily was for the first hour or so, when it reached 500°. After that point, we seemed to reach that max of about 700°, but it wasn't stable.
We then did some research into how thermocouples work and found this, we may be telling:
We'd need to do more research to find exactly what happened, but we take solace from discovering that this whole theme appears to be a very complicated subject - entire books have been written on thermocouple error and the difficulty of measuring fire temperatures. See here (a very good article) and here, if you like.
The cones? Although protected in the flower pot, they still did not bend one teeny bit.
And so, one way or another, we never reached an even ambient temperature of Cone 014, i.e. 807°. And on further reflection, it's not surprising. Because this oven was designed for baking, not for reaching very high temperatures. To do that, we'd need to control the draught, build a chimney, even create a separate fuel chamber, etc.
So it's been very interesting to determine that this oven does not, in any stable way, reach temperatures higher than 700°. How high it regularly gets, we don't know, but it's not around 700°.
The pieces we put in all fired very well. Some of them were made with mud-oven clay that we refined, so they copied better and came out very well, better than the last time (with the unrefined stuff). I'll post pics and a little summary of the refining process soon.
(The first Venus - top left - was more sandy, and came out great. The second also worked well, with less sand in the mix, and the third is the original unrefined mud/clay. The three pine cones are also unrefined, refined, and refined with sand. The two yoni-lingams are quite clearly refined and unrefined; and okay, so the salamander didn't come out too well, but actually it broke coming out of the mold first.)
From here on, we'll probably leave the temperature experiments and move on to refining our breads and pizzas, and making better salamanders...
We also used the cones again, except this time we covered them with a flower pot (like a saggar) so we could surround the pot with lots of hot coals and not worry about the cones getting broken. The thinking was that maybe maybe we would reach cone 014 if the fire/heat was a bit more protected from the cool air moving in, etc.
And we of course took advantage and fired up some more pieces.
Outdoors it was a sunny 26°C and we started up at 2:55pm. Getting to 100° took a little while, but after that things moved quickly.
100° at 3:25 pm
200° at 3:36 pm
400° at 4:00 pm
In just over an hour we reached 500° (4:10pm), but around that point the temperature readings started fluctuating wildly, jumping up and down in a flash by 60° or more, and then settling for a little while before jumping again.
We moved the pyrometer away from the side and towards the middle of the fire and hit 700° by 4:16 pm. But suddenly, eight minutes later, the reading dropped about 400° in one jump, and then stayed around 320° for the next half hour.
We moved the probe back to the side to see if that would have an effect, because it was clear that the fire was still going strong and that there was some reading error going on. We moved it back to the middle and lifted the tip out of the coals a bit, and hit 612° at 5:10.
And then again in one swoop everything dropped to 335°. It had now been about two hours of firing and the temperature reading wasn't really moving, so we let the fire die down to see what would happen. The pyrometer reading started dropping too (meaning the device wasn't broken).
At 288°, we closed the door and then the temperature started to climb about 1° every 4 seconds, reaching a max of 346°. Then it started dropping slowly again, and when I finally pulled the pyrometer out of the oven it took a bit of time but eventually read the correct outdoor temperature of 26 degrees.
So... foiled again! The only time the pyrometer climbed steadily was for the first hour or so, when it reached 500°. After that point, we seemed to reach that max of about 700°, but it wasn't stable.
We then did some research into how thermocouples work and found this, we may be telling:
"A thermocouple measures only its own temperature, which often differs from the gas temperature, mainly due to radiative exchange with its surroundings. It is common for gas temperatures and wall temperatures in a room fire to differ, which means that the measured temperature can differ from the gas temperature." [Fire Safety Search]
We'd need to do more research to find exactly what happened, but we take solace from discovering that this whole theme appears to be a very complicated subject - entire books have been written on thermocouple error and the difficulty of measuring fire temperatures. See here (a very good article) and here, if you like.
The cones? Although protected in the flower pot, they still did not bend one teeny bit.
And so, one way or another, we never reached an even ambient temperature of Cone 014, i.e. 807°. And on further reflection, it's not surprising. Because this oven was designed for baking, not for reaching very high temperatures. To do that, we'd need to control the draught, build a chimney, even create a separate fuel chamber, etc.
So it's been very interesting to determine that this oven does not, in any stable way, reach temperatures higher than 700°. How high it regularly gets, we don't know, but it's not around 700°.
The pieces we put in all fired very well. Some of them were made with mud-oven clay that we refined, so they copied better and came out very well, better than the last time (with the unrefined stuff). I'll post pics and a little summary of the refining process soon.
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From here on, we'll probably leave the temperature experiments and move on to refining our breads and pizzas, and making better salamanders...
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