Potters & Sculptors - Making Rock from Mud
I am working with an Orten controller on my electric kiln. In spite of low use (<30 firings) my thermocouple doesn't appear to read very accurately. The automatic cone fire programs consistently overfire.
I am trying to get the adjustments just right to get the combination of a good soak, slow cool and actually hitting cone 6.
My teacher really recommends I soak for 30 minutes and cool at 90 degrees and hour. If I set my controller to fire cone 6 with these parameters it will flatten a 7 cone flat as a pancake not to mention flattening all of my colors along with it.
I fiddled with a few firings and found a cone 4, slow ramp, with 30 minute soak, Thermocouple offset of -10 degrees and 90 degree cool down would put a cone 5 over and leave my cone 6 at about 2:30. Colors looked pretty good but my teacher is telling me the clay body feels underfired (using a cone 6 stoneware). I would like to hit the cone 6 on the nose but not sure which factor to tweak. My controller manual isn't much help since it pretty much only gives the conversion chart for what looks like a no soak firing and my temp is unreliable.
I use a computer and fire to cone 5 then I hold for an hour. (technically this is cone 6) Though I've never used an actual pyrometric cone to test it. I guess I should. I have been contemplating fooling with the cooling but generally I get pretty good results and haven't yet been willing to experiment again as last time when I went to 6 I got lots of pitting. Good luck on whatever you do!
What controller is it? Also what type of TC is it? If you get K & S mixed up, you are going to have problems. One thing that I have found to be helpful is to have a good pyrometer & track your controller as it fires to see how far things are off & then you can do an offset. Any time you do any kind of soak it is going to have an effect on the cones, possibly going up a whole cone with the heatwork, so if you are already too hot, that will make it worse. Fallonator Products makes a good, inexpensive pyrometer to help fine tune your controller. Let us know how we can help. Jeff
Editorial note by George Lewter added 4/18/2014 -- I tried numerous times to get in contact with Terry Fallon of Fallonator this winter and failed. I even knocked on the door of his building. Eric at MIY Ceramcs and Glass said that Terry had been very ill, and didn't think he was working. So his good pyrometer may not be available. Anyone have any good alternatives other than ebay. bought 3 heavy gauge typ K thermocouples at ebay about five years ago for $3 apiece. I wish I had bought a dozen, and they could last for the rest of my life. If anyone finds a good source, please give us a heads-up.
Sorry guys a little digit slip there. I've deleted all comments about 1130 degrees F. The reply should have read as:
My orton controller also overfires in the cone six range. I do 2130 deg F with 1 hour hold to get cone 6 at the same level as my single pyrometer. My kiln is not very even heating top to bottom, consistently 1 1/2 cones cooler at top. I don't use any offsets. I program my firing schedule rather than using the "cone fire" presets. This gives me all the ramps and holds I want on both sides of the heating cycle.
I believe if you fire and you put cone 6 touching down, the heat work is properly executed regardless of speed. If the clay isn't mature at that point, then it isn't actually a cone 6 clay, unless you are talking about a very thick sculptural type work.
It's a K thermocouple, factory supplied. I've considered replacing it but I think I will buy or borrow a pyrometer first. Looks like I can assemble one for under $25 with from a TC and a digital reader, just need to match it up to a ceramic disk, figure I can insert it into one of the spy holes.
Thank you george. I also switched to the ramps to try and get better control over the situation. I was also trying to do some #13 Tomato Reds (came out brown and green) so need precise control over the cooling. I'm just really struggling with getting that top temp right.
I think I need to set up a test and observe the cones during the firing while watching my temperature read out to see if I can get a feel for the calibration error. I think I am also going to pick up some cone 5 1/2's to help sort this out. Adding a separate pyrometer should help me with the calibration.
I get pretty consistent top and bottom cone bending, especially with the 30 minute soak. Part of that is I use split shelves that are stair stepped up so there can be pretty air good circulation.
This is an interesting problem, and one that I have been struggling with for some time. My kiln is a manual 7 cubic foot one with a downward vented circulation and a kiln sitter. I used to fire fairly quickly to cone six with a top temperature on my pyrometer of 1200 C - 1210 C. However to help create a more uniform temperature in the kiln both from top to bottom and from the side into the middle and indeed between the inside and the outside of pots, I now fire at at slower rate. Also every second shelf is now a split shelf to help circulation. I now find however that the kiln tends to overfire even though the peak pyrometer temperature is still just above the 1200C.
For me, it is a very slow process to find an ideal firing cycle. One is tempted to try various changes all at once, but to really test an aspect, I find it is more instructive to make one change at a time. Whatever I do next, I find it very helpful to watch the pyrometer closely during the final part of the heating cycle. The problem however is that the pyrometer and the cones, whether in the sitter or on the shelf, don't give the same information. To be able to make some consistent reliable link between the temperature as indicated on the pyrometer and the maturity as indicated by the cones, the heating rate and soak time have to be consistent from one firing to the next. Now that I have got a fairly uniform environment in the kiln, my next step witll probably be to reduce the peak temperature slightly, maybe even by putting a cone 5 bar into the sitter.
One question I do have concerns the difference between the small sitter bars and small sitter cones. The bars are more uniform and supposedly give more consistent results. The cones however can be placed in the sitter in a way that might alllow some ajustment to trigger temperature. Does anyone have experience with this?
I have fired a lot using a sitter and using both type of cones. I would suggest that really it isn't a very controllable situation. I do think that firing slow is always better (though more expensive) I think that not only does it let the glaze mature more fully it allows any gasses to escape the actual ware. I think it will actually cost you less money as you will have less seconds. I also find that re-fires are seldom successful. Though I have a glaze that I put on top and it will often smooth out the pits, making the piece acceptable.
Reply to Christopher Cisper's message
Yes I agree with you that slow firing is better. I get much better results and less failures after having decreased the firing rate down to about an average rate of 110 C per hour. This seems particularly so with red iron rich clays.
Manual kilns pose a particular challenge. As my kiln near peak temperature even after a standard heating cycle, I often ask myself which is a more reliable gauge to use in an attempt to control the firing to the desired maturity, the cones in the kiln sitter or the pyrometer reading. Both have their inherent uncertainties, but I tend to rely more and more on the pyrometer. Its reading its subject to considerable absolute error, but it should give consistent relative readings from one firing to the next. In that case, the sitter becomes more of a safety cut-off rather than indicator of maturity. However I am hesitant to abandon completely the use of sitter cones to trigger the kiln for maturity, although I usually continue the firing with a soak.
Hi David when I was referring to control-ability it was in regard to the two different cone types mentioned in the original post. The bar type verses the triangular cone type.
Thanks for that precision. David B.
In the end I switched from cone fire to absolute temperature and just kept adjusting my Thermocouple offset (TCOS) while running a pack of cones 5,6,7 to observe the results. I now have a user program that yields a perfect 3 o'clock on cone 6. I can drop it to 6 o'clock with just a 3 degree change to my TCOS
Using Witness Cones we discovered that the Bartlett Controller in our Cress E-23 kiln always fires exactly one cone too high, regardless of the cone. This can't be fixed with a temperature or thermocouple offset as the number of degrees between each cone is different.
If we want to fire to Cone 6, we program cone 5 and achieve a perfect Cone 6 witness cone. When we want to bisque to Cone 04, we select cone 05 and achieve a perfect 04 witness cone. We have confirmed cone 07 fires to 06, and cone 017 fires a perfect 016.
There is obviously a programming error in the chip these manufacturers use. It's easy enough to work around this, just insensible. Bartlett suggested the problem might be fixed if we return our CPU card to them to flash our EPROM with their updated program.
We haven't done so yet. But it's very difficult to explain to new potters in our studio why they need to choose one cone lower.