So - I have been doing some pretty good work on the wheel lately. When I finally realized that I was using too firm clay, the centering go a lot easier!
But - even if I have wedged the clay thoroughly just before slapping it on the wheel, I sometimes get this wobble in the middle. I feel it as I am opening out, that the middle just does not want to be the middle, if you get what I mean. I am thinking (gee! WOW!) that I might not have coned the clay up well enough in the beginning? Or are some bits of clay just not willing to conform? I have seen a lot of potters throwing pretty uncentered pieces, but I wonder if the middle is uncentered or just the outside?
Once again, thanks to you guys for being around. You have the experience that I can't find from anyone else around these parts!
Marianne
PS - asked my dealer about cones, and he doesn't carry them and doesn't find them useful at all. Will try to remember to talk with my teacher about it on Friday.
Remember that the clay on the spinning wheel will want to travel outward, so you need to keep squeezing inward as you raise the walls. Wobbles can be worked around, but it's always better to spend some additional time to minimize them -- they're becomes especially painful when you're trimming :-)
I don't like being blunt but your dealer's an idiot. There's no way to measure heat-work except through the use of cones. I use a pyrometer and cones, but the pyrometer's just to give me an estimate of heat, it cannot tell me the amount of heat-work being done on the clay -- that's what cones are for. They're invaluable in repeating firings, especially when the kiln load is significantly different between firings; the cones will allow you to consistently repeat your firings (plus alot of other things). I can easily fire wihout the pyrometer, but certainly not without cones.
I agree with Peter 100% on this. Sure you can fire without cones but it is not wise. For example you stated that you soaked for 40 minutes. There is no way you can know what the heat work was that you reached without cones. As I said, I dropped my temperture by 10 degrees F to get the heat work I needed with a 30 minute soak. You said that you suspected that you were firing 20 degrees C less than what your kiln said it was doing. That is a huge amount. This is something you should know and cones will tell you.
About centering - I can't imagine someone throwing an acceptable pot if it is not centered. First off, take as much work out of the process as you can by patting to center.
I love this UTube - his clay before he even begins throwing is a work of art.
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remember to remove your hands slowly from the clay when you have it centered or else you risk knocking if off center as you move away. When you think that the piece is centered check it with a needling tool - both sides and top. If it is centered, do your initial opening. Now check to see if the opening is centered with your needling tool (you are using your needling tool the same way you do when you are checking if your are centered for trimming. If both the outer an inner walls are centered and you are getting wobbles you have two things left. When you opened up the floor you did not maintain the level of your fingers as you pulled out and you opened slowly enough that your floor is not level (really hard to do but possible). The other is that you have air bubbles in the clay. Large enough ones will make your walls uneven.
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what it all comes down to is doing it over and over and over. 5000 hours you will be a professional (that is the time estimate from Psychologist Don Norman)!
While I pretty much agree with Peter and Donna on this, I have long noticed that Europotters give temperatures instead of cone numbers. It would seem that plenty of good work is being done without reference to cones.
From my own experience, if you fire the same schedule and load, the cones are more for peace of mind. I keep detailed time and temperature logs for each firing, and even though I do watch the cone, if I look back at the logs it wouldn't have made any difference. This is probably especially true if there is to be a soak after the cone goes, since soaking is adding heat work too, and could easily reach a higher cone rating.
I no longer bother with cones for bisque firing, since it is not all that critical. (Consider the wide range that people use for bisque firings anyway.) And I only use a single number 6 cone for glaze firing... no cone pack. I have no doubt that these firings hit cone 8 by the end of the soak. So? The important thing is to be consistent.
The advantage of cones is that they automatically tend to compensate for differences in kiln load, especially the thermal mass of the load. A bigger mass takes longer to reach any given temperature, so if you are just going by the kiln temperature you know that the clay temperature will lag behind. It's just like baking in the oven: In that case you start out with the oven at the target temperature and just go by time. You know that a big turkey takes longer than a little one. If you always roast turkeys of about the same weight, you can just use time and oven temperature. If you decide to change to a much bigger or smaller bird, and you don't have a handy cookbook formula, you should consider measuring the actual meat temperature. (Here in the States, turkeys come with the equivalent of a "cone" in the form of a pop-up device that is supposed to tell you when it is done. I don't use those... I pull them out as the first step to prepping the bird. They are someone *else's* idea of "done" and they don't apply to the way I prep my turkeys or how I want them to turn out.)
Pottery is the same way, except that it can be more finicky in some cases, and if you have variable loads the time before the cone tips (or the firing is "perfect") will also be variable. If you normally fire your pots "until the meat is falling off the bones", then having cones is even less important. By this I mean that if your glaze and clay are fine with a modest overfiring, and all you needed the cone for was to prevent underfiring, then simply soaking a bit longer may be enough to compensate for different loads, etc.
Just my 2 cent's worth...
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
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Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!
I think Europeans use more advanced thermal computers than Americans do. The next one I get will also let me control the cooling process.
I have seen so many lovely pieces made without using cones in firing, that I can't quite agree with you guys. Also when it comes to reproducing a firing to bring about the same colors/effects with the glaze.
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Like wow - this guy has done this a couple of times before! Thanks for sharing.
_snip great advice_
Okay - I'm about 400 hours in now, maybe! Only 4600 more to go! LOL!
Does you kiln instructions call these venting holes? Usually venting is done with really small holes in the bottom of the kiln with a fan pulling the air out. You typically get more than enough air for this venting from the crack opening in the lid. You don't need air for firing since you are not using a flame, you just need to get rid of the nasty chemicals that pottery puts off. The venting pulls out these chemicals and circulates the heat so you get more even heating. Leaving peep holes open is going to give you cold spots where are bad and could be giving you your blisters. The 3 holes in the side of the kiln are typically called peep holes (you should use glasses to look in them) and this is where you check your cones to make sure you haven't over or under fired. I kiln should ALWAYS be monitored in some fashion especially at critical temperatures (like when you expect it to stop heating).
Just something you might want to examine a little closer.
I do not use cones on my bisque either - every time though I think of skipping them in the glaze firing, I kick myself and do the right thing. If nothing else, it is going to tell me when my wiring is going and if my glaze comes out wrong I have at least the cones to tell me if under or overfiring was a problem.
No Europeans do not have more advanced thermal computers than Americans do. Again you are talking Temperature vs Heat work. They are critically different. I don't know how else to say it other than they are correlated but not the same.
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I have seen so many lovely pieces made without using cones in firing, that > I can't quite agree with you guys. Also when it comes to reproducing a > firing to bring about the same colors/effects with the glaze.>
And yes I believe Europeans can do beautiful work without the aid of cones - I have fired simply based on the color of the kiln with a gas or raku kiln. But we are not talking about people who are not having problems and trying to solve what is wrong with their firing. If they are getting perfectly beautiful pots then they are not having issues. What we are talking about here is a kiln where you don't know what the temperature is let alone the heat work AND you are having problems with your glazes. To me it seems critical that the first thing you find out is what heat work you are getting at various places in your kiln. JMO of course but it is difficult to give advice without the proper information.
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>> Like wow - this guy has done this a couple of times before! Thanks for > sharing.>
Are we having fun yet? Hope you get a good firing soon. We need the variable reinforcement to keep us addicted...
I do know about what temperature I am firing at. It is NOT out by 20 degrees, that has been established.
I have done only 40 firings (including bisque) with this kiln, so it still is taking some getting used to.
Even in that last kiln, I had a couple of pieces turn out really great. The clue is to figure out where I can fire so ALL (or almost all) the pieces turn out lovely. I don't use just one glaze, so my milage varies accordingly. The piece that bubbled the most was a combination I was trying for the first time.
I'm sorry, Donna, but the people I know here who have electric kilns of varying sizes never use cones and they produce beautiful sculptures and crockery and you name it. If they can manage without cones, I am not going to stand on my head and run around Europe to try to find some - I will try their suggestions about temperature increases first. Even if my thermostat is slightly off, it will always be off by the same amount or percent, so that I will refire at 5 degrees more, then 10 degrees more.
In simple terms: imagine a plot of temperature against time for the firing cycle. Now draw a line across at the minimum temperature that the glaze will flow. The relevant heat work is the /area/ under the curve but above that line, and is dependent on both temperature and time. Cones reflect this because they sag faster at higher temperatures, and sag more with time at any particular temperature above their characteristic minimum temperature.
Hmm, "diplomatic" is not something I'm often accused of!
Seriously, I think the point that folks are missing is that a good kiln controller *does* take heat work into account. By "good", I mean one that follows a specified time-temperature curve, so that if you set it to (say) "hold at 1000C for an hour" at one point in the firing cycle, the controller cycles the power to the elements just the amount needed to hold at 1000C. That part is no different from the "inifinity control" on an ordinary electric oven.
But in addition to this, if you tell it to ramp at a certain rate, it can adjust the power moment-by-moment to hold that ramp rate (within some tolerance, of course). This is critical, because it compensates for light or heavy kiln loads: If there is a light load, the elements (if unregulated) would heat it up more quickly than with a heavy load.
One can debate that the controller only knows the temperature of its thermocouple, which is not the same as the pottery itself. But on the other hand, the exact same argument applies to the cone. There is nothing magical about the fact that the cone is made out of a ceramic substance and therefore holds some secret knowledge about other (wildly different) ceramic substances in your clay and glazes. The cone is just a "cheap and dirty" way to get an estimate of heat work. Before the advent of computer controls, it was the best show in town. Now, I think it is more like a security blanket if you already have a computer controller. Barring things like power outages that might confuse a controller, I you could have only one or the other the controller wins hands down, every time. After all, the cones only tell you about end-points, they don't allow you to program cooling soaks for crystal growth, etc.
But I have a big box of number 6 cones, and they make dandy sacrifices to the kiln goddess!
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
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Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!
Sorry about the mistake - I could have sworn you had said that your were firing to 1270 but thought you were only reaching 1250.... I can only blame the memory on having a virus at the time. It seems we have only led you down wrong garden paths and have been no help at all. If you ever get it figured out, please let us know for our own education. Donna
I have a computer controller. Pretty basic, but it lets me set TIME to TEMPERATURE, TOP TEMP and SOAK TIME. When it is finished soaking, it shuts down and just shows the temperature until I shut it off.
My hubby is so wonderfully supportive and I am getting a bigger kiln and a wayyyy better controller for Christmas.That controller has about 30 different times and levels to program - including cooling time, which is why I will wait until I get that before experimenting again with my more difficult glazes - see my reply to Donna on thread "Bubbly update".
LOL! I made a huge sacrifice a few months ago - a sculpture of Patrick Stewart's face/head that had turned out pretty fine, but I didn't let dry by far long enough. Busted into a billion pieces on bisque firing. The gods should be sated for another year, I think, with that one!
I said I suspected, but the firing proved me pretty wrong there. And it is a good thing that you tell me what you think I said, because sometimes I just don't say things clearly enough!
In my other reply to you, I have outlined what I will do now, and I promise I will get back to you with results when I try out the new equipment. In the mean time, I am playing it safe(er).
You too! I'm being wild and crazy - driving to the Bronx to pick up materials. I do know how to have a good time ;)
Donna
P.S. If you decide to try our way of venting here is a doit yourselfer from Sondahl
Cheap Electric Kiln exhaust I recently added a cheap kiln vent to my electric kiln. I bought some flexible aluminum clothes dryer vent and attached it to the bottom peephole of my kiln with three sheetmetal screws with added washers to keep from ripping through the thin metal. At the other end of the flexible section I used standard rigid aluminum dryer vent tubing to get it out of a window. Outside I added a small fan mounted in an A-frame of plywood (so as to protect it from rain/snow). From inside I can push the vent tube closer to the fan to create more suction, or farther from it for less. When I fire a bisque, I want more suction, to vent the sulphur fumes. You can tell how effective the venting is by how hot the pipe is getting. It never seems to get too hot to touch. No longer the taste of sulphur when I fire... Do bear in mind that in venting out fumes, you are also removing some air from the room, which must be replaced somehow. If you are in a confined area with fuel fired appliances (gas hot water, furnace, or woodstove), the draft created by drawing out kiln gases can also draw into the room some of the exhaust gases from these other appliances, including carbon monoxide. So you may need to run a vent hose into the kiln space to equalize pressure.
What a cruel thing to happen to such a sexy actor! My kiln god the blew up was a dragon - wouldn't you know that my next glaze firing was for sh!$. Maybe I will just go Bob's route and treat my cones as my kiln god...
Congratulations on your new kiln!!! Can't wait to hear the next chapter.
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