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Joseph Fireborn

Upcoming 300th Firing - Kiln Repairs

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I was reading L&L's CNOS and TC offset page. They seem to think if your overfiring to adjust the TCs instead of the offset. They said the offset is usually used to change your final temp if you want to fire say: Cone 6.5 or 5.5. It is interesting. I peeped into my peep holes and I am closer. 6 is still slightly melted at the tip touching the shelf, but 7 isn't bent over completely anymore. I am going to leave the CNOS where it is and change the TC offset by 10 more degrees. I think this will put me exactly where I need to be.... Hopefully!

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When you change the TC offset, it changes it for all temperatures. When you do a cone offset, it only changes for that cone. So if you're kiln is running hot at all temps then you want to do the TC offset. If it's only running hot at cone 6, but fine at 04, then you want to do the Cone offset.

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@neilestrick That makes more sense about the TC vs Cone offset. Bisque I am not too worried if its under or over fired a cone there. So I have never checked it. I stopped bisquing for a while, but I am back doing it again since I am dipping some of my work. 

I do have some questions about the way the controller works.

If I fire to 2232F is that different than doing Other: Cone 6? Is the controller trying to measure heat more accurately with the Other: Cone 6. Compared to just firing to 2232F from 2032F at 108F/hour as to measure heat by Orton Cone Chart accurately?

I am trying to figure out if I should lower my CNOS(6) by another 10 degrees or if I should just adjust my TCs instead. I guess it depends on if the Cone 6 thing is different from 2232F. Because I have some other schedules where I fire to certain temps and hold for a period. Some of them are longer holds than others. 

Thoughts?

 

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3 hours ago, Sputty said:

When you say 'critical', how much difference do you think it really makes? I've been doing this (see pic ) for the last 35 years, in my slap-dash way (it takes maybe 30 seconds), and to be honest I've never noticed any problems at all. And I primarily fire by cone rather than digital doohickey, although I have that too.

Do you think it makes a practical difference where the clay wraps round? By what sort of margin of error? I've never really given it any thought, to be honest - ignorance is bliss, apparently! It's just what I've always done, and it's always worked (for me).

gallery_76831_1379_6526.jpg

I make them them in a slap dash manner too. I get the angle right but don't measure to check the height of the cone above the clay wad. Now I'm curious if the difference is significant or not, I'll make some up the proper way and my usual way and see if I can see a difference. Hmmm, good to know I've been doing it wrong all these years! :blink:

Edited by Min

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Well I peaked in my peeps. I arranged the cone packs so I can see them. I think I am on target now. So with the 15 degree CNOS and the 10 degree TCOS. I have some tiles to show the difference in the heat work from each glaze firing. I think it will prove interesting. But I want to get these last ones out so I can post the 3 firing glaze tiles to show the difference between 7, 6.5, and 6. It is interesting to see how much certain glazes can really change with just a little more heatwork. 

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If the tip of the cone is not 2" above the clay pat than it would not be reading according to the kiln chart for regular large cones.  If the tip is set at 1 3/4" above the clay pat than the selfsupporting  cone chart should be used. , these readings are based on 108  F ramp, or 60 C  ramp for the last 200 F or 100 C  temperature.

David

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Okay!

I didn't unload the kiln all the way. I just pulled out a tile to see if the glaze was right. The cone pack is still in there but it was perfect cone 6. The tip is just touching, not melted at all. 

The results of the three firings. I put 1 tile with the glaze that I am most interested in on each shelve. This is the top shelve tile. 

From left to right: Cone 7, 6.5 and 6. Notice the color differences and the melt difference. The cone 7 one is really runny and the 6.5 one is starting to do it, and has  white surface appearing that feels awful and bubbles and pinholes a lot... The last tile is perfectly butter smooth matte and feels wonderful in the hands. Ignore most of the large cracks. I was testing black slip variations on these tiles. Also if you look you can see the differences in the clay bodies as I over-fired. The red rock clay, turns into a muddy brown color when over-fired compared to the rich reddish brown color.  You can also see the start of bloating on the cone 7 tile if you look close.  It is much more apparent on larger pots.

59f4c6e94c004_IMG_20171028_135929(Copy).jpg.210cd7bdd65c34227b2f16827a8afdad.jpg

Thanks everyone for all your help along the way. I am happy to finally have my kiln back in full operation. Only took 3 firings to get it right. Now I can get back to focusing on making and spraying. Have a wonderful weekend everyone! Woo Hoooo!

 

Edited by Joseph F

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On 10/28/2017 at 1:07 PM, Joseph F said:

From left to right: Cone 7, 6.5 and 6. Notice the color differences and the melt difference. The cone 7 one is really runny and the 6.5 one is starting to do it, and has  white surface appearing that feels awful and bubbles and pinholes a lot... 

 

Off topic, but it seems to me that this glaze has a very narrow firing range. Stiff and matte at 6, runny and glossy at 7. The problem with a firing range that narrow is that slight variations in kiln temperature are normal, and so this glaze is going to be very inconsistent. It also makes me question whether the stiff one is making good glass. I'd prefer to see that kind of variation over 3 or 4 cones, not 1. Would you be willing to post the recipe?

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Neil:

I view the  glaze flow differently, due to the red clay body. Tried to have this conversation in the Red Clay thread. The nature of red clay body is causing the variances in out come in glaze.

1. Red clay/s have 6-8% naturally occurring flux! which is not factored into the glaze chemistry. As the temperature climbed, the flux level in the clay, coupled with the iron content (5-7%) also reacts as a flux.

2. Joseph: in red clay bodies/ slips- the sodium, calcium, and potassium molecules are chemically bond directly to the iron molecules .as the temperature rises, these fluxes directly reduce the iron- causing a shift from red to brown. Go up another cone, and it will be darker still. Judging from results, Banta or Red Art was used in this body. Combination of Red Art and Newman Red would be my guess.

3. In dark brown, grey, and red raw clay; the rule is- as the iron levels rise, so does the titanium, carbon, and sulfur content. It is the law of morphology( how it was formed). The white you are seeing is titanium bleed- gasses pushing it to the surface.

So I do not think your glaze is unstable. I do know that red clay brings a lot of it's own flux power into the mix. Same reason "Black" is having a runny glaze issue on his pieces in the "Red clay" thread. Something that is not discussed or taught, red clay bodies have their own unique chemistry that can add nearly 10% of flux before you even add glaze.
 

The white titanium is bleeding out from under the crystalline area: it is not a layered glaze.

image.png.f9668570972faa5631612a353737b046.png

Edited by glazenerd
Picture added.

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@neilestrick I felt the same way once I saw the results. I knew the glaze was overfired when I first put in the elements by the way it looked, but then I was surprised to see I was only one cone over 6. I mostly spray this glaze which leads to more uneven issues as well, unless I am careful. 

I probably will drop it for a while it's getting cold in my shed so I don't spray much during the winter anyways. Here is the recipe: It is a slightly modified version of a glaze from John Britt's Book. I ran a grid and found that I liked the grid cell 14:

image.png.3b7844092c25e9427f38f80a92b2d666.png

On a side note. It is a beautiful glaze. I might experiment with dipping it, but it won't give me the desired effect that I have been gaining from spraying. I am looking for a set of "winter" glazes currently so I can dip in my garage instead of spray outside in my shed. Running some currie test tonight/tomorrow to find a really rich iron heavy black semimatte glaze. 

@glazenerd

That is interesting stuff, but I haven't found that any of my glazes melt that much more on stoneware compared to porcelain. But you know a metric ton more than I do about all this. The body is designed only for cone 6 as the max temp. Of course I haven't gone over 6 very much in the 3 years of firing my kiln, only 2-3 times when I was experimenting with oil spots. 

It bloats at 7 even though you really can't see it in the tiles. I could take pictures of mug bottoms with little bubbles in them if I hadn't hammered them. The glaze itself is a titanium glaze. A lot of the difference between the tiles could be explained in variation of dipping. I might have dipped the first tile a half a second longer or something. Then combined with ^7 it could have changed everything up. I don't remember doing anything more than a simple 3 count, but I could have easily made a mistake. It isn't worth testing more for me as I will probably find a better glaze this winter anyways. = ) I like to do a lot of testing in the winter so that when I get up my studio is nice and warm with my kiln cooling down.

 

 

Edited by Joseph F

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Just now, Joseph F said:

@neilestrick

@glazenerd

 

It bloats at 7 even though you really can't see it in the tiles. 

 

 

Joseph:

That is the most telling feature: it takes certain chemistry for that to occur.

X amount of iron, Y amount of flux, and Z amount of sulfur. I know the numbers, but the general belief is that carbon is the cause of bloating. 

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