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Rockhopper

^5 On Sitter Bent To 90* - But ^5 On Shelf Barely Started Bending

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Just finished my first glaze firing in a manual/kiln-sitter electric kiln.  Everything went fairly well, except that it appears to have under-fired.

 

Used a 5, centered on the sitter, and self-supporting 5 & 6 near the center of each shelf. (3 shelves)  The sitter cone seems to be bent correctly, to approx. 90*,  but there is almost no bending in any of the witness cones, and some of the glazes are nowhere close to the expected colors.  (Various PC glazes.)

 

Is it unusual for the sitter to register a full cone higher than the rest of the kiln ? Based on the #5's just barely starting to bend, I'm guessing I pretty-much fired to ^4.  (and wishing I had put 4-5-6 instead of just 5-6 on the shelves, so I wouldn't have to guess).

 

I'm considering re-firing the same load, with a 6 on the sitter - but not sure whether that's a good idea, or if I should wait 'til I have some more stuff ready to fire.

 

Would appreciate any insights from those that have already been down this path. 

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post-19205-0-21724700-1473045392_thumb.jpg

post-19205-0-51439300-1473045438_thumb.jpg

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We definitely bracket our cones, like you said, using 4-5-6 if we're firing to Cone 5. We also use pyrometric bars instead of cones. With cones since they're thicker at the base and thinner at the tip you can put it in just a fraction too low or too high on the cone and have it affect the temperature the kiln fires at.  

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I think you should check the temp differences of the mini cone, compared to the self standing cone. It might help you understand the differences.

 

http://www.sheffield-pottery.com/v/vspfiles/downloadables/cone_wall_chart_horiz.pdf

 

 

 

best,

Pres

Thanks for the link...  The chart shows the small ^5 as 2230*, and the self-supporting ^5 as 2118-2205, depending on firing rate - which suggests that if the temp was uniform throughout the kiln, the witness cones would have bent before the sitter cone.

 

It makes sense that the sitter cone would need to have a higher melt-point, since it's closer to the elements, and would heat faster than the middle of the shelf - but is it typical for a lightly loaded kiln (pic below) to be that much cooler in the center ? 

 

Obviously, it's going to take more than one firing for me to learn how this kiln behaves.  I guess my main question at this point is:  What happens if I re-fire these pots with a 6 on the sitter ?  Do I have a reasonable chance of good results - or should I start with a fresh batch ?

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post-19205-0-96878600-1473088157_thumb.jpg

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Another question:  How important is shelf location in relation to the sitter ?  Should I expect different behavior with a shelf an inch above the sitter, vs having one just below it, and the next one 4-5 inches above ?

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Should I expect different behavior with a shelf an inch above the sitter, vs having one just below it, and the next one 4-5 inches above ?

 

Yes.  I have a digital controller, and if I put a shelf too near the thermocouple it doesn't fire properly.

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Re-firing: you can get away with a few but realize that the whole firing process is harder on already vitrified ware. Eventually the nicest pieces will start to crack. The good ones always seem to go first :-(

Usually people will only re-fire the pieces they feel really need it and fill the rest of the kiln with new work.

 

I liked to have a shelf ~2" above the probe and would apply the same philosophy to the sitter. I don't think it matters quite as much with a sitter but consistency does lead to consistent results!

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