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Liam V

Olympic kiln - bartlett v6-cf, did my cones get overfired?

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So I recently got a brand new olympic kiln and the guy who sold it helped me do a calibration firing. I believe these are cone 05 witness cones and the kiln was fired to cone 05. I'm still waiting on their response, but in the mean time, I was wondering if anyone here could give me an expert opinion. They look overfired to me, however, since this testing I've done a bisque firing to cone 06 which seems fine so far and a cone 10 glaze firing (It only reached 1270 degrees celcius before completing; I think cone 10 is closer to 1300 degrees celcius).

I've also noticed that I can see the heat inbetween the gaps of the kiln lid and around the plugs, not sure if kilns are meant to be completely sealed (I imagine this is why my kiln didn't quite make it to cone 10 before giving up)

I have yet to open the glaze firing as its all still cooling down.

The first image is of the cone from the bottom shelf and the second is of the cone on the middle shelf.

 

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67706516_2444293715802227_4059339063654612992_n.jpg

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Liam seeing heat from under lid is quite normal.

What do you mean it finished at 1270?

What was your firing schedule because that will affect the behaviour of cones..heat work done etc and so the temp the cones bend at.

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@Babs The v6-cf comes with a few default schedules, I used the one labelled 'fast glaze' so I'm not too sure about the firing schedule.

 

@neilestrick would I be fine with foregoing the offest and just leaving it as is? I've noticed all my pieces crazed, but I think that's related to the clay body used rather than the slight overfiring.

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58 minutes ago, Liam V said:

@Babs The v6-cf comes with a few default schedules, I used the one labelled 'fast glaze' so I'm not too sure about the firing schedule.

 

@neilestrick would I be fine with foregoing the offest and just leaving it as is? I've noticed all my pieces crazed, but I think that's related to the clay body used rather than the slight overfiring.

Overfiring can cause a slight bloat in some of my bodies (I'm looking at you red clays), and when it bloats it does cause glazes to craze that normally wouldn't.

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I haven't noticed any bloating, the glazes look fine as far as I can tell, however I have very little experience with kilns and glaze; I've spent the last year doing wheel throwing only.

These are cone 10 glazes and clay bodies, once I get through everything I have left, I'm going to swap to cone 5 or 6. I'm hoping I'll lucky dip on a combination that has the same shrinkage rate, not too confident with changing the temperature offset on my kiln just yet.

 

 

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Generally when glazes craze on a particular body they are not a close enough match in expansion characteristics of the body and the glaze.

A thick application of the glaze often exacerbates this problem where a glaze that seems to fit  all of a sudden in thick spots does not.  I mention this because your glaze appears most crazed in the thick areas of your pictures. If these are studio mix glazes then there is the possibility that they were mixed slightly differently.  This mix issue can  occur in commercial glazes as well on occasion.

In general if a glaze does not fit well enough with the body it will craze at some point in its life. Often years later. There are tests that shock from cold to hot or vice versa and they often will provide a reasonable expectation about the future  fit, but there are no absolute practical tests potters can do  presently to accurately determine when crazing might happen in the future.

Fortunately once you match a glaze to a body (Get workable combinations)  they generally are solid and workable for years so I don’t want to make this sound impossible.

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Isn't 1270C well short of cone 10? How did your cone 10 witness cones look - any bend at all in the guide cone (cone 9)? If you are reading 1270C on a pyrometer, well, that would be a guide and/or relative indication; cones tell the story. Place a cone pack on each level. 

Doesn't take much to push a cone from the proper bend to bent all the way over; more than slightly overfired would be slumped all the way to the kiln shelf, and very overfired - a pool of glass!

Would not a slightly overfired bisque result in lower absorption, hence thinner glaze layer? I'm checking specific gravity on my glazes now (see Tony Hansen's articles and video on the subject), each and every time; to limit the thickness, I'm running wetter now, ~1.41 to 1.46, depending, and "thickening" back using vinegar or Epsom salts. I get a reasonable glaze layer that behaves well (doesn't run and drip) without having to rush.

...good news, your kiln should perform just fine for mid fire (cone 5/6), which will be a whole new round of tests and trials; any reason to wait on the transition (how much cone 10 clay do you have; how much time to mess with cone 10 glazes)?

I'd be interested in cone 10 if I had a good gas fired kiln; I have an older electric, hence mid fire is the way to go for me. In electric, a newer kiln that's rated higher than cone 10 would be required (for me) to pursue electric high fire...

The craze lines depicted are far apart, hence, you are close to a fit; how far apart are the craze lines on the inner surfaces?

Any road, a year and a half in, I'm still more focused on throwing than glazing and firing! At least half of the raw clay pieces end up in reclaim bucket when it's time to bisque - as it should be, eh? That said, glazing is a skill, takes practice to get better; firing takes experience as well, onward!

Aforementioned article and video

https://digitalfire.com/4sight/library/thixotropy_and_how_to_gel_a_ceramic_glaze_73.html

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6 minutes ago, liambesaw said:

Just looking back at your cone picture are you sure that's a cone 10?  I usually see pictures of cone 10 cones being a glossy white and yours looks more like a bisque cone

That's an 05 in the picture.

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2 hours ago, Hulk said:

Isn't 1270C well short of cone 10? How did your cone 10 witness cones look - any bend at all in the guide cone (cone 9)? If you are reading 1270C on a pyrometer, well, that would be a guide and/or relative indication; cones tell the story. Place a cone pack on each level. 

At a slow final heating rate cone ten bends approximately at 1250 C. Cone packs good idea! Cone 10 electric kiln getting to cone ten faster than 40C per hour, probably not likely. Maybe only brand new and lightly loaded. So 1270C seems appropriate but agree with @Hulk hopefully you start working at cone 5/6. This is hard  on a typical electric kiln.

 96800799-785F-4643-9224-E283EBC7CD95.jpeg.0d2ae12facd12a704da02c6de3784d84.jpeg

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I didn't realise that most people fired to cone 5 and cone 6 until it was too late. I bought about 200 kgs of cone 9-10 clay before I actually had a kiln. Now that I've got a kiln, I've been told that it can be very hard on the elements with that kind of temperature :) I believe I only have witness cones for 05, as that was the temperature the technician I should do the calibration firing for.

@Hulk were you saying that specific gravity can influence whether a glaze crazes or not? I never actually measured mine, I used Jon the Potter's methods of unscientifically measuring specific gravity.

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Hi Liam,

Meant to say that higher bisque should be less absorptive, hence thinner glaze layer.

Glaze that's way too thick may craze where a thinner layer doesn't - or doesn't right away. A good glaze fit should tolerate some thickness. If you're doing test tiles, consider varying the glaze thickness as part of your testing? I dip my test tiles twice, at angles, so there's an overlap triangle in the middle, which gives me an idea of thickness tolerance of that glaze on that clay, for that body thickness and curvature...

Looks like you went a little hotter than 05 there, in your pic.

Yep, I jumped to specific gravity, the "thin" connection being controlling specific gravity and thixotropy is a good way to adjust thickness and have good glaze behaviour. If you already have a scale - you'll need one if you're going to mix glaze - a graduated cylinder is all else needed to measure density/specific gravity. Mix thoroughly just before!

The cloudiness and far apart (like your bowl, above, the middle piece) craze lines might very well clear up by going a bit thinner. A wetter glaze goes on thinner in the same dip time, but will run and drip - that's where thixotropy comes in! Dip/pour, shake, flip, shake ...done! Glazing got easier (and more fun) after going wetter and jellier, plus side benefit of staying mixed better.

200 kilos, oooh, that's a lot of clay! ...could last a while, 'specially if you reject and recycle a lot! Throw big pieces! ...know anyone with a big gas kiln doing high fire?

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