First kiln glaze fire - program recommendations to fix issues

[MCB]

Hello,

I just got my first test pieces out of a glaze fire in a new kiln, and need some help in directing me to fix some problems.

The kiln is an L&L easyfire E18S-3 with a genesis 2.0 controller. I ran a  slow glaze program to cone 6, with a 1 min preheat, no hold, and no controlled cool down. I now realize the slow preset was probably slower than necessary as it took 16 hours. In the future I was thinking of running a slow-medium or medium preset glaze program. 

Some of my test tiles came out well, but a few were blistered or had pinholes. One of the mugs I glazed came out very blistered, which was on the top shelf. My witness cones look like it got a bit hotter in the kiln than a cone 6. The witness cone 6 on the bottom shelf was more bent than the one in the top shelf. The kiln says both zones maxed out at 2224 F. 

My questions are... 

- Do I need a thermocouple offset? If so where do I start?

- Should I do a final hold? If so how long and what should I set as the maximum temperature?

- Would a slow cool help with blistering? 

- I bisque fired to cone 06. Would bisque to 04 possibly help with blistering?

Any advice is very welcome. Thank you!

 

[Bill Kielb]

Those are pretty close cone wise but a tad over fired or strong cone 6 so you could tweak slightly with offsets.  My guess is your glaze is happier at cone 5. Glaze firings generally can go fast. Since the ware is dewatered and all the organics have been removed many find their glazes work just fine with medium or fast firing. Having said that slow bisque is more popular to remove all organics. Darker clays generally slower than light.

Since you have so many pin holes there could be several causes originating with how well was the clay bisqued, how evenly was the glaze applied, is it really a cone 5 glaze. From the picture, it looks over fired, so to start properly bisqued test tiles, one two three coats of glaze etc…. Fire to different cones. For problematic or fussy glazes and clay body combinations they can be dealt with a drop and hold schedule or other methods as appropriate.

First thing IMO is get all the prep stuff right and test (using test tiles) to then see what is the best way to fire that clay and glaze. I believe that is a commercial glaze and some of my very first pieces used it in layers to get the look. Temperature sensitive, base glaze sensitive and thickness sensitive from my recollection if it is the same that I used. These inconsistencies drove me to learn as much about glaze as I could find time for…… so I could just mix my own.

 

 

Edited December 14, 2022 by Bill Kielb

[Kelly in AK]

Thank goodness you used cones! I agree with Bill here, he really said it all. I imagine your cones touched down quite a while before the kiln shut off.

I have one glaze in the school studio (cone 6 electric) that blisters if it soaks too long or is applied too thick.

Many years ago I had a clay body that disproportionately produced blistered glazes. Same glazes, same firing, side by side, pots from that clay blistered, others didn’t. It took a few kiln loads to see without a doubt it was the clay. I was trying all kinds of things to solve it, the clay body wasn’t even on my radar.

Bisque at 04 and try a shorter glaze firing. See if you can isolate certain glazes prone to blistering.

In my home studio I struggled a while with pinholes (not blisters so much). Dropping 100° at the end and holding 30 minutes helped more than anything else to solve that. 

[neilestrick]

I would try a cone 5 firing, on medium or med-fast. Slow is not necessary. Your glaze is probably over-fired, however bisque firing cone 04 might help.

[MCB]

Thanks everyone for your responses. The clay and manufactured glazes I am using are rated for cone 5/6. The glazes are from clayscape, and I have read that they often fire them to cone 7, but maybe the combinations I have do better at a lower temp. It sounds like it’s a good place to start by testing a bisque 04 and glaze cone 5 and see how they fare. Thanks again!

[Roberta12]

10 hours ago, neilestrick said:

I would try a cone 5 firing, on medium or med-fast. Slow is not necessary. Your glaze is probably over-fired, however bisque firing cone 04 might help.

I agree with @neilestrick  Your kiln has 3inch brick, it will hold the heat well.  Great idea to get cones in there.  Good information.  I would try firing to cone 5 next time. 

I say this because I have switched from cone 6 with a hold to cone 6 without a hold to cone 5 with a hold this last year.  My glazes are happier, and I am not getting some of the bloating I was getting on my dark brown clays before at cone 6.  I also realize that while my kiln might have been firing hotter than previously, I also was not properly making my cone packs.  I bought some of the pre made plaques and put kiln wash on them so they can be reused.    I feel like my cone results are more consistent.  Anyway, I see you have self supporting cones, and you have a great start!  Do you have a vent fan on your kiln? 

Roberta 

[MCB]

Hi Roberta - yes I have a downdraft vent so there should be good air movement going on. I turned the vent off when the firing finished thinking a slower cool would be better, but maybe that (marginally) contributed to my overfiring as well. Great to hear you’ve had better success going to cone 5 with a hold. How long do you do your hold for?

[Roberta12]

53 minutes ago, MCB said:

Hi Roberta - yes I have a downdraft vent so there should be good air movement going on. I turned the vent off when the firing finished thinking a slower cool would be better, but maybe that (marginally) contributed to my overfiring as well. Great to hear you’ve had better success going to cone 5 with a hold. How long do you do your hold for?

And I also have an L&L.  E23T   the 2 inch brick.  My beauty has 943 firings on it.  I have been programming slow glaze, cone 5, 10-15 min hold.  I have been putting in the 15 minute hold if it's a lighter load. My thinking was that the lighter load might need the longer hold because there wasn't as much work in there.  But 5 minutes probably doesn't make that much difference.  Just trying to problem solve on the go.   And like you, I try to turn the fan off right after it finishes firing.  If I am reading my cones correctly, I am getting about 5 and a half with heatwork.  And bottom and middle are hotter than top.