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Glaze test result - What would you do next?

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I don't know if other folks had a tough time learning to fire an electric kiln but I have had a rough go of it. After much testing (some of it asinine) I came to the conclusion that I was overfiring my glazes here at home because my kiln fires too hot at ^5 even with no hold, the 5 was all the way down and the 6 starting to bend, so I did a TC offset of 23f. 
Bisque fired to 04 brought down the tip of the witness cone just to the shelf after the TC offset of 23f, previous 04 fires had brought it all the way down and slumped. So far so good. Did a Cone 5 med, no hold and all but one of the glazes came out great, no blistering....BUT... The ^5 cone barely bent at the top of the kiln and the one on the bottom does not look like it bent at all.
The bottom shelf had a turq pot with blisters, which makes me think I was wrong about the overfiring causing the blisters, so weird, the middle and top turq pots were perfect.
I am thinking about doing the same fire, ^5 med but with a 10 minute hold, hoping that will even out the kiln temp and bring down the ^5 cone. Guess I'm just looking for a sanity check, I've ruined so many little pots, sigh... At least I have plenty of filler for the kiln for all these tests, lol. I have mugs that have been fired 5 times.


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Id be worried about the clay body not vitrifying if you're barely tipping a 5.  The glazes look good in that picture though.


How fast are you bisquing from 1200-1700?  Bloat and blister might be coming from the clay body?  Man I don't know but you're having a heck of a time.  Have you tried commercial glazes to see how they fire to cone 6?

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Two things:

1. Refiring won't necessarily fix the glaze problem. Sometimes it gets worse, sometimes it gets better, sometimes it's the same. Don't use refires as an indicator of whether or not the changes to the firing schedule are helping. Only fresh glaze will tell you that.

2. All the firing changes haven't fixed the problem, so maybe it's time to try the glaze formula. Try adding 2% each of silica and EPK, in increments up to 10% and see what happens. Wait until you get a solid cone 5 schedule worked out, though.

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I'm only using previously fired pots to fill the kiln for thermal mass, not using them to judge results. (I did get a few salvages though!)

Iv'e ruled out everything but the glaze firing schedule, between home and the studio, the only remaining variable is my fire. I just need to get that 5 cone down so the tip is at the same level as where the base starts and I think it will be OK. After that I will move to adding EPK and silica if there are still any issues.

I have water in 3 of those recent tests to sit overnight. I recently switched to Little Loafer's 5/6 and I love it!

Cone Shrinkage Absorption
6 11.0 1.87
5 11.0 3.4

From reading and my fire results, it looks like glaze always goes through a bubbling stage, then peak temp smooths it out, is this accurate?


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For the most part, the higher the LOI, the higher the amount of bubbles.  You can avoid high LOI by reformulating glazes.  That's just in the glaze though.  Bubbles also come from organics burning out between 1200 and 1700f, which is why I asked about your bisque schedule.  I don't get any bubbles now that I've switched to a super slow 108f ramp between 1200 and 1700 in my bisque, even with my high iron clay.

3% pretty iffy for cone 5, which you didn't reach, but if the glaze fits well you only have to worry about mold on the feet (not holding water) or later crazing from hot/cold cycles

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Thanks dh, I tried several times and could not find the right temperature, they always had blisters and felt like I was getting nowhere. I have since realized the kiln is firing hot and was laying down a 5^ cone at 2150f and 10 min hold.  So I decided to try the TC offset and cone firing mode to replicate the cone bend I got in the studio kiln, I will probably go back to programming in the firing schedule but it feels like too many variables right now. I am hoping the 10 min hold will get that ^5 cone down some and I can adjust the TC offset again based on what the cones do. 

Any bisque fire issues have been ruled out, I can bisque here, use my glaze and take them to the studio, let them fire them and they come out perfect. That is good to know though, as I do want to try a high iron clay body.

Neil, I think you are right, didn't think about it until now, but I remember reading different glazes can come out different at ^5 vs. ^6, so yeah, the Turq and Floating Blue seem to be real narrow


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Guess I will throw in because there are so many unknowns.

1) Let’s start here: do you have the glaze formulations and share them if you do. A few minutes on UMF will be helpful.

2) do you have a specific claybody and if little loafers, firing it to cone six should be your goal as 3% absorption is significant and since it more fully vitrifies at six it likely does not start expanding and bloating before cone seven.

3) kiln loading and kiln controller as well as cone placement have an effect. Can you clarify a bit on what you have and your firing philosophy.  Packed tight, offset shelves, bottom element, spacing of first shelf above if element ...... lots to think of here.

Even excess downdraft ventilation may be outside the PID controller programming in your kiln. (PID = proportional, integral, derivative)

4) is this a multi zone or single zone controller.

5) I cannot see a blister in the picture. Is a blister actually a pin hole?

a couple other  quick comments

glazes are simple, adding EPK and silica may be a good idea or the worst one ever. Get the formulation check umf chemistry or ask someone to do it.p or forward to me and I will do it for  you.

strive to fully vitrify your claybodies and minimize absorption, period!

glazes that craze are poor fits with the claybody. If they don’t fit, they will craze eventually. They just don’t fit well and Will craze, period.

a final comment, there are glazes that exhibit a packed bed porosity issue and do not heal well. Bisqued to 05 or 06 often solves this problem as well as some drops of Darvan believe it or not. Then I have seen the same issue in glazes that contain very little silica and fire somewhat matte even though they have a glossy silica to alumina ratio. These glazes don’t heal because they literally are always over fired and run out of available silica.

more info is definitely needed here. Just my opinion though.


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@shawnhar I haven’t been following all the details of your experiements, so this may have already been covered. I recall you said the group studio uses no vent and keeps all of their peep holes closed. I don’t recommend copying that, because it doesn’t sound healthy for the elements. But if you are using an undermounted vent, your cool down is much faster than theirs. This could explain the differences from your kiln vs. theirs. I would suggest a much longer hold than 10 minutes. Maybe 30 minutes or even an hour would simulate what’s happening in the studio’s kiln. 

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Babs, I hope to fiddle, one day! Glaze composition is something I have spent no time on yet, I don't have a place to do dry ingredients.

 - Finally, cone 5 with 17 minute hold, all glazes came out right, 5 cone tip was even with the base. Now I guess I need to find a clay with less absorption.

Thanks all!

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