Kiln creating bubbles in clay

[SaeChoi]

I bought my first kiln for home use, an Olympic Doll-E kiln, after someone recommended it.

The pottery studio I used to go to has a Skutt kiln. My main clay bodies have been Laguna Clay speckled buff and b-mix speckle. Whenever I fired at the studio, the pieces came out great- no bubbles, smooth finishes, bright underglaze.

However my kiln at home, the speckled buff comes out slightly discolored (more of a grayish-yellow than tan) and the b-mix speckle surface bubbles on the glaze and unglazed surfaces. Even underglaze I use comes out much darker than they’re suppose to. And all the textures come out tougher than they’re supposed to.

I bisque fire at cone 04 slow and have tried different cone 5 firings for glaze- slow, medium-slow and medium, some with holds and some without. But every time, the results come out the same.

I know the much smaller kiln size is a factor to the results, but it’s getting discouraging to try new test fires when they’re all coming out the same.

Has anyone else have this issue with a small kiln using those clay bodies? Or any solutions to how to remedy this?

(Additional info: I use Amaco Velvet Underglazes and Amaco Sahara H-9 Zinc-free Clear glaze. I’ve used these for 3 years with the same clay bodies. At the studio, the results always came out great)

[Hulk]

Hi SaeChoi,

Welcome to the Forum!

Two suggestions:
i) if you can get samples from the Studio, fire side by side, just to confirm the clays are the same
ii) place a cone pack on each level, to confirm the heat work

Your new kiln may be running hot?

A small kiln will likely cool much faster.
You might program a slow cool to emulate the slower cooldown of a larger kiln.

I tried a bag of Laguna's Speckled Buff clay last year.
It was labelled Cone 5.
I hadn't tried firing it hotter than Cone 5; the pieces on the shelf that was closer to Cone 5.5 were a bit more toasted looking.
I really liked the clay and will definitely buy more! It was easy to work, takes my glazes well, and there were less defects.

Some clays tolerate overfiring better than others.
Some of the red, black and dark brown clays I've tried misbehaved (bubbling, bumps, fizzing) when overfired.

Edited March 25, 2023 by Hulk
added

[SaeChoi]

54 minutes ago, Hulk said:

Hi SaeChoi,

Welcome to the Forum!

Two suggestions:
i) if you can get samples from the Studio, fire side by side, just to confirm the clays are the same
ii) place a cone pack on each level, to confirm the heat work

Your new kiln may be running hot?

A small kiln will likely cool much faster.
You might program a slow cool to emulate the slower cooldown of a larger kiln.

I tried a bag of Laguna's Speckled Buff clay last year.
It was labelled Cone 5.
I hadn't tried firing it hotter than Cone 5; the pieces on the shelf that was closer to Cone 5.5 were a bit more toasted looking.
I really liked the clay and will definitely buy more! It was easy to work, takes my glazes well, and there were less defects.

Some clays tolerate overfiring better than others.
Some of the red, black and dark brown clays I've tried misbehaved (bubbling, bumps, fizzing) when overfired.

I thought if the speckled buff was fired higher it would be more toasty, but it ended up being a gray color. So I’ve been firing to cone 5, still not producing the results I’m used to from the studio I used to fire at.

and examples of the issues I’m having with speckled b-mix fired at cone 5 (it recommends not exceeding past it) with all the bubbles.

[Kelly in AK]

You weren’t kidding, those are definitely bubbles in the clay. Like @Hulk said, you ought to get some cones to see what’s really happening in there. Kiln’s getting too hot or soaking too long. 

[Roberta12]

20 hours ago, SaeChoi said:

I thought if the speckled buff was fired higher it would be more toasty, but it ended up being a gray color. So I’ve been firing to cone 5, still not producing the results I’m used to from the studio I used to fire at.

and examples of the issues I’m having with speckled b-mix fired at cone 5 (it recommends not exceeding past it) with all the bubbles.

Oooooo I know this one.  I had exactly the same issue when I used speckled bmix.  It's overfired.  That was part of my descent into the rabbit hole of "your kiln may be overfiring".  I fired to cone 6 as I normally had been and had the same result as you did. Lots and lots of little pimples.    My supplier had said that clay was cone 5/6.  I called Laguna.  They said that clay is only happy at cone 5.  I started putting cones in every single load and discovered cone 6 on my kiln was more like cone 7.  after more than a year of personal kiln research, I have happily fired speckled b mix at cone 5.  Your photo looks just like my overfired pieces.  As @Hulk suggested  cones on every shelf.  That will give you good information.  I have used Laguna Speckled buff for years.  It is a very forgiving clay.  It can easily go to cone 6 if necessary.  Speckled B mix is a different story.  Cone 5 only.

Roberta

[Pres]

You definitely need to get some self standing cones and calibrate the new kiln. I bought a kiln a few years ago and had to go through this process. Technical, but not complicated. When you bought the kiln did you get the optional Genesis controller, or does it have the Bartlett controller? My solution to my problem turned out to be the kiln was overfiring cone 6 about 35 degrees. I put a -35 degree offset in the cone offset section of the Genesis controller, after 2 smaller tries working up to that. The cone offset is only for that cone, so I also set an offset for the 06 cone. This is different from a thermocouple offset as that is just when one area is over/under firing. I used self standing cones for my tests, as it was just easier. Since I was targeting only two cones, bisque and glaze I only bought a box of each.

Good luck, get the cones!

best,

Pres