[Clayart] Hold temp to allow outgassing?

[Robert L. Johnson]

I'm struggling with blisters and wondering whether I'm firing too fast to
allow complete outgassing of my faux B-mix before the glaze begins to melt.
Or, should I hold the kiln at a specific temperature on the way up?

My target is cone 6. And I do seem to have more trouble with two tenmoku
glazes that I love, but I have found blisters on several other glazes, from
time to time, as well. (I do use the tenmokus most often, so that may bias
my observations.)

I see on Digital Fire that there is a recommended firing schedule for iron
reds, which I am guessing are close cousins to tenmokus. That schedule
recommends a hold on the way down at 2100 F. to let blisters heal.

Your suggestions will be much appreciated.

Bob

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[David Finkelnburg via Clayart]

Bob,
Blisters are caused when a glaze seals over (melts at its surface)
before the body has shrunk or otherwise ceased to produce gases that will
cause blisters in the melted glaze. Please accept this as a starting point
for problem solving, not an answer to all issues. A glaze blister presents
a complex problem.
Ideally all of us would use draw trials to determine exactly at what
temperature and when using a particular firing schedule a given glaze seals
over.. Such trials can also reveal when body densification/maturity is
reached. It should be before the glaze seals over.
Molten glaze viscosity, firing atmosphere, firing speed are all
variables that influence blistering. A soak at peak temperature that holds
the glaze molten and allows surface bubbles to escape can solve a lot of
blistering provided glaze viscosity is not too high to prevent bubble
escape nor too low to prevent the molten glaze from running off the ware.
Intentional outgassing of oxygen, btw, is a critical feature of oil
spot/hare's fur glazes that makes use of the melting of a surface glaze
BEFORE a later melting glaze beneath it.
All the best,
Dave

On Mon, May 13, 2024 at 8:08 PM Robert L. Johnson <impal...@gmail.com>
wrote:

URL: <https://lists.clayartforum.com/pipermail/clayart/attachments/20240513/c8dfc18a/attachment.htm>

[joel joelfink.net via Clayart]

Bob,
I had this problem when I was using an interpretation of Laguna's B-Mix the manufacturer called Fool's Gold. We researched, asked, and experimented. One potter noticed that there could have been a seasonal pattern to it, but we moved to a Flint Hills porcelain and the problem went away.

Blisters can be a funky problem. I solved it temporarily by mixing red iron oxide into the clay and double-pugging it. That wasn't a solution, but it was a very interesting cause of the elimination of the problem. We never actually solved it. My theory was that the Fe2O3 was decomposing into FeO and the extra oxygen atom was suppressing gas formation by making CO2, which evacuates the mass much earlier than glaze maturation temps.

We altered bisque, we altered soak, and we changed some glaze chemicals for more "clean" chemicals.

I don't have an answer for you, I wish I did. I only have these anedotals and some thoughts that lack a test bed to grow in, and I don't want that test bed again, lol.

In the end, I don't think any of the recommendations, and several were very good recommendations, addressed the problem, for lack of an understanding and definition of the actual cause. I looked at the blisters for any information, like direction of flow of a bubble, exposure of the clay substrate, size, completion (fully popped), for information. What I had were bubbles that could be a quarter of an inch across down to almost nothing, along with partially sealed over and newly formed.

My real problem was the 'why' of bubbles seeming to defy gravity and burst away from the surface (perpendicularity) and not up, with more of a teardrop shape.

Best of luck on this,
Joel.


________________________________
From: Clayart <clayart...@lists.clayartforum.com> on behalf of David Finkelnburg via Clayart <cla...@lists.clayartforum.com>
Sent: Monday, May 13, 2024 8:38 PM
To: Clayart international pottery discussion forum <cla...@lists.clayartforum.com>
Cc: David Finkelnburg <dfinke...@gmail.com>
Subject: Re: [Clayart] Hold temp to allow outgassing?

URL: <https://lists.clayartforum.com/pipermail/clayart/attachments/20240514/55d12710/attachment.htm>

[David Woof via Clayart]

Hi Bob,
David Finkelberg said: "A soak at peak temperature that holds

the glaze molten and allows surface bubbles to escape can solve a lot of
blistering provided glaze viscosity is not too high to prevent bubble
escape nor too low to prevent the molten glaze from running off the ware."

This true and good advice if one understands that a hold at "peak" target cone 6 maturity is now going to continue the maturation of glaze and body, and as David said, one may well produce "the molten glaze from running off the ware"

The physical actuality then is that now one is really progressing the firing toward cone 7.

But the conundrum is: This "top temp hold" does help seal glaze zits and pox if your glazes are broad range enough to take the abuse, and some experienced folks do have success.
However, this is not a true cone 6 firing, which sensitive glazes cannot take the abuse, and can yield the post firing grinding of shelves and foot bottoms.

​Please Note:***I recommend, from many years of experience, a simple hassle-free alternative for control and crater healing.

One can initiate a simple programed hold on the way up, instead of a hold at peak temperature.

Example:*** an hour hold initiated as cone 4 is beginning to soften will give the zits time to seal over and heal as the firing heat work progresses through cones 5 and 6.

Added benefit:*** a hold in this temperature range works to even out the kiln temperature in zones top to bottom if one is experiencing uneven zone maturity. This hold can cause cones 5 and 6 to go down much more evenly, many times during that same initial hold as well.

The progression is steady so Don't go for a cup of coffee or get distracted because this will all happen fast.

Misneach,

David Woof.............................my Muse says that I forget to tell folks that sometimes one needs to "grow a new pair" to try something unusual and new. I suggest that perhaps she has "grown a pair" because today the spritely little Raven-Haired Vixen seems to be getting too big for her britches.
She just chuckles and gives me that "challenging look" when she is trying to manipulate me for how she thinks the day should go.
************************************************************************************************************
________________________________
From: Clayart <clayart...@lists.clayartforum.com> on behalf of Robert L. Johnson <impal...@gmail.com>
Sent: Monday, May 13, 2024 3:02 PM
To: cla...@lists.clayartforum.com <cla...@lists.clayartforum.com>
Subject: [Clayart] Hold temp to allow outgassing?

URL: <https://lists.clayartforum.com/pipermail/clayart/attachments/20240516/b6a73ca7/attachment.htm>

[Terry Lazaroff via Clayart]

Down firing may be great for coloured, matt, and crystal glazes. I believe that for transparent, clear glazes, the faster the cooling possible is necessary to ensure the glaze does not develop crystals.

I have seen the Jingdezhen kilns being opened right after the gas shut down. I was told that this ensured a clear transparent glazed surface. Apparently there are rejects, but that does not seem to bother them.

Terry
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________________________________
From: Clayart <clayart...@lists.clayartforum.com> on behalf of David Woof via Clayart <cla...@lists.clayartforum.com>
Sent: Thursday, May 16, 2024 10:29 AM

To: Clayart international pottery discussion forum <cla...@lists.clayartforum.com>

Cc: David Woof <woof...@hotmail.com>
Subject: Re: [Clayart] Hold temp to allow outgassing?

URL: <https://lists.clayartforum.com/pipermail/clayart/attachments/20240517/fe193541/attachment.htm>

[Snail Scott via Clayart]

> On May 13, 2024, at 4:02 PM, Robert L. Johnson <impal...@gmail.com> wrote:
> I'm struggling with blisters and wondering whether I'm firing too fast to
> allow complete outgassing of my faux B-mix before the glaze begins to melt.
> Or, should I hold the kiln at a specific temperature on the way up?

To eliminate pinholing and blisters from outgassing, do not slow during rise or soak at peak; that usually just does extra heat-work and causes even more outgassing. Instead, do a hold after the temperature has dropped a bit - maybe about 50 degrees F/25C - then hold for around 20 minutes. This keeps the glaze within its molten range long enough to flow and heal most blisters and pinholes, but without adding to the cumulative heat-work and overvitrifying the clay. You can check the witness cones…as long as they don’t bend any further, you are fine.

It’s not the temperature of the hold that matters; it's the margin between peak temperature and the hold that is important. Whatever your firing temperature, use enough of a drop to prevent more heat-work and outgassing, while staying hot enough to remain within the melt zone. If a longer soak is needed, do a lower drop, too, to compensate. Exactitude is not required; just an awareness of the parameters.

-Snail