Disappearing Glaze

[Malcolm]

I have an open top kiln and fire in oxidation at 1240c. I glazed both inside and outside with the same commercial blue glaze, quite thickly. The interior of the pot came out as I wanted, but on the outside the glaze had virtually disappeared. I am very pleased with the result as I like the contrast but I would like to control the glaze better. I must admit that I am using the preset temperature controller and not cones, which would be more accurate. Is the interior of a pot cooler in the kiln and do I need to decrease the temperature? Or is it something else?

[Chilly]

Pictures might help @Malcolm

[Malcolm]

[liambesaw]

Looks like the glaze didn't disappear but the colorant did.  Looks like a copper blue to me, maybe it flashed off?

[Malcolm]

I think it is a copper blue. But the body is white and feels like the original clay body with no glaze on. Ignore the other glazes at the bottom as they have fired as predicted.
I have heard of flashing but associated it with flame burning kilns, can this happen in electric kilns? What causes it?

[Pres]

Absolutely, electric kilns, Oxidation atmosphere, will flash just as much as a gas kiln. I used to use a tin base glaze, with copper in my stains with over glazing. The reason was that I wanted the pink flash to appear in areas. This effect worked well for a few years, then I go tired of it and moved on. Of late, I use a combination of zircopax and tin for the color I want, then soften it with a slight percentage of rutile. This does not flash at all.

 

best,

Pres

[liambesaw]

22 minutes ago, Malcolm said:

I think it is a copper blue. But the body is white and feels like the original clay body with no glaze on. Ignore the other glazes at the bottom as they have fired as predicted.
I have heard of flashing but associated it with flame burning kilns, can this happen in electric kilns? What causes it?

Copper carbonate loses its carbon and water at around 550f, and the copper oxides start vaporizing towards cone 6.  

However this doesn't appear to be that, was there a second glaze on the outside? If so, it could have just pulled the copper glaze into it, especially if it wasn't well attached to the clay (such as when glazing the inside and outside at the same time, or putting a high clay glaze on top of a low clay glaze).

[Mark C.]

OK everyone has great ideas but how about maybe the glaze gods snuck in and wiped it clean after the lid was shut?

I normally do not drink the cool aid 

[Babs]

How are you glazing?

Interior first then the outside?

Think your vessel is too wet on outside for enough glaze material to fix.

Stirring glaze constantly essential.

  Looks like there has been a layer of almost water at top of your bucket and a  wet pot immersed into this watery glaze.

Does this glaze settle to a hard pan?

Yeh, withMark.not for consumables

 

[neilestrick]

What is that thick layer of white around the middle of the pot? Is that as it should be? It looks like maybe your glaze slid down.

[Mark C.]

That glaze does not look food safe at all-hope you know about that.

[Babs]

In another post you mention you are spraying glaze...are you doing this here?

[Malcolm]

I make purely decorative vases and so I don’t worry about food safety, but I guess it would be unsafe. This is hand built from a rolled slab. I know that the middle section looks like the glaze has slipped but it was a deliberate  effect.The middle section was tightly bound with string and covered with white slip then bisque fired. When I glazed I poured the inside first with a turquoise glaze then dipped the bottom in the same. Poured the central (slipped) section  with a white semi matt Then double dipped the top down to the middle section with the disappearing blue glaze.
It obviously got inside and worked there (and on top of the turquoise) I mix all of the glaze thoroughly before applying and don’t glaze on saturated bisque ware so I don’t think that the problem is in the application. The other glazes seemed to fire ok. This is a commercial glaze and so I don’t know the ingredients. My workshop is planned for working at this temperature (in England what we refer to as stoneware) and all of my glazes should fire together.

[Babs]

Have you had success with this glaze before using same ap.method?

 

If you are over dipping a damp i.e. pot which has been glazed on inside, then you will prob have to hold pot in glaze longer to get the layer of glaze you need on the outside.

Test tiles or pots. ..dip small pot in glaze to see how that turns out...all being glazed at same time.

My money is still on the dampness of the clay. You can see that effect on bottom application of your pot.  v. Different from inside.

This method works for you with different glazes?

It also looks like some attempts to reglaze glaze fired stuff.  The clay is not accepting the needed thickness of glaze for some reason.

After glazing middle section did you wipe outside of top area at all?

You didn't dip it in the slop bucket by mistake :-))

Clay work never boring eh?

[Malcolm]

Hi Babs, Everything you say is logical. I really do think that the pot was dry when I glazed the outside. It was a long while before each procedure and I always stir my glazes well before glazing anything. I have test tiles that work but I will do some small pots, dipping the inside and outside together and see if theres a difference. It does look like the glaze has been rejected by the ware. My thought that it was hotter on the outside of a pot than the inside and that I was over firing the glaze because the pyrometer is not registering the correct temperature. But that doesn’t seem to be a theory that anyone else supports, so I guess that’s wrong. Anyway more testing is the way forward. Thanks.

[liambesaw]

37 minutes ago, Malcolm said:

Hi Babs, Everything you say is logical. I really do think that the pot was dry when I glazed the outside. It was a long while before each procedure and I always stir my glazes well before glazing anything. I have test tiles that work but I will do some small pots, dipping the inside and outside together and see if theres a difference. It does look like the glaze has been rejected by the ware. My thought that it was hotter on the outside of a pot than the inside and that I was over firing the glaze because the pyrometer is not registering the correct temperature. But that doesn’t seem to be a theory that anyone else supports, so I guess that’s wrong. Anyway more testing is the way forward. Thanks.

If you fired too hot the glaze wouldn't disappear!  

And if it crawled off entirely you'd see a puddle of glaze on the shelf, so it's not that!

The copper didn't volatilize because you said that area feels like there's no glaze, and if the copper had cooked off it would still be a glazed surface, just colorless.

So I'm out of ideas, gonna have to chalk this one up to kiln gremlins

[Bill Kielb]

10 hours ago, Malcolm said:

When I glazed I poured the inside first with a turquoise glaze then dipped the bottom in the same

Interesting that the bottom dipped section is super thin, just like the disappearing top. Might give you a clue since both were dipped and both seem extremely thin and maybe even crawled a little. My thought is this is likely a result of application.

[Malcolm]

I believe you’re right, the bottom glaze does look thin and it must be a fault of glaze application but as there is no evidence of the glaze running off, it must have burnt off or vaporised or something, it was well covered when it went in the kiln, and a bit of a shock when I opened up. I will do some more tests and post the results if anyone is interested. Thanks

[Babs]

Good luck Malcolm. Aside from colourants , most g. Ingredients will run oof your pot, not just vanish.

Some folk measure glaze thickness.

Wht cone are you bisquing to?

Test ing only way.

Hopefully a one off...

Copper wi drop to bottomof jar/ bucket and requires stirring between dips unless glaze is formulated to stop this.  

[Bill Kielb]

2 hours ago, Malcolm said:

, the bottom glaze does look thin and it must be a fault of glaze application but as there is no evidence of the glaze running off, it must have burnt off or vaporised or something

My thought would be bad mix of glaze / settled by the time you dipped. Whether the colorant is there or vaporized is almost irrelevant to the rest of the components that will not vaporize but are obviously not present in the quantities to form a nice glass. Both dip lines are present so I see it as some way this glaze became unusable. Maybe needs  lots of stirring or someone thinned it or bad mix or all of the above would be a guess from the picture.

[neilestrick]

When you pour the inside first, the pot can get pretty saturated with water, so then when you dip the outside it's no longer absorbent. So regardless of how well you stir the glaze it's not going to apply well. It's got to be absorbent in order to take glaze. Next time let it dry for a while before you dip the outside. How long depends entirely on the pot and the glaze, but as dry as it can get without the first dip totally drying out. Could be hours. I often pour the inside then lit it sit overnight before doing the outside.

[Malcolm]

I am firing to cone 9. I recall double dipping the glaze but I am not sure that I stirred between. I bet it was a bad mix of glaze and that the surface wasn’t covered properly Even though it did look it.  After I glazed the inside I know that I didn’t wait overnight before glazing the outside. I will make that part of my practice in the future. I thought I could recognise when the bisque was damp and couldn’t take the glaze but it looked thick when I placed it in the kiln.  As you can see nothing ran down the pot so any glaze must have gone into the atmosphere. Thanks, anyway more resting is needed.

[Sorcery]

It's just too thin.

There was turquoise under the bottom and the inside, not there.

It reads one dip of that section. The "double" was an adjective for the inside.

Sorce

[Malcolm]

Yes, I guess the inside was applied too thin but the disappearing glaze was double dipped on top of that. I must have wiped the rim for any overspill before double dipping, I am sure that I let it dry before I did. I will send a pic of the inside.

[Malcolm]