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Glaze/Undeglaze Problem


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#1 BethR

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Posted 07 September 2012 - 08:11 AM

Hi folks,

I'm new to this forum as a member but have been signed up via Facebook for some time and have found the forums useful for finding all sorts of information. I thought I'd better say hello as I'm now looking for help!! :D

So, here's the problem.....

I've just taken some pieces out of the kiln after a glaze firing. The pieces are made from earthenware clay with an underglaze decoration that had been applied to bisque and fired on prior to the application of the glaze. After the glaze firing areas of decoration have chipped away taking glaze and underglaze off to leave bare clay. Please see the pictures for examples.

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The damage seems to have occurred mainly on handles and edges. There were also some flat pieces in the kiln where the flat surface has fired with no problem but there are odd areas of chipping round some of the edges. I have used this type of decoration in the past and there have occasionally been issues with small areas of glaze crawling to leave bare underglaze but the underglaze has never come away like this. Other pieces in this firing that did not have an underglaze decoration have fired perfectly, as have some small dishes that were painted with underglaze on the upper surface only so I don't think it's a glaze fit issue. It seems to be related to the underglaze where it is on areas such as handles and tight edges in some way, I'm just not sure how :huh:

The underglazes I use are powdered which I mix with a medium to apply to bisque clay. I bisque to 1000C and fire the underglazes on to 1120C. I had tried firing on underglazes to a bisque temperature previously but found it didn't fix them sufficiently. The glaze is brush on and was fired to the top middle of it's range at 1120C with decent temperature ramps on either side and a 45 minute soak at the top. This usually gives reliable results.

One theory we have come up with is that as I've been handling the pieces while painting on the underglaze my skin has left oils on the clay which may have interfered with the underglaze bonding to the clay. Is it possible that this could create such drastic damage? If not then what could??

Any help and advice is most gratefully appreciated!!

#2 Chris Campbell

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Posted 07 September 2012 - 09:51 AM

I don't know anything about powdered underglazes but it does look like there was no absorption ... question - are you using anything like a wax resist to protect areas as you decorate others? The fact that it is happening on the parts of the object you handle as you decorate suggests a transfer of oils or resist that is setting up a barrier. The exposed clay also looks under fired ... Are you firing it to maturity?

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#3 BethR

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Posted 07 September 2012 - 10:09 AM

I don't know anything about powdered underglazes but it does look like there was no absorption ... question - are you using anything like a wax resist to protect areas as you decorate others? The fact that it is happening on the parts of the object you handle as you decorate suggests a transfer of oils or resist that is setting up a barrier.


Thanks for replying Chris :D

I've used no wax resist on these pieces at all, so the more I think about it, the more I'm starting to think that it could be this issue of oils from my fingers. Though I am amazed that it has had such an effect!!

The exposed clay also looks under fired ... Are you firing it to maturity?


The clay's firing range is 1060C to 1150C so it's well within the maturing range by the time it's gone through all the processes. We think the rough surface on the exposed clay has been caused by the way the underglaze and glaze has come away as where there are unglazed bases everything is as you would expect.

#4 TJR

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Posted 07 September 2012 - 10:55 AM

Beth;
Your problem is with your underglaze fitting the body. I am assuming that the colour on your pieces comes from the underglaze and then you are applying a clear glaze? Are you using underglazes and not slips? I have seen this problem with slips not being fluxed enough, so they don't adhere to the mug and fall off.
One idea I am having is for you to apply your underglazes to the leatherhard pot, then bisque and then apply your glaze.
Without knowing the recipe of the underglaze, I couldn't give you a suggestion about increasing the flux in the underglaze. Complicated? I hope not. Trying to be helpful here. Great pots by the way.
TJR.

#5 Chris Campbell

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Posted 07 September 2012 - 10:58 AM

You might want to find a way to prop the mugs so you can decorate with less handling or wear gloves if you need to hold the handle for a long time.
You might also want to review the mixing directions for your powdered underglaze to make sure they have not changed something without your noticing. The color results in other areas of the pots look very uneven ... thin and thick, pale and dense ... so uneven application or bad mixing could also be at fault.

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#6 BethR

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Posted 07 September 2012 - 11:52 AM

Your problem is with your underglaze fitting the body. I am assuming that the colour on your pieces comes from the underglaze and then you are applying a clear glaze? Are you using underglazes and not slips?


Thanks for replying TJR :)

Yes, the colour is all from underglaze onto white earthenware, covered with a clear glaze. And yep, definitely underglazes and not slips - I buy them in powdered form and mix them with a universal medium (they can be mixed with water, but the medium is meant to give better results. I've tried both ways and it's definitely better with the medium :D ) to create a liquid that can be painted on.


One idea I am having is for you to apply your underglazes to the leatherhard pot, then bisque and then apply your glaze.
Without knowing the recipe of the underglaze, I couldn't give you a suggestion about increasing the flux in the underglaze. Complicated? I hope not. Trying to be helpful here. Great pots by the way.


I will give your idea a try. When I first started using underglazes I did try painting them onto greenware, but they didn't seem to fix evenly at bisque temperature and I then found they were moving around in some places as I applied glaze. However, that was when I was also trying to use them by mixing with water, so it's definitely worth another go. It's odd though, that this is only happening on areas where there is either the sharp curve of a handle or on lips, edges where there is a sharp corner. On the rest of the piece it is fine.

Don't worry about complicated - it makes me go away and learn :) . Every day I realise how much more there is to understand if I want to be able to solve my own problems! You've been very helpful - thank you :) .

#7 BethR

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Posted 07 September 2012 - 12:17 PM

You might want to find a way to prop the mugs so you can decorate with less handling or wear gloves if you need to hold the handle for a long time.

Thanks Chris. I will try this, as I think there is certainly an element of this oil/resistance factor going on. I'm not sure whether it's the whole problem or part of it, but is certainly worth taking some extra measures to eliminate the effects of this and those are good suggestions.


You might also want to review the mixing directions for your powdered underglaze to make sure they have not changed something without your noticing. The color results in other areas of the pots look very uneven ... thin and thick, pale and dense ... so uneven application or bad mixing could also be at fault.

I quite like the painterly effect of underglazes and so tend to use them as fluidly as possible rather than in blocking colour, which is some of the reason for the uneven application. Having said that, I have noticed that there are some particular colours where the powder is more resistant to mixing well, and this does affect the level of control over their application. I'm not sure whether this is playing a part in this particular problem as the underglaze has been applied in this way all over and yet the problem is just in specific areas, though I will bear it in mind as I do some more tests.

Thanks again :)

#8 Marcia Selsor

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Posted 07 September 2012 - 02:39 PM

i think you need to have these underglazes fit your clay better. Why it happens where it does is a mystery.Is ther anyway you could add 0.2 to .0.5% of siliica to increase the expansion tolerance in the underglaze?
marcia

#9 Marcia Selsor

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Posted 07 September 2012 - 02:40 PM

i think you need to have these underglazes fit your clay better. Why it happens where it does is a mystery.Is ther anyway you could add 0.2 to 0.5% silica to increase the expansion tolerance in the dry underglaze.
marcia

#10 neilestrick

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Posted 07 September 2012 - 04:09 PM

Some underglazes, when applied too thickly, will flake off of bisqueware. A.R.T. underglazes had this problem. Apply them to greenware instead.
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#11 Idaho Potter

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Posted 07 September 2012 - 04:49 PM

I use ready-made liquid underglazes on the majority of my pots and the only time they have chipped was when I first started using them and applied them too thickly. I put underglaze on greenware, bisque fire to cone 05-04, and clear glaze to cone 5 or raku with clear crackle (cone 06). Found that underglazing on bisque ware needs more liquid and more coats to give the depth of color I want.

You mentioned that the underglazes moved around when glaze was applied--yet, you also say you bisque after applying underglazes. What cone are you bisque firing to, and what cone are you glaze firing? If the underglaze shifts even after a bisque fire, perhaps you actually need some amount of water so it can help by getting the underglazes to adhere. The medium you are using could possible be sealing the clay so the underglazes are actually floating rather than penetrating.

#12 BethR

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Posted 12 September 2012 - 03:36 PM

Thanks everyone for such helpful replies :) . I am taking on board all the suggestions and am going to incorporate them into a thorough set of tests to see if it will help me pinpoint exactly what I need to change. I'm guessing it may be a combination of factors, but hopefully by trying all these things out I will find a way of working with the underglazes that will be more consistent and reliable. Thanks again for all your input :) .

#13 scoobydoozie

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Posted 26 September 2012 - 08:05 AM

The term for what is happening is called "shivering". Here's a link to a topic from February with lots of great information....

http://ceramicartsda...__fromsearch__1




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