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Glaze not adhering to Underglaze


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Hi everyone!

I'm fairly new to glazing pottery, and have just unloaded a kiln full of pieces I had painted with underglaze, and applied a clear glaze over. Some pieces had been bisque fired with the underglaze already painted, and others were painted onto bisqueware, but all seem to have the same problem of the glaze avoiding areas where the underglaze is. In some places it has caused the underglaze to chip off, but mostly it's just all very rough feeling and patchy looking.

There are a few pieces where the rough patches are a bit annoying but not too bad, but the majority have deemed the pieces unusable :-( 

I used the Amaco velvet underglazes, and a clear glaze from Scarva. The glaze required the kiln to be fired to a pretty high temperature - cone 1 at about 1137 celsius - so I'm wondering if this is the issue or if I've done something else wrong. I'm looking into finding a different lower firing clear glaze but am worried the same might happen again - and I would rather not attempt making my own glazes as I'm not experienced enough - so any recommendations would be much appreciated! 

I'll try to attach some photos to explain better what I'm talking about (might have to be in another post).

Thanks!

Amber :-)

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1. white/red plant pot - clear glaze only visible in patches around the pot, rough, bisque-like texture all over 

2. spotty mug - clear glaze not adhered to majority of underglaze spots

3. large spots pink handle - underglaze cracked off in places, patchy clear glaze over underglazed sections

4. rim of blue/green mug - clear glaze fine over most of the mug, except for the rim where it is still a rough texture

5. spotty bowl - clear glaze not adhered to majority of underglazed areas

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I'd guess just too little clear, as is a usual beginning problem, it just looks different with clear. 

Any correlation between previously bisqued or not and chipping? I believe the chipping is from adding the clear on top the unfired underglaze.

It may also have to do with the pot being under fired, especially the bisque texture all over, a Cone 1 clay is uncommon.

Sorce

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Two things strike me:
one very important  is certain underglaze colors when painted solid or heavily to be let’s say 100 percent coverage make the area more refractory so the overglaze does not melt. There appears to be areas in your pictures where glaze has melted smooth and then underglaze areas that are not fully melted. This would seem to be consistent with that type of failure.

The second is an application issue where the areas that are heavily underglazed do not take the overglaze well. They are far less porous than regular bisque and even dry super slow when applying. This leads to too thin an application of the overglaze.

Both are common problems with some very solid underglaze colors. The first requires testing of the underglaze and clear to ensure  the clear overglaze used will fully melt and the second is a learned and observed thing most often revealed while waiting for the ware to dry sufficiently for a second coat of overglaze.

Testing on test tiles is often the only way to figure this out making sure the test tile has a very solid application of the underglaze. In the end you should find some underglaze colors more refractory than others when  painted on heavily. Since you are using already formulated overglazes you will need to find the best glaze underglaze combination.

Edited by Bill Kielb
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On 8/17/2020 at 1:00 PM, Sorcery said:

I'd guess just too little clear, as is a usual beginning problem, it just looks different with clear. 

Any correlation between previously bisqued or not and chipping? I believe the chipping is from adding the clear on top the unfired underglaze.

It may also have to do with the pot being under fired, especially the bisque texture all over, a Cone 1 clay is uncommon.

Sorce

That's what I thought too - I was going to attempt painting some more glaze just on the areas where it hasn't taken and re-firing soon, as they're not usable at the mo so there's not much to lose! 

The pieces which had been painted and then bisque fired before glazing seem to have worked lots better than those that weren't, but there are still lots of patchy areas where the glaze hasn't taken. Think in the future I'll fire the painted pieces before glazing as a precautionary measure, and see if that does help.  

Thanks for your reply! 

Amber :-) 

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On 8/17/2020 at 1:00 PM, Bill Kielb said:

Two things strike me:
one very important  is certain underglaze colors when painted solid or heavily to be let’s say 100 percent coverage make the area more refractory so the overglaze does not melt. There appears to be areas in your pictures where glaze has melted smooth and then underglaze areas that are not fully melted. This would seem to be consistent with that type of failure.

The second is an application issue where the areas that are heavily underglazed do not take the overglaze well. They are far less porous than regular bisque and even dry super slow when applying. This leads to too thin an application of the overglaze.

Both are common problems with some very solid underglaze colors. The first requires testing of the underglaze and clear to ensure  the clear overglaze used will fully melt and the second is a learned and observed thing most often revealed while waiting for the ware to dry sufficiently for a second coat of overglaze.

Testing on test tiles is often the only way to figure this out making sure the test tile has a very solid application of the underglaze. In the end you should find some underglaze colors more refractory than others when  painted on heavily. Since you are using already formulated overglazes you will need to find the best glaze underglaze combination.

Both sound very probable, I painted quite a few layers of underglaze hoping for the colours to be as even and bright as possible. I'm very new to underglazing so wasn't entirely sure what problems to look out for until they came out the kiln!

The glaze does seem to work over the underglaze in some cases, though still patchy, so it seems likely that it's an issue of the thickness of the layers of both. 

Thank-you very much for your advice - I definitely should have made test tiles before going all in but felt like a waste of a firing to only have a couple things in at once! Lesson learnt though :-) 

Amber :-)

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On 8/17/2020 at 1:26 PM, neilestrick said:

On some it looks like the underglaze is too thick, and is soaking up the glaze. Others look like your clear glaze is too thin. How are you applying the clear glaze?

I applied the recommended 3 coats of underglaze, but it does definitely seem to be soaking up the clear glaze.

I dipped everything in a bucket of clearglaze, left for a few seconds and took them out - I actually was worried the glaze was too thick and would drip everywhere, but that definitely wasn't the case! 

Will try again and learn from my mistakes :-) Thanks for your reply!

Amber :-) 

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4 minutes ago, ayembr said:

I applied the recommended 3 coats of underglaze, but it does definitely seem to be soaking up the clear glaze.

I dipped everything in a bucket of clearglaze, left for a few seconds and took them out - I actually was worried the glaze was too thick and would drip everywhere, but that definitely wasn't the case! 

Will try again and learn from my mistakes :-) Thanks for your reply!

Amber :-) 

3 coats may or may not be too much. It depends on how dry/absorbent the pot is, and how thick the underglaze is. It'll take some practice to get it right.

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Just adding.

If underglaze damp when dipping in glaze the glaze will not lay down as thickly on those areas.

Test tile time.

1,2,3 coats of underglaze and dip

As above with longer holds in the glaze bucket

Some underglaze still problematic..add a bit of frit to it..

Applying u.glaze pre bisque fire means you can carve through etc but also won't smudge when glazing.

Test time 

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3 hours ago, ayembr said:

I actually was worried the glaze was too thick and would drip everywhere, but that definitely wasn't the case! 

To make this easier on yourself, most washed or water color effects work much more easily. With your test tiles make sure to have some areas painted solid and trail off to a water color effect. This way you can try one two three dips of overglaze and see more readily what you can achieve with the combo.

I have never seen underglaze absorb the overglaze. What I usually see is the areas painted solid are less porous because the underglaze fills some of the pores of the bisque. A very light sanding before glaze  can often open this up but you risk removing the color. We worked on this for about two years and decided to engineer a clear glaze that worked fairly well in  extreme refractory cases so my wife could paint as she desired. She has serious portrait / brush artist skills.

In the first pic below the glaze needed to cover many layers of the painted gem. It was an exercise in refraction of light in gems. The sugar bowl below needed very dark fully filled black petals and  very strong red so we developed a gloss and matte that melted  nicely over these. This became our worst case and she decorates within these limits. Just to note when I spray these the underglazed areas take about three time longer to dry because they are less absorbent. As a result to avoid runs and keep the entire layer the same I might spray 3-5 light coats waiting for the previous coat to dry enough to receive the new coat.

 

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satin matte 3.jpg

Edited by Bill Kielb
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  • 10 months later...

Ayembr, were you able to successfully refire.  I am suddenly having the same problem and have been using underglazes for a while.  I was thinking of refiring.  I originally fired at cone 5 but was thinking of adding another coat of clear glaze and firing at cone 6.  This happened with both the gloss and satin Amaco clear glazes.  I sometimes have one or 2 pieces that come out this way but this was almost the whole kiln.  I thought maybe something was wrong with the kiln.  

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just read this whole thread.   daleann, did you read the dates of the previous posts?   it is hard to realize that the date matters.   amber has not been using this website since last october.  if you want to send her a message, she may be notified about your post.

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7 hours ago, DaleAnn said:

Ayembr, were you able to successfully refire.  I am suddenly having the same problem and have been using underglazes for a while.  I was thinking of refiring.  I originally fired at cone 5 but was thinking of adding another coat of clear glaze and firing at cone 6.  This happened with both the gloss and satin Amaco clear glazes.  I sometimes have one or 2 pieces that come out this way but this was almost the whole kiln.  I thought maybe something was wrong with the kiln.  

hello! wow i haven’t been here in a while, forgot i posted this question!

 

so it turns out I didn’t follow the instructions properly when I first tried using the underglazes - i painted straight onto bisque ware + tried glazing them without firing the underglazes pieces first. a bit silly of me but i’ve learnt since then :-) 

 

a while after they came out all weird + unglazed on the painted areas, i got round to refiring them with lots of clear glaze painted onto those areas + it seemed to work for the most part :-)

 

hope that helps a bit! amber :-) 

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