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Underglazes Flaking Off


Benzine

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I've been having issues lately with the underglazes chipping off, student's finished projects.

The underglazes are both Amaco (Velvet Series and LUG Series), and I've used both extensively for years, with almost no issues.

The underglazes seem to take application well, and come out fine after the bisque. It isn't until the glaze firing, that they flake off. In one case, the underglaze was left bare, on the flange of a slab box. In the other case, it had a glaze over top part of it. It seems to be separating from the clay body, almost like old paint from a surface.

I fire low fire so Cone 05. The clay is the same low fire white, that I've used for years. The only other similar issue I've had, was with a brown underglaze last year, that did something similar. But now, it's two different colors, from two different product lines.

 

I have my students rinse their project after firing, to remove any residue they might have accumulated. But I am thinking it could be oils from the hands. Thus far, it has happened only to one student, on two different projects.

 

Any ideas?

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Ben,

If this problem were more widespread across projects of other students, I'd bet on a mismatch of the clay and slip firing temperature. Oil, or misplaced wax resists sounds more like the culprit.  I have had this happen before when the claybody was a recycled mix of clays...and adding Gerstley Borate to the slip at something like 4:1 fluxed the slip enough to stick.  I'd certainly experiment with that before putting on a student's pet project.

-Paul

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Paul,

 

I'm still inclined to think it's maybe oil from someone's hand.  I do find it odd, that it doesn't flake off, until after the glaze firing though.  It goes through the bisque fine, but only starts to separate, after it is fired with the glaze.

 

In one case it was just a clear glaze.  In the other, it was a colored glaze.  For the former, I had them try the combination again and am refiring it.  For the latter, I just had them use the colored glaze alone, as you couldn't see the underglaze through it anyway.

 

It's definitely not a mix of clay issue, as I only use one type.

 

Mark, a fit issue crossed my mind, but I've been using this clay body and underglazes together for years.  The project with the colored glaze, over the undergalze, is still possible, because I don't believe I've used, or had students use, those two specific things together before.

 

We'll see how the refirings turn out.

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Was it fresh clay or recycled throwing clay?

 

Low fire white clays are notorious for having random glaze shivering problems, usually on rims and edges. I once had it happen on a batch of kids pots that had never been problematic before, and then it went back to normal. Try refiring and don't worry too much about it unless it becomes a common problem.

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Neil, on one project, it was recycled clay, the other was fresh from a box.  I'm trying to remember where the former flaked.  One the latter, it was on outside of the rim.

 

Yeah, I'm not too terribly concerned at this point, as it has only been two projects, out of near one hundred.  But it is any issue that I want to get ahead of, if it does reoccur.  

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If you apply some underglazes, maybe higher clay content, to pots when they are too dry, they look fine and still look fine after a bisque but on going highrer they may flake or bloat off under a glaze . I take a brush with water on it and coat the entire pot inside and out then apply underglazes.

I have had this happen when the pot is just a shade too dry.I apply water to even out the moisture content of the pot prior to applying the underglazes on every pot, looks a bit too dry, I don;t try anymore

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As of right now, I'm treating it as an isolated incident. It has only happened to one student, on those two projects. I do feel bad, because she has tried to touch up the spots, and it still seems to come back, when refired. i've got one to refire, and we'll see how that goes.

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*Sigh*  I pulled the students bowl, out of the kiln today.  The same spot, that they touched up twice, was bare.  This time, they only glazed the spot, no underglaze.  There were bits (chips) of the glaze, on another project next to it.  The glaze apparently popped off.  So the clay is shrinking more than the glaze it seems, shivering right?  This is odd.  I've never had this happen, in the ten years I've used these glazes/ underglazes and this clay.

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Neil, I'm inclined to agree, that there is something on those spots. I haven't the foggiest what it could be. What would not have burnt put after the first firing, that could still be causing glazes to pull away from the claybody? Angry Kiln God magic, is the only conclusion I've reached. I would leave a shot of alcohol out, but the school will probably frown upon that, and my answer of "It was for a kiln god offering, will not probably help matters...

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if you want to attempt it again with this piece, i'd try sanding down the area slightly with a dremel tool to remove the outer layer of clay that's causing the issue.  if that doesn't work, i'm out of ideas.

 

on a side note, fingernail polish looks pretty darn close to glaze when you need a touch up :)

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  • 5 months later...

I have been seeing this problem for the last 4 years. I fire at cone 04 and also at cone 6. I use the same underglazes for both on red clay and white clay at cone 04, on Bmix5 at cone 6. The cone 6 work just fine but the low fire flake/ shiver at cone 04. Sometimes the underglazes flake off the flat areas on tiles as well as the sharply curved areas like rims and handles. I think it is due to two seperate issues. Firstly the glaze for both temperatures contains Gerstley Borate, more at cone04 than cone 6. GB can help solve crazing issues in a glaze but if added to excess it can cause shivering, the opposite to crazing. If you add to this physical characteristic of the glaze, the underglaze issue of flaking, you can see how the underglaze can be forced off. However, underglaze should be adhered to the clay body by melting to it. Therefore it seems reasonable to assume, since mine work fine at cone 6, that at cone 04 the underglaze is not melting to the clay body. I tried firing to Bisque 01 and the issue mostly disappeared, the deep greens still flaked as if unmelted to the clay. I realized that these two colors were very old pint jars that had settled out repeatedly over the years. I add water and stir them but they settle every time I open the jar. I think that old underglazes may change compositionaly as they are used, heavier particles settle, lighter particles get used up since they are at the top of the jar. I have been guilty of not reading the label instructions, which, clearly state to stir thoroughly before using. To sumarise I think that underglazes should be bisque fired higher to melt them to the clay body. Secondly glazes with GB should be balanced by replacing the GB with Fritt 3195 to reduce the glaze comression on the clay body. Thirdly the underglazes should be stirred vigorously every time used. Old underglazes should be modified by adding fritt 3195 if they seem to be settling out and flaking. Adding a suspending agent to the underglaze should also help with settling issue.

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. I realized that these two colors were very old pint jars that had settled out repeatedly over the years. I add water and stir them but they settle every time I open the jar. I think that old underglazes may change compositionaly as they are used, heavier particles settle, lighter particles get used up since they are at the top of the jar. I have been guilty of not reading the label instructions, which, clearly state to stir thoroughly before using. 

 

Not stirring properly is a real problem in a community setting too.  I liken glaze to a fruit cake mix.  If you make it and leave it to stand for a few days, the heavy items sink and the light ones float.  Now tip it into three separate cake tins and cook.  You'll probably get "cake" from all three, but they will have differing amounts of fruit, nuts and peel in each one.

 

I'd throw the old jars and buy new.

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  • 4 months later...

We had a spectacular "show" on Thursday.  A slip-cast door number type plaque had been out of the kiln for 1 or two weeks.  The owner put it on her table and called me over.  The underglaze around the edges was flaking off.  The same yellow underglaze on a separate flower was not flaking at all.  

 

Other colours on the plaque were also flaking a little bit.  I gave her an old toothbrush and suggested she brush off the loose pieces (while I thought "what next") and then when she was nearly finished I sat down opposite her.  

 

She was idly brushing around the inside border when......... the underglaze on the flat surface started to crack and then each piece curled up.  Our "oohs" and "ahhs" brought the rest of the group to watch the show.  Eventually the whole flat surface had no underglaze attached at all.  The fancy edges stayed covered, it was just the flat surface that was now completely bare.

 

Our old tutor left four weeks ago, so I have no idea what products this lady used, she says "Miss" gave her underglazes and told her to use "???" as a basecoat first.  No, no idea what the basecoat was, and no chance of finding out.  The average age of the group when I'm not there is nearly 80, and despite many years of trying, we cannot get them to stop saying "paint" when they mean underglaze or glaze.  It all happened so quick, that I didn't even think to take photos.  Well, not until I got home, but by then - too late.

 

I'm guessing that like vsmithpots above, these products are so old and they've never been stirred properly, that they are lacking some magic ingredient.

 

I think I'll have to do a mini-teach next week on the importance of stirring thoroughly.

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  • 3 years later...

Recently I witnessed two failures to properly shake / mix glaze jars.  One occurred when a very experienced potter friend of mine purchased glazes from Europe.  She had several firings where the glazes did not come close to their advertised color.  We thought it was a temperature problem, but later she discovered and inch of solid material at the bottom of the jar.  She was very embarrassed :-).   On another occasion I was reviewing ceramic videos and the person grabbed a jar gave it three shakes and then opened the jar.  It still had the foil inner seal on.  So a brand new jar was only shaken three tImes!  Highly unlikely they will be getting consistent results.  (Better to learn from others mistakes)

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