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Underglaze pinholes and flaws - I don't understand why?


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Hello everyone,

I've been having glaze (or underglaze?)  problems and thought I had it figured it out - but the last firing says I haven't.  I had to transition from a large home studio to a small bedroom studio, when I downsized from a house to a condo.

I handbuild in this bedroom studio and it's been working pretty well.  Firing seems to be the problematic side of the arrangement.  I have a  110V test kiln in this studio that I've been using to bisque fire - open window, lots of fans, air cleaner running - and yup, I know it's far from ideal.  I have my larger kiln in a friends' studio and I usually take the bisqued pieces  over there to glaze fire.

And I've been getting  bubbles, bumps  and holes after the glaze firing and think that the clay was not properly outgassing in the test kiln.  So I started using the larger kiln for both firings.  I used ^5-6 Laguna stoneware, Mayco underglaze and Mayco clear, glossy glaze.  I bisque'd to ^04 (according to commercial glaze directions) at SLOW speed.  Then, I rinsed the pieces under running water, brushed on 2-3 coats of glaze and smoothed over irregularities with my finger.  I  fired to ^6 at MEDIUM speed.

Some pieces came out okay - a few tiny bumps usually in clusters - but not as awful as like before.  BUT, some of the pieces had bumps everywhere.   I'm attaching 2 photos of 2 plates, underglazed and glazed exactly alike.   Please help me figure out what's going on!!  Thank you!

IMG_3669.jpeg

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No, I put the underglaze on greenware and then bisque.  I was having problems with Amaco underglazes and switched to Mayco, but seem to be having similar problems.  I do sgraffito and Mishima, but rinse after the bisque & before I brush on glaze.  Thanks!

 

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1 hour ago, neilestrick said:

Try not rinsing it. Could be something in your water, and it's not necessary. 

How thick is the underglaze- how many coats?

Neil, 

I'll stop rinsing.  I was concerned there might be dust but since I glaze almost as soon as it comes out of the bisque, i didn't think that's an issue. 

I try to keep the underglaze thinned with water, because it  gets pretty  thick when the lid is kept off.  But the black was pretty thick, I think, when I used it.  I've had problems with it not covering well, so I use somewhere between 3-5 coats depending on the coverage. 

If it's thick when applied, would that be a problem?  I've seen people apply it when it's really goopy and thick (yogurt); others who like it thin,  almost dripping off the brush (skim milk).

Thanks, Neil, I want to get this resolved.

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3 hours ago, carolross said:

I try to keep the underglaze thinned with water, because it  gets pretty  thick when the lid is kept off.  But the black was pretty thick, I think, when I used it.  I've had problems with it not covering well, so I use somewhere between 3-5 coats depending on the coverage. 

If it's thick when applied, would that be a problem?  I've seen people apply it when it's really goopy and thick (yogurt); others who like it thin,  almost dripping off the brush (skim milk).

Before doing more work I'ld run some tests with thickness / number of layers. 5 coats seems excessive. Have you tried Spectrum Black 515? I use it watered down and 2-3 fairly thin coats for solid coverage. (it's on the bowl below)

If there is dust on your pots if possible try using compressed air to blow it off before glazing if you don't need to wet the bisque. This might be another thing to test, unwashed, wiped with a clean sponge, compressed air, dipped in water etc. 

IMG_1408.jpeg.3b353d4c109b000cf43df4b7473db64a.jpeg

 

EDIT: I'm going to edit the title of this thread to better reflect it's contents. Going forward it will make it easier to find it when doing a search.

Edited by Min
Title edit by Min for clarity of topic
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2 hours ago, carolross said:

hmmm, okay, but the area feels bumpy and ridged?  So, if those are pinholes, what can I do differently?  Thanks!

Something to consider:
I do see bumps and we have seen this with many underglazes when applied thickly, usually uniform coverage thick. So the possibility to consider is the areas where underglazes are applied thickly can become refractory and the over glaze simply does not melt In these areas. After testing for over a year we found thickness as well as certain colors were more refractory.

The end result was to formulate a clear gloss and matte that would melt better over these areas so the artists were free to create as they preferred with underglaze. It’s fairly easy to test for this condition and wash style painting rarely bubbles and is much more porous and less refractory than the heavy areas. So if all the wash areas are fine and many of the heavy application areas bubble, this is likely your issue.

To make matters more confusing

When underglaze is applied solid it also affects the amount of glaze that can be applied over it. Heavily underglazed  areas will close off the pores in the bisque which will then take longer for the overglaze in that area to dry which means if double dipping, wait until the underglazed areas dry, they will take longer than the rest of the pot. Dip too soon and you likely remove as much overglaze as you put on. A thinner than normal layer of overglaze definitely makes the bubble condition more likely.

Easy to test if this is your issue though.

Edited by Bill Kielb
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  • Min changed the title to Underglaze pinholes and flaws - I don't understand why?
4 hours ago, carolross said:

I'll stop rinsing.  I was concerned there might be dust but since I glaze almost as soon as it comes out of the bisque, i didn't think that's an issue. 

I try to keep the underglaze thinned with water, because it  gets pretty  thick when the lid is kept off.  But the black was pretty thick, I think, when I used it.  I've had problems with it not covering well, so I use somewhere between 3-5 coats depending on the coverage. 

If you're covering the piece with wax during the decorating process, you will have a little bit of dust/ash on the piece when it comes out out bisque just blow it off or brush it off with a dry brush.

You'll get more consistent/even coverage with 3 thin coats than 3 thick coats. Most underglazes come too thick and need to be thinned out. The dryness of your work will also affect how evenly they apply. I prefer a little on the dry side of leather hard.

Are any other colors doing it, or just the black? Or are you only using black?

 

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2 hours ago, Bill Kielb said:

The end result was to formulate a clear gloss and matte that would melt better over these areas so the artists were free to create as they preferred with underglaze.

Lovely that you found a solution that works for the potters in your studio but from reading the original post from @carolrossit sounds like she is using a commercial brushing glaze. I think we need to work within the parameters of the the op's usage insofar as commercial versus studio mixed glaze.  Like many things in ceramics there are times we can make things more complicated than they need to be. 

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1 minute ago, Min said:

Lovely that you found a solution that works for the potters in your studio but from reading the original post from @carolrossit sounds like she is using a commercial brushing glaze. I think we need to work within the parameters of the the op's usage insofar as commercial versus studio mixed glaze.  Like many things in ceramics there are times we can make things more complicated than they need to be. 

Makes sense, it was not offered to complicate things it was offered as a diagnostic not presented in the other threads. If that is the issue, then there are other remedies than formulating your own glaze. We worked on this for over a year before concluding it was the central issue.
Had someone pointed out the possibility it would have saved us a year of testing. Along with all the typical remedies everyone was speculating on at that time.

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carol,   using a brush to apply underglaze to a piece is a very difficult thing to do.   it does not appear to be, just pick up a brush, dip it in a jar and put that on the clay.  simple.

except  underglaze does not melt.  it stays where you put it.  if your brush picks up a thick liquid and the clay is dry, the liquid turns to a solid quickly because the dry clay sucks out the water before the brush gets very far.   what to do?   think about the possible solution.   make the liquid wetter and make the surface it touches wetter.  use a brush that is wet before you put it into the jar.  the lumps you have applied to the piece will still be there when covered by glaze.   do not have lumps in the first place.   remember, underglaze does not melt.

if someone who is working on a melting underglaze please do not confuse the answer by saying it can.

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15 hours ago, Bill Kielb said:

So the possibility to consider is the areas where underglazes are applied thickly can become refractory and the over glaze simply does not melt In these areas.

If the underglaze is refractory and the overglaze doesn't melt, why are there bubbles/roughness? Clearly things are melting. Every time I've seen underglazes get all rough and bubbly it's because they are fluxing out too much, not because they are too refractory. You can glaze a firebrick, after all. If you look at Speedball Red and Royal blue, they have this exact problem, where the glaze comes out rough and bubbly looking. If you fire them alone, you can see where they're starting melt out and gloss over, whereas the underglazes that don't have this problem stay matte. So I think it's more of an issue of the underglaze mixing with the overglaze and messing up the melt, like when you overlap two glazes that aren't compatible for whatever reason and you get bubbling.

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1 hour ago, neilestrick said:

So I think it's more of an issue of the underglaze mixing with the overglaze and messing up the melt,

Maybe. I can tell you what the testing revealed. Simple tests, where a light coat of underglaze is connected to a very thick coat of underglaze and the overglaze bubbles in the heavy area and stays smooth in the light area.

Follow that with a better glaze melt (increased boron slightly) and all of a sudden the glaze begins to  finish smoothly in the heavy areas. This proved true for matte and gloss overglazes btw. 

We share those results so folks can consider the result. I have learned though that there is often speculative disagreement and contention over this to which I try and phrase things as ………consider this possibility……. Simple to do your own test. The tests we did indicated something counterintuitive and helped us solve our problem.

Edited by Bill Kielb
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2 minutes ago, Bill Kielb said:

Simple tests, where a light coat of underglaze is connected to a very thick coat of underglaze and the overglaze bubbles in the heavy area and stays smooth in the light area.

Follow that with a better glaze melt (increased boron slightly) and all of a sudden the glaze begins to  finish smoothly in the heavy areas. This proved true for matte and gloss overglazes btw. 

My thinking is that a thin wash of underglaze simply don't have enough material to affect the glaze melt, but the thicker areas are being taken into the glaze a bit and messing up the melt. That blend is not a good proportion of fluxes to get a smooth finish, as when a glaze bubbles before smoothing over. The underglaze, being more refractory than the glaze, is keeping the glaze from completing its melt. It bubbles but doesn't smooth over. By increasing the boron in the glaze it's allowing the glaze to finish. So yes, the underglaze is more refractory than the glaze, but the underglazes that have this issue are less refractory than underglazes that don't have the problem. Underglazes are not supposed to go into melt.

That said, I have also seen this problem with glazes that are quite fluid, so I think it also has something to do with the glaze formula, not just the melt.

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1 hour ago, neilestrick said:

Every time I've seen underglazes get all rough and bubbly it's because they are fluxing out too much, not because they are too refractory.

Thank you, everyone!  There's so much to know, so many possibilities to explore and so many ideas and opinions.  I've found that clay people, almost without exception, have been incredibly generous over the 20+ years I've been making.  It's one of the things about working with clay that always surprises and delights me.  And that wealth of info can be befuddling and overwhelming!

Okay, what I see is that some underglaze colors, like red and black, bubble.  But I do generally apply them more thickly than absolutely necessary, I think, because they are so lovely when they're opaque.   So... hopefully, lesson learned - they don't need to be that thick.  I'm going to test, as Min suggested, by increasing the thickness of the glaze first.  Then by the number of coats of the underglaze.  I've also noticed that I get hardly any bubbling when I use satin matte glaze...

I'm wondering if I can do this in a test kiln (isn't this what it's for?) or will I have trouble with outgassing because the small kiln fires off so quickly?

I've also wondered about using slips with mason stains instead of commercial underglaze.  I haven't worked with slip - is it necessary to add flux, which is what I learned to do in college?  If so, which fluxes will adhere without bubbling (or overfluxing?)  My studio space is within my home, so I try not to do any mixing.  That said I have a couple of options.  One is that I have a patio which would be great in good weather.   Second a local supplier who will mix ingredients for me;  it won't be less expensive that way but at least I might have better luck!  

I've also seen sgraffito done using glaze on nearly-dry greenware.  Can it then be once-fired without great risk?  It would limit sgraffito work to the exterior of vessels and to non-functional pieces because the carving would leave bare clay exposed.

Thanks again for your suggestions!  I hope you'll answer these additional questions!

 

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3 hours ago, oldlady said:

except  underglaze does not melt.  it stays where you put it.  if your brush picks up a thick liquid and the clay is dry, the liquid turns to a solid quickly because the dry clay sucks out the water before the brush gets very far.   what to do?   think about the possible solution.   make the liquid wetter and make the surface it touches wetter.  use a brush that is wet before you put it into the jar.  the lumps you have applied to the piece will still be there when covered by glaze.   do not have lumps in the first place.   remember, underglaze does not melt.

Thanks, @oldlady.  I try not to apply underglaze to hard leather hard surfaces, but know that some potters do so on bone dry clay.  I don't understand why or how that works so well, but the work is lovely.  The colors aren't as vibrant as I prefer, but the work appears pristine!

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6 minutes ago, carolross said:

I'm wondering if I can do this in a test kiln (isn't this what it's for?) or will I have trouble with outgassing because the small kiln fires off so quickly?

To get accurate tests from a little test kiln you really need to put a digital controller on it so you can slow down the firing both going up and cooling down, so it mimics the cycle of your big kiln.

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@carolross  I use a fair amount of underglaze.  I have stuck with Amaco simply because I have their color palette dialed in.  I also have a little problem with black and red.  I try not to apply either of those colors too thickly. 

I started out using underglaze on bisqueware.  Then spray with clear and fire. For the last 3 years I have been trying to do more underglaze work on greenware (leatherhard or bone dry) in order to speed up the glazing process.  With the bone dry clay, it sucks the UG in rapidly so you can layer other designs on top of it if you choose.  If I am putting UG on leatherhard, I get more of a soft look that (I think that is what you were describing)

I was originally taught to wash my bisqueware under running water and immediately glaze.  Glaze results were interesting.  Then  I started using a damp sponge to wipe down the pots.  My gut tells me that allows the bisque to absorb more of the glaze or underglaze.  Perhaps the experts here will have a different opinion, but that has worked better for me rather than complete submersion of the pot. 

If you are going to once fire, you will need to test your clay and get your firing schedule dialed in.   @oldlady is a single fire potter.  She can give good tips on that process.

Roberta

 

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@Roberta12  Thanks for the tips!  I've always liked Amaco's colors much more than Mayco's.  I was using Amaco when I started having glaze problems and called Amaco.  They couldn't help and my supplier suggested Mayco.  He loves it, probably for considerations other than the color! :rolleyes:

My studio is inside my home, in an extra bedroom so I can't spray glaze.  Since I brush it, I take a lot of time to smooth over it with my fingers, eliminating pinholes.  I've seen a nice demo on bisque, with wax and glazes.  Might try that.  if I try once-firing, i will check with @oldlady, thanks!  

Thanks, Roberta, I appreciate your help!

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Sue McLeod is doing an underglaze research group. It might be good for you to check out what she's doing and add your input. Go to  Sue McLeod Ceramics and find "Troubleshooting Commercial Underglazes" . She has charts you can fill in to track your testing and then share with her to compile the results. I believe she has a facebook group as well where you can discuss these issues. 

I apply underglazes to bone dry work. I also mix up my own underglaze like or wash like formulas with mason stains and flux, sometimes adding other things as needed such as a bit of my clay body slip, CMC gum, or white underglaze. I notice that the mason stains act differently from one another too just like the commercial underglazes do.  I fire to cone 5. I experimented a bit with different fluxes mostly just to get the mason stain to stick to the surface of my work after bisque. I've used frit 3124, Gerstley Borate and Wollastonite as fluxes.  The ratio is about half flux and half stain, but do some experimenting if you add things like your clay body slip because it already contains some kind of flux. 

It almost looks like the black is trying to pop itself off the clay but the glaze is sort of holding it on. I've had big flakes of underglaze come off after bisque, I think when it was applied too thickly all at once. It seems like  a good way to get it to adhere better to the clay is to use a more watered down first layer so it sinks into the clay instead of sitting on top of it. 

 

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Re the debate of a refractory underglaze or not it would be fairly simple to test this theory without altering glaze recipes. Since we know adding flux will make things less refractory and adding alumina the opposite, using this logic I'ld run 2 parallel tests, one adding flux, Gerstley Borate should be a good choice, and the other adding calcined kaolin.  Say roughly 1 Tablespoon liquid underglaze + 3/4 teaspoon of Gerstley Borate or calcined kaolin plus enough additional water to make them brushable. Apply to both a vertical and horizontal surface with the same number of coats and fire them both the same and see what comes out of the kiln. 

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19 hours ago, carolross said:

@Roberta12  Thanks for the tips!  I've always liked Amaco's colors much more than Mayco's.  I was using Amaco when I started having glaze problems and called Amaco.  They couldn't help and my supplier suggested Mayco.  He loves it, probably for considerations other than the color! :rolleyes:

My studio is inside my home, in an extra bedroom so I can't spray glaze.  Since I brush it, I take a lot of time to smooth over it with my fingers, eliminating pinholes.  I've seen a nice demo on bisque, with wax and glazes.  Might try that.  if I try once-firing, i will check with @oldlady, thanks!  

Thanks, Roberta, I appreciate your help!

I should say I brush clear or dip clear now.  If the sprayer is up and running for whatever, I will use that, but I just unloaded some small dishes with underglaze that had been previously  bisqued  and I applied the clear by dipping.  They look great.  

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16 hours ago, Min said:

Say roughly 1 Tablespoon liquid underglaze + 3/4 teaspoon of Gerstley Borate or calcined kaolin plus enough additional water to make them brushable. Apply to both a vertical and horizontal surface with the same number of coats and fire them both the same and see what comes out of the kiln. 

If one is able to modify their glaze then testing as we did  for color and thickness toleration is fairly easy especially with a boron based cone six glaze. In the end we had a tolerant glaze and  the ease of testing led to being able to test whole panels of thickness, color, and manufacture easily, hundreds actually including combinations of thickness and color while minimizing the amount of underglaze used. If unable, there was the practice by some of adding a bit of Fritt to the underglaze and maybe not consuming a bunch of underglaze to get it to work.
Gerstley and epk contain silica, alumina and flux,  Gerstley not so much on the alumina, ……………… so maybe a good indicator, maybe not.

Edited by Bill Kielb
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