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I am making plates with plainsman clay and the ^6 clear glaze. The problem I am having is with the under glaze painted on the rim.  The clear glaze does mot seem to get thick enough and has a sandpaper like finish after fired to ^6.I dip the plate in the glaze, tied a longer and shorter dip time with no difference.  The rest of the glaze on the plate is fine.  Any ideas how to solve this problem.

 

Edited by ronfire

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is this something new?   have you changed brands of underglaze?   are you using underglaze to get a special color or could you use slip made from the claybody and some coloring stain or carbonate?

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3 hours ago, ronfire said:

Using PSH under glaze. This has been doing it for awhile but seems to have gotten worse. Maybe the wife is painting more under glaze on

We had this problem and it was color specific as well as the heaviness of the underglaze. All colors worked fine as a light wash but as their thickness increased the area above appeared to became more refractory and would not melt. We reformulated our glaze with additional boron to melt and ended up with a decent compromise with how thick certain colors could be applied. We then developed a gloss clear and matte clear that worked in this fashion

The picture below is the result. Although there is heavy black over orange, the matte glaze appears fully melted. In the past the heavy areas of coverage would not fully melt, even the random dots of yellow splatter would struggle to melt. This works for the clear as well, just don’t have a picture handy. We confirmed this with a number of test tiles before settling on the final recipe’s.

Let me know if you want the recipe’s to try, message me. No guarantees but they work for us on our porcelain.

7DA4899F-BD61-4133-BF06-16A0940B83FB.jpeg

Edited by Bill Kielb

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5 hours ago, neilestrick said:

When are you applying the underglaze- before or after bisque?

We have had that happen green and bisque, so from many tests it  appeared to be thickness  as the main cause and definitely more predominate with certain colors.

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10 hours ago, Bill Kielb said:

We have had that happen green and bisque, so from many tests it  appeared to be thickness  as the main cause and definitely more predominate with certain colors.

If the underglaze is being applied to bisque ware, the binders in the underglaze can prevent  getting a good glaze layer when dipping, regardless of the thickness of the underglaze.

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45 minutes ago, neilestrick said:

If the underglaze is being applied to bisque ware, the binders in the underglaze can prevent  getting a good glaze layer when dipping, regardless of the thickness of the underglaze.

Could be, just have tried many  bisque and green samples and from that experience bisque had no effect, before and after. In our case bisque only sintered the thick underglaze which remained refractory in the thick areas when fired.

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11 minutes ago, Bill Kielb said:

Could be, just have tried many  bisque and green samples and from that experience bisque had no effect, before and after. In our case bisque only sintered the thick underglaze which remained refractory in the thick areas when fired.

We see a huge difference across all brands of underglaze on how the glaze goes on depending on whether or not the underglaze was bisque fired. Amaco seems to be the worst, but it happens with all of them. The binders that make the underglaze brushable act like a shell, making it less porous than the raw clay, and therefore giving a thinner glaze layer. If you bisque fire the underglazes, the binders burn out and it's not an issue. Thickness can definitely be an issue, too, but ronfire could be getting a double whammy if the under glaze is too thick and not bisque fired before glazing.

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3 hours ago, neilestrick said:

Some underglazes can also get rough because they flux out too much at cone 6. Speedball red and royal blue do that. If you fire them raw, they actually start to gloss over a bit. With glaze on them they bubble and come out rough.

We noticed that in many of our tests but after looking at the vertical surfaces, no movement so we changed our thought that the area was too refractory or not melted.. Tests with various matte clears revealed these areas when melted would go from matte to glossy where the thickest underglaze was and of course often color / brand dependent.

That was a reasonable indication that for that local area the silica was raised enough to move from (5:1 ish towards 7:1ish) matte to gloss. Some folks actually counted on this happening for the effect. Anyway, almost all these issues went away after upping the boron until a clean melt was achieved. Adding high boron fritt to the underglaze seemed to back up the theory as well.

We did experience a reasonable limit though especially with the true matte clears.There is only so much you can make them melt without destroying the matte as well as causing plenty of other issues. Our limit ended up to be: it had to melt well over a drop or splatter of the underglaze which actually ends up quite thick.

In the end for us it was easiest to develop a matte and gloss that worked every time over moderate underglaze.

Edited by Bill Kielb

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I just had a conversation with a teacher about Amaco's Warm Gray ug.  I had several pots that the gray and only the gray, crawled in the glaze firing. She said that particular color will give a person problems if you put it on too thick or too many coats.  It is rather thin out of the bottle, so I gave a 2nd and perhaps even a 3rd coat.  And it crawled on both stoneware and porcelain.  And my application was on greenware.  I have previously used it on bisque without  a problem.  Perhaps that is because the bisque absorbed more than the greenware?  Not sure, but note to self....thinner application of Warm Gray.  

And...That could be the problem for you as well @ronfire   Just a thought!

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2 hours ago, Roberta12 said:

I just had a conversation with a teacher about Amaco's Warm Gray ug.  I had several pots that the gray and only the gray, crawled in the glaze firing. She said that particular color will give a person problems if you put it on too thick or too many coats.  It is rather thin out of the bottle, so I gave a 2nd and perhaps even a 3rd coat.  And it crawled on both stoneware and porcelain.  And my application was on greenware.  I have previously used it on bisque without  a problem.  Perhaps that is because the bisque absorbed more than the greenware?  Not sure, but note to self....thinner application of Warm Gray.  

And...That could be the problem for you as well @ronfire   Just a thought!

Excellent catch.!  Underglaze definitely clogs things up so your glaze will end up sort of thicker in those areas and therefore enhance the potential for micro cracks as it drys.  If your overglaze does not possess the healing power necessary then crawling likely.   If you ever spray glazes this becomes an obvious difference as the underglaze areas tend to pool and not absorb so we are allowing a considerable amount of  extra time for those areas to dry.

So thinner underglaze is always less troublesome than thicker.

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@ronfire, could you clear up if the u.g. is bisque fired on or if you are glazing over unfired u.g.? Narrowing down the possible problematic areas would help here. Also, which of Plainsman's clears are you using? Are the underglazes settling out over time and not getting stirred up? Since you said the problem is getting worse I'm wondering if some of the heavier ingredients (like a frit) are settling out in the bottom of the jar.

30 minutes ago, Bill Kielb said:

Underglaze definitely clogs things up so your glaze will end up sort of thicker in those areas and therefore enhance the potential for micro cracks as it drys.  If your overglaze does not possess the healing power necessary then crawling likely.   If you ever spray glazes this becomes an obvious difference as the underglaze areas tend to pool and not absorb so we are allowing a considerable amount of  extra time for those areas to dry.

You lost me here Bill, the glaze is thicker over the underglaze like in your first sentence or not absorbed like in the last sentence? I'm not getting how sprayed glazes can pool on a pot like in your example above.  I've found that with underglazes that flux enough, during the bisque fire, to cause a thinner layer of glaze being absorbed onto those parts of the pot that a cooler bisque fire helps. 

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10 minutes ago, Min said:

 

@ronfire, could you clear up if the u.g. is bisque fired on or if you are glazing over unfired u.g.? Narrowing down the possible problematic areas would help here. Also, which of Plainsman's clears are you using? Are the underglazes settling out over time and not getting stirred up? Since you said the problem is getting worse I'm wondering if some of the heavier ingredients (like a frit) are settling out in the bottom of the jar.

You lost me here Bill, the glaze is thicker over the underglaze like in your first sentence or not absorbed like in the last sentence? I'm not getting how sprayed glazes can pool on a pot like in your example above.  I've found that with underglazes that flux enough, during the bisque fire, to cause a thinner layer of glaze being absorbed onto those parts of the pot that a cooler bisque fire helps. 

Every time I spray, the least absorbent part of the pot is where the underglaze is. Where the bare pot absorbs and drys quickly I need to wait for the areas with underglaze to dry.  The areas that dry slower pool, drip, run much more so than the areas that absorb the glaze quickly. In effect it can end up effectively thicker while drying on a more impervious area. This is super obvious while spraying as runs and drips become a concern. So maybe potential for more uneven coating of glaze if you will.  Assuming the layer takes longer to dry rather than be partially absorbed it is likely more predisposed to crawling. I believe  @Roberta12 makes a great point.

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The under glaze is done on the  bisque fired plates of plainsman m340 clay then the dip in the glaze after it has dried.The clear glaze is the plainsman premade glaze mix  just add water.

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1 hour ago, ronfire said:

The under glaze is done on the  bisque fired plates of plainsman m340 clay then the dip in the glaze after it has dried.The clear glaze is the plainsman premade glaze mix  just add water.

The problem is as I described above. The additives in the underglaze that make it brushable are preventing a good glaze coat. Try applying your underglaze at bone dry and then bisque firing. The underglaze will go on very similar to on bisque since bone dry is very porous.

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Thanks for providing more info @ronfire, that was really helpful. I agree with Neil, apply the u.g. to dry greenware then bisque fire it. BTW I think the clear glaze that Greenbarn sells for M340 is this one from Tony Hansen. 

Ron, I looked up PSH underglazes and they do say to put them on bisque then glaze when dry so I'm wondering if they flux out a bit at typical bisque temps. Like everything ceramics I would suggest running a few test tiles before doing actual work.

Edited by Min
added a thought

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11 hours ago, Babs said:

What is a substitute for Fusion frit in this recipe Min? F524?

It's getting harder to find info on frits that is up to date. The best I could come up with is this substitution chart from Fusion but I don't think it's much help as Pemco frits are no longer widely available and the Ferro one I couldn't find either. I think you could use a combination of Ferro 3286 and 3289 to get an equivalent of Fusion F524 but it would take some tinkering.

 

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23 hours ago, Bill Kielb said:

We noticed that in many of our tests but after looking at the vertical surfaces, no movement so we changed our thought that the area was too refractory or not melted.

 

My thought on the bubbly underglazes is that they're in that in-between stage where they are melting more than the others, but not enough to melt out and smooth over. I think stiffening the underglaze could also solve the problem. My concern with adding frit to the underglaze is that it may reduce the porosity after bisque firing, making glazing difficult. I'm going to run some tests both ways with the Speedball red and see what happens.

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