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


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1 hour ago, Bill Kielb said:

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.

Hi Bill,

The point I was trying to make was if one wanted to test for an underglaze being too refractory or overly fluxed adding a material to increase or decrease the melt would be a simple test. Again, it's lovely that the glazes you use work. Given that altering glazes is out of the reach of some potters adding Gerstley Borate (or a boron frit like you mentioned some members in the studio you are member of do) is an easy way to do increase the melt of an underglaze. Adding kaolin to to do the opposite. 2 materials only, a measuring spoon and no scale needed. A very simple old underglaze recipe is simply kaolin, Gerstley (or frit), stain plus water. Not the best recipe but it demonstrates the core materials.

1 hour ago, Bill Kielb said:

Gerstley and epk contain silica, alumina and flux,  Gerstley not so much on the alumina, ……………… so maybe a good indicator, maybe not.

Yes, thank you, I am aware of the oxides that make up Gerstley Borate and EPK. Just as we refer to potassium or sodium feldspars doesn't preclude the fact that both contain other oxides also; I assumed it was a given that we are supplying more than just a pure flux oxide or alumina oxide.  Adding Gerstley Borate will increase the melt, adding kaolin the opposite.

1 hour ago, Bill Kielb said:

... hundreds actually including combinations of thickness and color while minimizing the amount of underglaze used.

If my suggestion of using 1 tablespoon of underglaze seems to be wasteful then by all means scale it down. 

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2 hours ago, carolross said:

That's great news :)  What clear are you using, and to what cone do you fire?

I mix my clear.  But I have friends who love Amaco's clear.  I fire to cone 6.  Although I have been experimenting with cone 5 just a bit.  I can send a pic?  The ug colors are bright and vibrant. 

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20 hours ago, Min said:
22 hours ago, Bill Kielb said:

 

If my suggestion of using 1 tablespoon of underglaze seems to be wasteful then by all means scale it down. 

Yeah, thinking about this a little further, if they are using 2oz bottles then a tablespoon is probably a bit much for a test sample  as that is a 1/4 of their stuff and UG is likely expensive for them. I see lots of folks with 2oz bottles for variety in color and pints for base colors they use.
Actually just running this in my head with the tablespoon of underglaze assuming about 65% is raw materials with a likely  percentage of 20% or more clay already in the formula, this probably kicks the clay up another 20% or more using the teaspoon of epk so maybe scale this all back considerably as well to start the test.

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well well well, someone shouldn't have commented on this thread at all (me!)  When I started looking at all the little bowls I pulled out of the kiln, imagine my surprise when every single dish with Intense Yellow or Flame Orange had some sort of issue.  I used Turquoise and Avocado on the other bowls. They were fine.  That tells me it was something to do with the intense colors of the yellow and orange. And....I put more than one coat on.  At least 2 maybe 3.  I have to say I love the orange peel affect on the orange, but it was not what I was intending.  For testing purposes, I put more clear on a couple of the bowls and put them back in a glaze fire this morning.  I do think the thickness of the underglaze on the intense colors, had something to do with this issue.  

Lesson learned.  Clay craft is humbling.

Roberta

Orange with citrus.jpg

blue with clouds.jpg

yellow with citrus.jpg

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1 hour ago, Roberta12 said:

I do think the thickness of the underglaze on the intense colors, had something to do with this 

If it helps we found color and thickness to be a primary cause. In our case we needed a tested glaze that worked pretty much no matter how the artist wanted to paint it else be accused of stifling creativity!  Hence the glaze solution. I think I only increased the boron a few points, so .15 to .19ish. Marcias matte is on glazy if you want to double check what it was brought up to. Anyway, IF this is your issue folks have been successful at adding a tiny amount of boron fritt or GB to their underglaze colors prone to this. Tiny amount seems to be key else the glaze chem locally is a mystery. Hope that helps.

As usual tesssssst.

 

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53 minutes ago, Bill Kielb said:

If it helps we found color and thickness to be a primary cause. In our case we needed a tested glaze that worked pretty much no matter how the artist wanted to paint it else be accused of stifling creativity!  Hence the glaze solution. I think I only increased the boron a few points, so .15 to .19ish. Marcias matte is on glazy if you want to double check what it was brought up to. Anyway, IF this is your issue folks have been successful at adding a tiny amount of boron fritt or GB to their underglaze colors prone to this. Tiny amount seems to be key else the glaze chem locally is a mystery. Hope that helps.

As usual tesssssst.

 

yes, more testing will happen.  It was such an easy project.  I used paper stencils on bone dry clay.  I have done that many times, but I usually thin my ug a bit.  And this surface issue didn't happen until glaze firing.  After bisque it was smooth.  I also discovered in that load that the 7 cone was down!  I haven't put cone packs in for awhile because I was out.  I will start doing that with more frequency.  

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