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Underglaze Flaking Off


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#1 Lucy

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Posted 31 March 2010 - 05:17 PM

Greetings and much thanks for this forum! I am using underglazes on a white earthenware clay body and often the underglaze flakes off after firing slowly to 05 and cooling slowly. I've been applying the underglaze to very leather hard clay. I'll do some experimenting, but might anyone save me some time by pointing me in the right direction re: apply it when it's wetter? drier? after bisque firing? Or are there other variables I should manipulate first? Thanks.

#2 Linnet

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Posted 31 March 2010 - 05:27 PM

Hi Lucy, I'm not an expert but I do some underglazing and terra sigs.
If the application is too thick this can sometimes happen. I have applied underglaze both on unfired and bisque pieces and prefer unfired due to my clear glaze over top sometimes combines with the underglaze and produces fury edges. Hope this helps. Would like to have some others view on this.
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#3 Rondas Originals

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Posted 31 March 2010 - 07:18 PM

Hi Lucy, I'm not an expert but I do some underglazing and terra sigs.
If the application is too thick this can sometimes happen. I have applied underglaze both on unfired and bisque pieces and prefer unfired due to my clear glaze over top sometimes combines with the underglaze and produces fury edges. Hope this helps. Would like to have some others view on this.


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#4 Rondas Originals

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Posted 31 March 2010 - 07:25 PM

Lucy,
I have had the same thing happen. I was unwittingly applying three coats of a one stroke underglaze, trying to get an opaque coverage. It peeled everytime during firing, greenware or bisque it did not matter.
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#5 Linnet

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Posted 31 March 2010 - 08:37 PM

There is another option. That is to mix a little of the same clay as the piece you made with the underglaze before applying to the piece just before leather hard, so they dry at the same rate and combine better.
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#6 Cindy in SD

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Posted 02 April 2010 - 03:31 PM

I don't have much experience with low-fire clays, but I prefer applying underglaze to unfired bisque, too. Or to bisque and then refiring it before glazing. If you apply the underglaze too thickly, it will crack--you can often see this happening as it dries. In addition, some underglazes are extra refractory (copper comes to mind) and will resist your glaze if applied too thickly.

#7 LindaSFG

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Posted 02 April 2010 - 09:18 PM

Greetings and much thanks for this forum! I am using underglazes on a white earthenware clay body and often the underglaze flakes off after firing slowly to 05 and cooling slowly. I've been applying the underglaze to very leather hard clay. I'll do some experimenting, but might anyone save me some time by pointing me in the right direction re: apply it when it's wetter? drier? after bisque firing? Or are there other variables I should manipulate first? Thanks.


I have been encountering this problem more frequently in the last few years although it has happened off and on over 15+ years. There doesn't seem to be any consistency. I've been watering down the first coat and careful not to apply subsequent coats thickly. It obviously is a fit problem which occurs on curved surfaces most often. I've been advised to add a little glaze to the underglaze but I don't care for a glazed finish and this is hard to control. Could it be the new talc formulas in white clays? It has happened to me and my students using different clay bodies, various underglaze products, and on both bisque and greenware applications. The majority of the time I apply color around leather hard or before. I would love to know the cause and solution!!!!

#8 Sam

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Posted 29 September 2010 - 05:08 PM


Greetings and much thanks for this forum! I am using underglazes on a white earthenware clay body and often the underglaze flakes off after firing slowly to 05 and cooling slowly. I've been applying the underglaze to very leather hard clay. I'll do some experimenting, but might anyone save me some time by pointing me in the right direction re: apply it when it's wetter? drier? after bisque firing? Or are there other variables I should manipulate first? Thanks.


I have been encountering this problem more frequently in the last few years although it has happened off and on over 15+ years. There doesn't seem to be any consistency. I've been watering down the first coat and careful not to apply subsequent coats thickly. It obviously is a fit problem which occurs on curved surfaces most often. I've been advised to add a little glaze to the underglaze but I don't care for a glazed finish and this is hard to control. Could it be the new talc formulas in white clays? It has happened to me and my students using different clay bodies, various underglaze products, and on both bisque and greenware applications. The majority of the time I apply color around leather hard or before. I would love to know the cause and solution!!!!


Hi

I too have been having a very similar problem and really wish I could find out the causes. I have been using a fritt based underglaze (Spectrum) on small pieces cast from white earthenware casting slip. On firing some of the underglaze flakes off, but only from small concave recessed areas that have been pressed into the piece as low relief decoration.

The ideas that I am currently working on as the possible cause are:

1. There is dust or something in the recessed areas causing the underglaze not to "take" properly
2. The underglaze is building up thicker in those areas as it sort of dams up in them
3. I have not dried my ware properly and the water has mainly stayed in the recesssed areas so steam is blowing those bits off (I do fire very slowly and vent up to 200 deg,but maybe not enough??)
4. I have almost abandoned this theory, but possibly it has something to do with casting slip having a higher co-efficient of expansion than pug clay and the underglaze not being compatible with slip-cast.... I have contacted the manufacturers but not heard back yet.

For now I am going to focus on the drying issue, and if that doesn't work may try adding some clear glaze or possibly high expansion fritt... although I have no ideas of the quantities/proportions that would be required... any info on that would be welcome.

thanks
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#9 Deb Evans

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Posted 30 September 2010 - 04:05 PM

Hi all, really hard to give you a fix because you have different materials/firing temps and we don't know what they are ( receipes or name brand helps/cone).
I've always made slip from the clay body I w (white clay) as using and then add mason stains or metals - that way you have the "fit". For darker bodies on bisque - a flux ( like GB) + color .

Nice white slip reciepe I 've used for all firing conditions
Neph Syn 625
Tenn Ball 500
EPK 500
Flint 750
Zircopax 200

I make a this batch and dry mix well, then wght out smaller portions like 200 gm and add colorents (lots of charts for % in books and web) and enjoy.
Steve - I'd bet on #2 - concave is getting too much slip - a little abrasion before firing should solve that problem.
Good luck.




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