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Glossy Spots On Unglazed Matte Colored Porcelain

porcelain unglazed troubleshooting pigments stains

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#1 Patrickhartog

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Posted 07 April 2017 - 04:23 AM

Hi everybody,

 

This is my first post here, nice to meet you all. 

 

I have a question about some result of fired colored porcelain. I am making some unglazed porcelain objects, with the goal of having a very matte finish. Now I have made a test with porcelain colored with a black stain, I have added 4 % to the slip. The stain is based on Co-Fe-Cr, it's standard stain I purchased at a ceramic supplier, the same goes for the slip. What I did:

 

  • First did a bisque fire (electric kiln) until 950 degree Celsius (1742 degrees Fahrenheit)
  • I removed the objects and finished them (sanding) to a more smooth finish 
  • In the kiln again, fired until 1230 degree Celsius (2246 degrees Fahrenheit)

 

The result is a matte looking porcelain, but it has some unwanted glossy spots on the surface (see attached photos). I do not understand why this is happening. Maybe somebody has an idea what could be the cause of this?

 

Thank you in advance.

 

greetings, Patrick Hartog

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#2 Chilly

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Posted 07 April 2017 - 10:09 AM

Are the glossy spots where you sanded?  Maybe the clay got burnished and stayed that way.


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#3 neilestrick

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Posted 07 April 2017 - 10:42 AM

Could be that the stain is fluxing out a bit, or that you're getting some soluble salts coming to the surface and fluxing it out. Are you mixing the stain in the porcelain body, or just making a colored slip and applying it to the surface? If you're making a colored slip, try  making it with distilled water and see if that helps.


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#4 Patrickhartog

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Posted 07 April 2017 - 12:35 PM

Hi both, thank you for your replies, just to give some more information:

 

- I sanded the whole object after the bisque firing, and the spots only appear on some areas, it seems random. Off course it could be, that I sanded a little bit more on those spots...

 

- I mixed the stain in the porcelain body. 

 

I think the cause must be the sanding in between, because before I didn't had this problem. But I still need to finish (sanding) the pieces before the final firing. When the porcelain is fired high, it's just to hard for sanding. Could it be that if I sand with a coarser sanding paper, this burnishing (what it could be) wouldn't happen?

 

Kind regards, Patrick



#5 neilestrick

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Posted 07 April 2017 - 02:45 PM

I don't think it's burnishing. That kind of polish will usually burn off by cone 04. I think there's something chemical going on here. How are you adding the stain to the body?


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#6 glazenerd

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Posted 07 April 2017 - 09:50 PM

I smell "self" glazing due to high feldspar content: although in this case the feldspar was not mixed into the body properly. Make a 4 x 4 test tile and fire it to maturity without doing any prep work on it. If the gloss patches show up; then it is self glazing due to feldspar levels.

 

Are these glossy patches closer to a satin finish instead of gloss by chance?

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#7 Patrickhartog

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Posted 13 April 2017 - 06:44 AM

Thank you for the replies. The patches are closer to a satin finish. What I forgot to mention, and is probably the cause of this problem. The form I am slipcasting is difficult to get out of the mold, therefore I used some talc powder in the mould to make it easier to get out of the mold. But when I bought the talk, there was no ceramic store nearby, so I bought baby-powder , lol.... I just read the ingredients list on the packaging of this powder, apparently there's not only talc in there, there's also some other ingredient's: zinc-oxide and hydrated silica! Probaby not the ingredients you want to have on your unglazed porcelain, right? 



#8 neilestrick

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Posted 13 April 2017 - 08:08 AM

Yes, that's probably the cause. Are you press molding or slip casting? If the form won't come out of the mold and you're slip casting, then you should either leave it in the mold longer to set up more, or check for undercuts that are causing it to bind. If you're press molding, don't use talc or anything like that. Use corn starch- it will burn away.


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#9 Patrickhartog

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Posted 14 April 2017 - 03:23 AM

Hi Neil,

 

I am slipcasting it. The form doesn't have an undercast, but it has a center core, the object shrinks around this core (you can see it in the photos). It shrinks itself stuck in the mold, if I leave it in the mold for too long, it will shrink until it breaks.

 

I cleaned the molds, and soon I will see if the problem is solved.

 

Patrick







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