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Gray Areas In Fired Glazes


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I was wondering why glazes sometimes have grayish areas after firing?  This has happened with a couple of different clay bodies (standard 112, 211 and 225) and two different glazes (coyote oxblood and amaco oatmeal). I'm pouring the glaze onto/into the bisqueware

 

I'm firing to ^6 with a sitter

1 hour on low

1 hour on meduim

remainder on hi-fire

holding for 8 minutes after the sitter falls

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This was the only one I could quickly find and photo's color rendition is not good. Also, its not one of the glazes I mentioned, I'm not sure this is.

 

post-80153-0-33751200-1494718465_thumb.jpg

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The body underneath probably has titanium in it. Which is causing the glaze above to have slight variation as well as microcrystals which might be causing the greyish areas. For your pots is it only happening on darker stoneware bodies and not white bodies? That would be my guess. However without a good picture I can't tell what is going on at all. 

 

The reason I say this is because I have a glaze that looks similar to that orange cream glaze, when I put a titanium glaze under or over it, it forms a greyish area. I can attach a picture and you can see if it looks similar.

 

Edit: Picture Attached. 

post-63346-0-46284600-1494720603_thumb.jpg

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In my picture or his. I am just guessing here. I can't see the grey areas in his picture. In my picture that is a spot of titanium glaze sprayed which is what turned it grey like that.

 

Edit: in my picture it looks like a band of white glaze, but that was just the rim diped for some reason there. The white glaze and the rust colored underneath it are both the same glaze. The only difference is the titanium glaze sprayed in that area to produce that mottled crystal greyish area. This is 100% from the other glaze. The creamwhite glaze doesn't run at all.

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Now that I am thinking about this it could just be thick areas that are more mottled?. Not 100% on anything without a good picture to see. I am not the best at this stuff. I am sure someone who understands more than me will chime in.

 

Edit: Reading your last post below this, ignore everything I said. If it is in the thin areas. I have no idea and all my post are incorrect in assumption. I will go back to my corner now.  :unsure:

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I was thinking it might be to thin. I have to look for the one with the oxblood glaze, if I didn't toss it out I'll post that one.

 

I'm going to seriously start considering spraying on the glaze, maybe I could get a more consistent layer.

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Ron:

 

Brown / dark stoneware bodies have high levels of iron / titanium / and magnesium. It is also common to find large particles of feldspar minerals: along with high levels of carbons (primarily sulfur from lignite coal). The very same elements that make these bodies all warm and toasty; are the very same things that cause problems.

 

 
In testing magnesium levels in clay, you can see the grey cast in bar #3 and titanium in bar 5. High levels of these elements can certainly leach into a glaze causing discoloration/s. Magnesium levels are the more probable cause as seen here:
 

Mgo Test 2

 

There can be other reasons, but if you are using a dirty/dark stoneware body; then Mgo is the most likely culprit.

 

Nerd

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[snip]

Brown / dark stoneware bodies have high levels of iron / titanium / and magnesium. It is also common to find large particles of feldspar minerals: along with high levels of carbons (primarily sulfur from lignite coal). The very same elements that make these bodies all warm and toasty; are the very same things that cause problems

[snip]

 

So is it possible that I am heating up the kiln too fast by not letting the carbons burn out? Maybe instead of...

 

 

1 hour on low

1 hour on medium

remainder on hi-fire

holding for 8 minutes after the sitter falls

 

...I should go 2 hours on low then directly to Hi-Fire?

 

The schedule I'm using is what the original owner suggested, the kiln's manual suggests the later.

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I find speculation on what may cause something to happen in a kiln interesting...

after a glaze firing in my Ceramics 1 class a number of years ago I was looking on the shelves for my Christmas red vase but couldn't find it on the first go around. I figured it would stand out like a sore thumb. On the second look I found it...but it was pure white instead of red. When I asked the professors what happened, they speculated that the piece might have been placed in the kiln next to something that might have had magnesium on it. If I can remember correctly, the piece was fired in one of the schools electric kilns. It still remains a mystery as to what happened.

JohnnyK

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I believe the cause is glaze thickness.  Some glazes are very sensitive to thickness of application, with the result being cloudiness or different color.  For those glazes that are application sensitive, the clay body or firing schedule is immaterial.  Thicker/thinner areas will appear to be different colors/shades.  Comes down to the ingredients in the glaze . . . which are uncertain here because they are commercial glazes.  Work on getting your glaze to a better application consistency so there are not overlaps or drips that cause unevenness in application. 

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I agree with Bruce that it likely is because of glaze thickness. Those tin/iron oatmeal glazes are always more orangey brown where thin and more white / oatmeal where thicker. Chrome tin reds aren't red if the glaze is too thin, they fire out a greyish clear. Would be fairly straightforward to either rule in or out. Take some test tiles and apply 1 thin quick dip or pour, then double dip the top half of tiles then triple dip one top corner. Fire them the same way as your usual glaze firing.

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