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rayaldridge

Small Pink Glaze Mystery

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Okay, I just opened a test firing in my tiny kiln, mostly testing a new glaze with various slips and double dipped other glazes.

 

I had a couple pieces come out with pink flashing on this glaze.  I'm familiar with the problem caused by chrome and tin, but there is no tin in this glaze, and no strong source of chrome in any of the other glazes (small amounts of chrome in some glaze stains, used in very small amounts, resulting in very pale blue-green glazes.  Also, no tin in the other glazes.

 

There were a couple pieces with glazes carrying 2 percent cobalt carbonate.  But that's the only strong colorant in the batch.

 

The glaze in question is a fairly simple glaze, designed to be active and pick up color from underlying slips.  It has 10 percent gerstley borate, 6 percent titania, and 6 percent lithium carb.  My best guess is that something is reacting with the titania.

 

The problem (actually I like the effect and would like to figure out a way to reliably produce it) is obviously from something fuming off a nearby piece, because of the way the affected pieces were only colored pink on one side.  It's not from underlying slips, as the piece I'll post a pic of was not decorated with slip.

 

Anyway, I'd appreciate any insights or observations regarding this.

 

post-65900-0-28731300-1427928407_thumb.jpg

post-65900-0-28731300-1427928407_thumb.jpg

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Mark, I do have one glaze with a fair amount of copper in it, but this is oxidation.

 

Neil, I have noticed that the pink is more fluorescent on thicker glaze, where the glaze movement has caused the glaze to pile up, but it's only flashed pink on one side.

 

It's occurred to me that I can probably figure this out by putting tiles with the new glaze, closely facing tiles with my old glazes that might be suspect.

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Well, okay, but why is it pink on only one side?  The glaze is dipped on, so should be pretty even in thickness.

 

Here's a mug with blue slip, where the pink appeared only on the handle.

 

(I'll add that the handle is not as enormous as it appears in the shot, taken with a phone camera, too close.)

post-65900-0-42722400-1428090865_thumb.jpg

post-65900-0-42722400-1428090865_thumb.jpg

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Wow,that IS delicious!

Copper can still flash in oxidation, but my observations of flashing from other pots is that it hits the high points on the recieving pot, not so much the valleys. The pink colour is strongest in the "valleys" in your pictures. This makes me think it's not flashing. If you wanted to eliminate it as a possibility, you could just paint a test tile with the oxide you suspect and put it next to a pot with your glaze. It'll either eliminate it as a variable, or provide you with a way to ensure the flashing.

 

-I have some glazes that are quite picky about their application. Do you remember if the bucket thoroughly sieved and mixed? Does it settle out fast? Is it a glaze ingredient distribution thing?

-You said this glaze is good at pulling colour from underlying slips. Is the Titanium pulling things out of the clay body? Iron or impurities in feldspar maybe?

-If directionality is a factor on the pink colour, is it maybe a matter of a few degrees temperature difference caused by proximity to the thermal mass of another pot or an element in the kiln?

Just some thoughts.

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Great questions!

 

The color seems pinkest where the glaze is thickest, and because this glaze is fairly fluid, the valleys get a deeper layer, as well as on horizontal ridges where the glaze piles up a little. But on the second mug, only the handle got pink.  Still, I wonder if you might be on to something, since I also got pink inside a shallow bowl, where you might not see flashing ordinarily.

 

I think I will test the glaze by facing it to tiles coated with a couple of the other glazes in that load.

 

I just mixed up this glaze, and sieved it twice through 80 mesh.  It has a fair amount of clay in it so doesn't settle quickly, and I keep a whisk handy to stir my buckets just before I dip.

 

The glaze is on porcelain, so very little iron, and the mug in the first image has no slip on it.

 

I wondered about firing temps, but I got the same effect on pots at the top and bottom of the kiln.  Also, I burned out an element in midfiring of this load, and had to replace the element and re-fire it.  The first time it got hot enough to melt the glaze smooth, and the pink was present on the surface after that underfire.  I can't remember if I faced the piece the same way for the second firing.

 

I'll post the recipe of the glaze, which is for Cone 8, but could easily be converted to 6 by increasing the gerstley borate by 2 percent, and cutting the kaolin by the same amount, I believe:

 

45       Custer Feldspar

14       Silica

11       Whiting

14      EP Kaolin

10      Gerstley Borate

  6      Lithium Carbonate

__

 

100

 

+ 6 Titanium oxide

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not a chemist but looking at your recipe what is your intention in using the titanium oxide?  i use rutile for the sliding effect without movement of the glaze, does the titania do the same but leave no color? :wacko:

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Titanium is one of those minerals that has all kinds of minute impurities that give interesting colour responses. It is an opacifier, but if titanium is in a glaze, and you layer it with other glazes you can get all kinds of fun things happening. Put it with the Gerstley and the Lithium (and to some degree, the Custer) and it's not too surprising the OP's glaze is doing *something* interesting. Exactly what's happening...testing will tell.

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I like the effect of titania, because in the glazes In which I use it, it promotes some crystallization of the surface, in an interesting manner.  The effect requires that cooling be a bit slower than would be the case in an electric kiln that is shut off when the target cone is reached, so that the crystals have time to develop.  In the glaze we're discussing, the crystals seem to develop mostly in areas where the glaze is thicker (though if I slowed it down even more, I suspect there would be even more crystallization, probably to the point of an overall satin matte.)  I even like to use titania in slips, because some glazes will pick up enough titania from the slip to verge on matteness in areas where there is slip.

 

As you said, the advantage over rutile is that there is little iron and therefore no dulling of the color.

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