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Post by nikolaybg on Aug 9, 2021 5:08:06 GMT -6
Hello,
These two samples have the same glaze, from one firing, from one shelf, next to each other. The left one has needle-like crystals and the right one has spherulitic crystals.
The difference between the two is - that the right sample was once over fired. After this over fired, there was not a single crystal,
so I covered it again with the same glaze and put it in the kiln and added another new test tile with the same glaze.
I expected the type of crystals to be the same because they are fired in the same conditions.
Have you had similar cases and how do you explain the differences?
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gczop
Full Member
Posts: 202
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Post by gczop on Aug 9, 2021 8:37:35 GMT -6
While perhaps not the explanation for this result, since the piece is twice fired the clay is no longer interacting with the glaze in the same way if at all.
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Post by mohawkpiper on Aug 9, 2021 11:17:33 GMT -6
I misread this at first... but i would guess it is because the two are no longer the same thickness. it would be hard to get the thicknesses the same but i would guess the refired piece has a thinner glaze than the other one.
It is also hard to say, the sample sizes are so small. you might want to try larger samples. The left one has the crystals
coming from the edge while the right one has a crystal in the center. that might also have something to do with it.
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Post by mohawkpiper on Aug 9, 2021 11:45:03 GMT -6
edit: i keep looking at this lol. but i got it backwards. the one on the left is THINNER. you can tell because the background starts to opacify.
I am no expert btw... i just like to chime in. not saying there isn't any truth to gordon's input. I just feel refiring glazes changes the thickness a lot. My wife refires pieces all the time. sometimes you can totally tell, sometimes you cant really tell. some glazes change a lot more, some almost not at all. but also, she refires on vertical.
some glazes she almost has the intent to refire, because it helps keep a more consistent background all the way to the top. but often, the crystals will come out the same shape as the non-refired ones.
some glazes the crystals will come out extremely needly. I always attributed it mostly due to the thickness.
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Post by nikolaybg on Aug 10, 2021 0:21:53 GMT -6
While perhaps not the explanation for this result, since the piece is twice fired the clay is no longer interacting with the glaze in the same way if at all. Some folks deliberately want the glaze to interact with clay for an effect. Others will coat with two similar glazes, say with more or less TiO2, one over the other to modify the result. G ( You don't say if they are seeded.) Тhis moment with the interaction with the clay I had missed  I need to think about this in more depth. There are seeds in both pieces, on the left example they are placed on the edge, and on the right they are in the center.
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Post by nikolaybg on Aug 10, 2021 0:33:10 GMT -6
I misread this at first... but i would guess it is because the two are no longer the same thickness. it would be hard to get the thicknesses the same but i would guess the refired piece has a thinner glaze than the other one. It is also hard to say, the sample sizes are so small. you might want to try larger samples. The left one has the crystals coming from the edge while the right one has a crystal in the center. that might also have something to do with it. You are absolutely right. Тhe refired piece has a very thin new glaze, because it is difficult for the liquid to stay on the glass. And the most interesting thing I noticed minutes ago. The crystals on the refired piece are only on the surface, where the new glaze was. The crystals have not grown down to the old glaze layer. In the left piece, the crystals are in the entire depth of the glaze. Which means that their shape seems to depend more on the thickness of the glaze than on the baking temperature. This is a bit of a shocking thought for me 
p.s. thickness of the glaze after first fire is about 1mm and litle more.
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Post by mohawkpiper on Aug 10, 2021 11:37:31 GMT -6
I would say glaze thickness definitely plays a part in the shape of the crystals. on a tall, vertical piece, you can get different shaped crystals between the top and the bottom. If you crack it open, the top is usually noticeably thinner than the bottom in finished glaze thickness. However I wouldn't say glaze thickness plays more a part in the shape than the baking temperature. I feel like the baking temperature plays the biggest role in the shape. But also, the glaze chemistry can change the shape too. In my experience, a very basic glaze with no colorants added usually gets the weirdest, most needle like crystals even at lower baking temperatures. colorants always seem to mellow them out and round them out. some more than others. different frits also can do this.
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Post by nikolaybg on Aug 11, 2021 4:57:07 GMT -6
When I tried to control the shape of the crystals only with the temperature, I reduced the peak temperature because I thought it was lower, the more widely crystals they would get. And I was very surprised when I downloaded the peak temperature of 1200 to 1160 degrees Celsius I received only many rude needle crystals. And despite the disappointment, the experiments are very entertaining. |
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I need to think about this in more depth. 
