Dark color result on clay body in electric kiln

[Caroline R]

Hi all, 

I have a question about a test one of my students had with her Juicy Fruit test glaze fired in cone 6 oxidation. We were testing colorants on the Juicy Fruit base glaze and got some interesting results but noticed some intense dark coloring on the clay that appeared something like flashing as the colorant increased. What could this be? Please see photos. We had a kiln full of glaze tests and noticed that it happened to some other people in a less extreme way, but there was no common denominator that I can think of. Unfortunately, I am not sure if the pieces were next to each other because I did not load/unload the kiln. We are using a cone 10 stoneware clay body recipe mixed in house. 

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Juicy Fruit Base Glaze Recipe:

Neph Sye- 44.5

Silica- 10.9

Whiting- 10

Kaolin- 9.1

Frit 3124- 8.2

Lithium Carb- 8.2

Soda Ash- 9.1

Bentonite- 2

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Colorant in order from left to right:

1. Chrome Oxide- 0.125%
2. Chrome Oxide- 0.5%
3. Chrome Oxide- 1%
4. Chrome Oxide- 2%
5. Chrome Oxide- 4%

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Please let me know what yall think. 

Thanks!!

Caroline

[Min]

It's flashing from the super high amount of sodium in the glaze. (and probably the cause of the crazing too)

[Kelly in AK]

Particularly from the soda ash, which is soluble and migrates through the bisque ware when you glaze. 

[Caroline R]

@Min thanks. I learned something new today then. I was aware you could get flashing like that in an electric kiln! It definitely got worse with the increase in chrome oxide but it doesn’t sound like it has anything to do with the colorant but more with the soda ash. 

[Min]

Chrome is volatile, it will flash too. Think of a white glaze opacified with tin, if you have a glaze containing chrome near it while firing the white can flash pink from the chrome.

Edited Tuesday at 04:50 PM by Min
typo mistake