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redbourn

Redart Slip Trouble

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I recently made and used Mary Barringer's cone 6 redact slip recipe (Spodume 10% Cedar Heights Redart 80% Lithium Carbonate 10% add 2% red iron oxide). I glazed with a Spectrum satin clear glaze.  In the glaze firing the slip turned a very nasty dark brown.  Any ideas why this happened?

 

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Might it be a reaction between the slip and the clear glaze? Does she fire oxidation or reduction -- that could make a difference. From pictures, she seems to use only the slip/engobes on surfaces with no covering glaze, and letting the kiln atmosphere do its work.

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I use redart clay mixed with a tad of baking soda on raw clay; also without the baking soda.  Applied to green ware at the leather hard or bone dry state, and to bisque ware.  On bisque ware, the application must be thin until it dries, or the shrinkage will cause it to flake off.  Multiple coats is the way to build up the thickness.  My work is usually fired to cone 10 in a gas kiln with mild reduction. 

 

If the redart is under a glaze, the glaze will significantly change the character and color of the fired marks. 

If the redart marks are left bare, then the redart usually fuses to the substrate and is red to brown depending on where in the kiln and the local oxygen fugacity.  My latest adventure rendered the marks as an dark orange-red color. 

 

The color also depends on the composition of the underlying clay body.  A dark burning stoneware will be react differently than a very white porcelain.  Part of the difference will be due to the contrast between light and dark, and partly due to the reactions between the redart and the substrate composition. 

 

At cone 5 electric I would expect it to be a nice red.

LT

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i am not sure which RIO engobe we use. seems like similar recipe.

 

even our test tile shows under glaze the engobe turns a dark brown. 

 

actually any form of iron under clear glaze changes in oxidation. if i put RIO on my bisqueware and then apply clear glaze on it, nothing or very little of the RIO will remain on the pot. but if i put RIO on top of clear the colours remain. 

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"actually any form of iron under clear glaze changes in oxidation. if i put RIO on my bisqueware and then apply clear glaze on it, nothing or very little of the RIO will remain on the pot. but if i put RIO on top of clear the colours remain. "


 


Very interesting. Thank you.


 


--redbourn

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i am not sure which RIO engobe we use. seems like similar recipe.

 

even our test tile shows under glaze the engobe turns a dark brown. 

 

actually any form of iron under clear glaze changes in oxidation. if i put RIO on my bisqueware and then apply clear glaze on it, nothing or very little of the RIO will remain on the pot. but if i put RIO on top of clear the colours remain. 

 

It all depends on how much iron there is in the engobe/underglaze, and how thick the iron is applied.

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