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preeta

Cone 7 Gas Reduction Claybody

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West coast here.  

Our school is set up for cone 6 gas reduction. Sometimes the kiln reaches cone 7.

when I go to buy my clay, I either find ^5 or ^10. 

How do I find a ^7 body? Or is the implication here ^5 is actually a range from ^5 to ^7? 

Before I start my research I was wondering if anyone else has had this problem and already found the answer? 

I know ultimately this means I have to test test test to figure out my clay body.

I am going to talk to the manufacturers to find out about range. 

another line of thinking. Could I wedge well a ^10 body with ^5 proportionately, and “create” a cone 7 body? Sorry if I make some of you cringe .

Perhaps East Bay Clay might have answers since they make so many different recipes, but then again they might make only ^10 bodies. 

FYI I have taken a few claybodies to ^6 electric with no slumping in the electric kiln.

How do you tell if the body is over Fired? The slump?  Would an L shaped slab test tile show the slump? Or do I actually need to make a small bowl or bud vase to see the slump? 

Thanks so much for your guidance.

 

 

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Preeta:

Most porcelain and stoneware bodies will tolerate an additional cone above recommended peak. Part of the chemistry behind translucent bodies is firing them higher to develop more glass. Since you are firing in reduction: iron levels will be critical. Iron is a flux in reduction: so pick either porcelain or white stoneware. An L- shaped test tile will work just fine: but put the upright at a 20-30 degree angle. Just like a cone, so that it more closely mimics bowls, plates, etc. I would do one at. 60-70 degree angle to mimic plates. I would also recommend you bisq first: then take an exact measurement from the base to the top tip of the angle. Then after peak firing, measure again and compare. There should only be a slight difference (5% or so), which would reflect fired shrinkage only.

nerd

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I fire bmix 10 at ^12 (bottom is a smidge hotter, fireboxes closer to 14), and have some issues here and there, but overall(96%) its just fine. I however would not fire mid range bodies a number of cones over recommended. More, and different feldpsars are used in mid range/low fire clay bodies than high fire. They usually are less happy with being over fired.

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Most cone 5 bodies will handle cone 7 without any real problems. However, brown bodies can have issues because the amount of iron needed/used to make brown in oxidation is greater than what would be need/used in reduction. So brown bodies may flux out or become very dark and brittle. As Nerd said, white bodies or porcelain would be a better bet, or be sure to test any brown bodies before making work with them.

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