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mrcasey

Making Pit Fired Pot Safe With Liner Glaze

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Suppose that I throw a raku clay pot, put a liner glaze in it, and fire to cone 6 
electric.  Suppose that I then put a clay cover over the pot rim and pit fire it. 
Would that give me a food safe vessel?  What if I just used regular food safe cone 6 clay?    

 

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The clay over the lip will  not  adhere to the fired pot as it will shrink  at different rates-also the pay firing will most likely crack the fired pot or at least the glaze. Either way I suggest trying it and see what happens. That way you will learn for sure.Maybe it will work perfectly-always that chance.

Edited by Mark C.

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OK give it a try-see if the liner glaze holds up (waterproof) after pit firing. If it does you are golden. 

The clay and glaze each need to be mature to not weep water.This is one of those deals you should just try.

I have a good idea whats going to happen but you need to learn about it by trying-I'm a big believer in the school of hard knocks and test limits is whats it about. Let us know the outcome please.

Mark

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The cone 6 pot will crack in the pit firing, because it can't handle the thermal shock. And once a pot is fired to cone 6, it won't take in the effects of the pit firing because it's no longer porous. They are two separate processes that can't be combined.

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I wanted to ideally have a nice glaze on the inside of my saggar pots but gave up on the idea.

Saggar is about the same in heat range as a raku. That said, the only chance is to use a raku glaze on the inside of your pot.

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14 hours ago, neilestrick said:

The cone 6 pot will crack in the pit firing, because it can't handle the thermal shock. And once a pot is fired to cone 6, it won't take in the effects of the pit firing because it's no longer porous. They are two separate processes that can't be combined.

Neil he really wants to try it-I knew its not going to work but sometimes you just let them stick a finger  into the fan to see what happens-I think this may be that deal.

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22 hours ago, Mark C. said:

Neil he really wants to try it-I knew its not going to work but sometimes you just let them stick a finger  into the fan to see what happens-I think this may be that deal.

*Slaps hand and points to the fan*  No, Mark, that is owies!  No, No!

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I have two comments: 

1. I have successfully fired stoneware to cone 10 -- flasks -- and then glazed on the outside with low-fire glaze and conducted a Raku firing of the flask for treatment with  ferric chloride spray.  All worked fine, except that the color effects of the ferric chloride were nil (probably pilot error) - no cracks or mechanical failure from 'thermal shock'.  Therefore the technique of firing to cone 6 followed by a Raku firing is possible. 

2.  I am currently working with laminated clay bodies.  The structural layer is a cone 10 stoneware or porcelain.  The applied laminate is a "clay body" created from mixtures of coarse dry raw clay from my pond, coarse dry fire clay, grog, sand, low fire (04) scrap clay (dry or moist), and combustibles such as sawdust or chopped pine needles.  The laminate layer is constructed of either stiff moist pastes of the afore mentioned ingredients or with the those ingredients embedded in a layer of scrap clay.  So far, the laminate has been added to the exterior of the bowls and cylinder forms to produce decorative texture and color. The laminate layer is on the order of 3-5 mm.  

What is relevant to this discussion is that the clay from my pond, when fired to cone 10, produces a very porous layer bonded to the non-porous mature substrate (bisque porosity after a cone 10 firing).  Therefore, it seems reasonable to expect that an open cone 10 clay body laminated to a cone 6 clay body glazed on the interior and fired to cone 6 would produce an object with a mature interior and an exterior that would absorb Raku reduction treatments.  

LT

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2 hours ago, Magnolia Mud Research said:

2.  I am currently working with laminated clay bodies.  The structural layer is a cone 10 stoneware or porcelain.  The applied laminate is a "clay body" created from mixtures of coarse dry raw clay from my pond, coarse dry fire clay, grog, sand, low fire (04) scrap clay (dry or moist), and combustibles such as sawdust or chopped pine needles.  The laminate layer is constructed of either stiff moist pastes of the afore mentioned ingredients or with the those ingredients embedded in a layer of scrap clay.  So far, the laminate has been added to the exterior of the bowls and cylinder forms to produce decorative texture and color. The laminate layer is on the order of 3-5 mm.  

What is relevant to this discussion is that the clay from my pond, when fired to cone 10, produces a very porous layer bonded to the non-porous mature substrate (bisque porosity after a cone 10 firing).  Therefore, it seems reasonable to expect that an open cone 10 clay body laminated to a cone 6 clay body glazed on the interior and fired to cone 6 would produce an object with a mature interior and an exterior that would absorb Raku reduction treatments.  

LT

Is the outer layer porous because the clay in it has not vitrified, or because there are so many voids due to the organic material burning out? There's a difference.

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38 minutes ago, neilestrick said:

Is the outer layer porous because the clay in it has not vitrified, or because there are so many voids due to the organic material burning out? There's a difference.

The clay from my pond fired to cone 10 is not vitrified.  There is very little organic material in the mix.   

The idea here is that a very high temperature commercial clay can be used to laminated over a mid range clay body inherently will be significantly underfired when fired to a cone that matures a mid or low range and that underfired clay layer will be porous enough to absorb soot during the reduction step of Raku or during the soot stage of a pitfire.   

Edited by Magnolia Mud Research
additional comment

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