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Raku Clay Vs Stoneware


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Morning all,

 

I've coloured some porcelain with a green mason stain and am wondering about the effect during a raku firing.

 

I'm going to do a raku workshop soon and need to make some pieces to fire. I have a buff stoneware and porcelain. The firings covered in the workshop are: naked, horsehair, obvara, traditional and copper fuming. Suggestions please about what clay body would work with each of these methods.

 

Thanks

Andrea

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Porcelain will crack in a raku firing.

Stoneware will work fine.

 

The horsehair and naked will work best on a smooth surface. Burnish with terra sig if you have it. A bright white body is more dramatic.

The others (cannot say I am familiar with obvara) will work fine with any surface.

 

Most raku pieces are evenly and thinly thrown. Evenly is more important but being thin does help. Closed, round forms generally do well. Open shapes and bowls are more likely to crack.

Places that are unglazed will be black-- give yourself accent lines to make that transition pleasant.

Don't try to be be too fancy with decorations; let the firing do the work.

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I've used both stoneware clay and specified Raku clay for the process.  Both performed about the same.

 

As Matthew mentioned, any unglazed areas will be stained by carbon.  This includes areas that are simply underglazed/ stained.  You can put a clear on top, like you would in a normal oxidation firing.  The carbon will affect the clear a bit, but it's a nice effect in my opinion.

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I have successfully used porcelain for Raku.  Uniformity in thickness, and avoiding sharp corners or rapid changes in thickness is one key to avoiding cracking with any clay body. 

 

My earliest foray into Raku was with low fire red clay.  It worked ok, especially with bright color glazes.

I now use what ever clay is available when I make work for Raku firing. 

 

LT

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I raku a lot using laguna b mix with grog. bisques up white so a clear crackle results in white crackle.

I make a lot of animal figures and have found it important to make sure the design includes a little channel so the air inside can escape to the outside. I had some cracking trouble with seated yoga figures until i figured this out. Seems to help with the temperature shock when the kiln is opened.

 

I have done some porcelain but only small things. Soldate 60 works pretty well. Los Altos works fine and with a clear glaze it retains its piggy pink color. I also use a lot of low fire glazes for color accents. they can be alone or covered with clear. 

 

Definitely do some terra sig pieces if you are going to do horsehair. Colored terra sig can work well. 

 

I usually let stuff pretty much air cool and am not in a big hurry to drench it as cracks happen in that process. 

Lots of funky pinch pots are pretty fun to raku. Have fun and post results.  Rakuku

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Laguna Soldate 60 for raku here. Well, actually she uses that and I use whatever left over-used up stuff I pug together. I have no idea exactly what's in my mix or what ratio. I know there is some porcelain in there in varying amounts due the the fact the scrap is all in different buckets. We've had some clay here for 13 years and I'm not gonna use "store bought" till I go through the 300 or so pounds of garbage we have laying around. The idea is mine clay is "free"... or I'm making the pugmill pay for itself; one of the two ....or both. :D

 

My stuff usually never cracks but I take a long time getting to temp and do other things while it's slowly climbing to temp. It's very anti-climatic. :lol:

 

I spend a lot of time making things so I'm in no hurry to break them. :)

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Thank you all for your advice.

 

I've got some stoneware bisque pieces that haven't had terrra sig applied, what would the best technique to use for these?  I have the opportunity to hire a kiln prior to the workshop and thought Id do some experimenting on my own.  

 

The workshop itself covers the following firing:

 

Obvara, horsehair, copper fuming, traditional and naked.

 

There is a pre firing day for throwing the forms correctly and where terra sig and sodium silicate will be shown and applied and the second part is a two day firing period.

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That sounds like one of my workshops on Alternative firing. I use porcelain for all those techniques but raku. I usually make large slabs or tiles for Raku and use a raku or stoneware clay for those. Much less warping.

Enjoy your workshop and be sure to post results of your firings.

 

Marcia

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I have used both B-mix with sand and Soldate 60 Cone 10 clays. I have had some cracks here and there (larger pieces 3-5lbs) over the years. I have also used The Laguna raku clay with good results but you have to keep tabs on what's what and you don't want that stuff in a cone 10 firing. If the pieces are larger I would use your stoneware clay or groged or sanded clay. You are in S. Africa so the clay bodies mentioned are more than likely not available. Use what you have and test. You could wedge some sand into your stoneware body to help with cracking. You will want to make a terra sig slip. No experience with porcelain. Good luck.

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If you have time to prepare, I would test your porcelain and see if it can handle the thermal shock.  Assuming you can take it out of your kiln and it survives the unload and quenching, then I would use porcelain for Horsehair, naked, and obvara. I think the white clay body will give you much more striking contrasts.

 

For raku using opaque and copper glazes, I think the color of the clay body will play less of a role, and your stoneware may be a better choice. If you want a white crackle, then use the stoneware and a clear crackle glaze.

 

Good luck it sounds like a very intense and interesting workshop.

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If you can, ask your clay manufacturer if they have COE values for their clays available. The lower the better. Porcelain has different properties from continent to continent, so examine the qualities of your own materials for best results.

Raku is done at such low temperatures (relatively speaking), how refractory the clay is matters less than its tolerance for thermal shock. Slip or terra sig can be added to anything to adjust surface colour: use the clay body less likely to break, and decorate from there.

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If you can, ask your clay manufacturer if they have COE values for their clays available. The lower the better. Porcelain has different properties from continent to continent, so examine the qualities of your own materials for best results.

Raku is done at such low temperatures (relatively speaking), how refractory the clay is matters less than its tolerance for thermal shock. Slip or terra sig can be added to anything to adjust surface colour: use the clay body less likely to break, and decorate from there.

Can you please explain what the COE value is?

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My earliest foray into Raku was with low fire red clay.  It worked ok, especially with bright color glazes.

 

How did you get your colours to work? i have had to stop using red clay in raku because all my colours disappeared. in one i used at least 7 different colored glazes but they all turned out mostly black with a hint of color here and there which you could only see if you stared hard enough (like a red turned into a dark brown). one of the glazes used did have copper in it. no underglazes.

 

i have made tall slab tile type stuff including box lids using ^6 clay and they all cracked in half in the kiln. smaller size stuff stays fine. 

 

i am now going to try make the same things using stonewear ^10 and also specifically clay planets raku sculpture mix cone 10.

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Stoneware will be somewhat heavier than porcelain and respond differently to the coloring. This will happen if the firing goes high enough to where the fluxes and colorants in the clay will start to melt and interact with the surface treatment.

Porcelain has very little to give other than whiteness. If you like black and white or brown and white, I think you may obtain good results from  naked raku and maybe obvara,  

Other than what many people believe, porcelain will not crack that easily, due to the fact that the fluxes in the clay will start to melt at much higher temperatures ( the temperature at which the fluxes start to melt) than a bisque and a raku firing. It will only get in shock once silica turns into glass. So your chances for stoneware to crack is higher than porcelain. 

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