Thickness Of Raku Pieces

[mrcasey]

Is there any evidence that raku pieces need to be thicker than normal cone 6 functional ware?   Several folks in our studio suggested that raku pieces should be a bit thicker to withstand the thermally violent process of the firing and quick cooling.  I guess I'm thinking of normal functional ware as being 3/16" to  5/16".   

[Callie Beller Diesel]

The big caveat to anything raku is that for every rule, there’s an exception that proves it. But in general:

Thickness isn’t a guarantee that something will live. A poorly constructed but chunky piece can still break.  You want to think clay body, and construction methods. 

Clay body can have a big influence on your results. If you’re using a clay that is sold as a sculpture or raku clay body, it’ll withstand a lot more thermal shock than one that isn’t. I’ve seen people create small, very thin-walled pinchpot style animal figures out of a groggy raku clay with minimal problems. I’ve also seen thick walled stoneware pieces shatter within the first few minutes of the firing. I wouldn’t suggest using a typical talc based casting body: they’re relatively friable after the bisque compared to their green strength.

You want to think about construction methods that are as structurally sound as the design will allow, and have as even cross section as possible. Curves>90* angles. You want to allow even heating/cooling over the whole piece, and avoid one side of the piece heating or cooling faster than the other. If cracks appear in a drying piece, I would not recommend repairs: start again. It will forever be a weak point, and throwing pieces into a reduction barrel can enlarge it. 

Some of the ideal thickness decision is going to depend on the scale of the piece too. 3/16” for an object that fits in your palm could be just fine if you’re working with a groggy open body. If you’re using a porcelain (which is doable) you’ll possibly want to go with a thicker cross section. Additional thickness is only useful if it adds reinforcement, not just for its own sake.

[neilestrick]

It will actually survive the thermal shock better if it is thinner, because it will cool more evenly. The thicker the walls, the greater the difference between the temperature of the inside and outside surfaces. That difference is what causes cracking because they are contracting at different rates.

IMO, functional work should never be more than 3/16" thick unless it's very large.

[Pres]

I remember classes from Penn State in the Summer with David Dontigny where he stressed thinness of the raku, but insisted on thicker rim areas as the tongs would sometimes abuse the rims. We threw an entire Summer with that heavily grogged clay and I had permanent sores on my first, third and thumb knuckles from the sanding they got. Never kept a single piece of that summer. Throwing  loosely was counter intuitively to me at the time, and I hated the pieces. Still see some around town, but don't admit to them.

 

best,

Pres