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Another Probably Basic Question About Sculpting / Thickness


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#1 Rebekah Krieger

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Posted 20 February 2014 - 10:54 AM

I would like to make some sculptures - larger ones.  I know that clay should be not more than 1/2 " thick to prevent exploding in the kiln. It seems many sculptures of life size busts etc have clay that is over an inch thick in spots. Are they using special clay or can I use the stuff I throw with?  Is there a special rule for firing thicker sculptures? If I am using a lunch bag stuffed with newspaper scraps as a base form for a sculpture would it damage the sculpture when I fire it? Could it damage the inside of the electric kiln? 

 

Some of you make architectural pieces that seem thick.. perhaps you can help?


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#2 Davidpotter

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Posted 20 February 2014 - 11:11 AM

i try to not go over half a half inch but sometimes i do because the clay simply can't support it's self. just make sure it's completely dry when fired and you should be good. I look back at my first projects that have 2 inch thick bases and i wonder how they didn't explode. i figure it's because my pieces dry for at least a week when it's thick. sometimes 2 weeks


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#3 bciskepottery

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Posted 20 February 2014 - 11:12 AM

Thickness is not so much the issue as dryness; thicker work needs much more time to become bone dry.  And, while the old trick of holding a piece to your cheek or seeing if it sticks to your tongue works for mugs, bowls, and similar thin-walled pieces, a thick sculpture may feel dry on the outside but still be wet on the inside of the wall.  So, you need to give sculpture lots of time to dry. 

 

Another key to firing sculptures is going slow . . . really slow on firing.  Long pre-heat to make sure the piece is dry; slow increase of temperature to allow the piece to heat evenly while ramping up the temperature.  During firing, your clay expands and contracts, so you also need to account for movement while firing -- especially if the piece has bulk and weight.  Firing on a bed of grog, or on cookies/coils/etc. to allow the piece to easily move is a consideration. 

 

Busts and objects should be hollow.  When I fired sculpture pieces at the community studio, I preferred any paper, etc. on the inside be removed . . . yes, it is combustible and burns out, but it can make a mess inside the kiln . . . you don't want charred pieces of carbon left over from the sculpture firing floating onto a subsequent glaze fire load of functional ware.  What you want to avoid is any carbon build up or oxidation on the elements; so if you do burn the paper, the next load should be a regular bisque or glaze that burn out any build up from the sculptural load. 

 

Think about how you are going to fire the piece as part of your process of designing and constructing.  Those who don't fire their own work often neglect to think about that part of the process. 



#4 neilestrick

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Posted 20 February 2014 - 11:31 AM

Evenness is important, too. Drastically uneven areas of thickness are more likely to have cracks.

 

Long preheat, slow climb.


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#5 JBaymore

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Posted 20 February 2014 - 01:53 PM

You can fire clay that is inches and inches thick. I believe there was a kind of misleading thread on here a while back that seemed to point to some magic "limit".

 

It is not about the clay thickness.... it is about firing cycles and specific clay bodies.

 

A firing cycle is developed to FIT the wares being fired. No such thing as a "one size fits all" for either bisque or glaze fire.

 

General thought........ the thicker and the less "open" the clay body,..... the longer the up cycle will have to be to not cause problems caused by water of formation, chemically bound water, organics and oxygen penetration, and thermal lag through the walls.

 

This kiln is used to fire pieces that JUST fit in there height wise...and maybe 3-4 pieces per load. You can bet that they are thick.

 

http://kcaiceramics....238605525_n.jpg

 

best,

 

............................john


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#6 Rebekah Krieger

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Posted 20 February 2014 - 03:19 PM

Thank you for all of the useful information.  John, that kiln is divine!! 


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#7 Mug

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Posted 20 February 2014 - 06:26 PM

I seen Philippe Faraut...sculpting god, uses a low fire clay...I cant remember the exact type, but I'm sure with a search you will find it. The clay he uses stays damp longer than normal, giving you a longer time to work the clay. He lets it get firm then scoops it out then takes a fork, poking holes to the inside of the sculpture to reduce the chance of exploding in the kiln. This might work well for sculptures that are not functional.

Sadly the clay he uses is not much good to me. I have been doing some tests on cone 6 paper clay for sculpture. The paper clay has been nicer to work with than expected. When the moisture content is perfect it seems to work almost like Roma Plastilina.

I'm not sure what type of sculpture you are going to create, or the level of detail you are trying to reach, but I hope this helps



#8 Benzine

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Posted 20 February 2014 - 06:44 PM

I tell my students, that anything over an inch thick, has to be hollowed out.  But of course, that means that I will fire things that are an inch thick.  I just make sure they are dried a while.  For me, it's not so much a worry of the projects exploding, as it is that for the size we generally work with, anything that thick, is just needlessly heavy.

 

This kiln is used to fire pieces that JUST fit in there height wise...and maybe 3-4 pieces per load. You can bet that they are thick.

 

http://kcaiceramics....238605525_n.jpg

 

best,

 

............................john

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#9 Rebekah Krieger

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Posted 21 February 2014 - 07:10 PM

My cousin is a sculptor and teaches anatomy at MIAD , he told me that he makes pinholes inside his work when completed to help with thickness and dry time. Nice tip I thought.  


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#10 High Bridge Pottery

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Posted 21 February 2014 - 08:07 PM

I put some pots in the kiln that were 1/4 to 1/2 an inch thick on the base but only just dry on the outside. All of them started exploding in the kiln at 100-200degC. I think it is all to do with the steam generated that makes clay explode.

 

As long as they are dry they will not explode and the slow firings allow the temperature to even itself across the thick walls.

 

After reading Johns post again there is a lot more subtlety to it but I have never really had problems. Also I have not fired many large sculptures.



#11 Mug

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Posted 22 February 2014 - 03:03 PM

Here is a link for a book I found on paper clay, it has a lot of material on sculptural work. It's pretty good, thought you might want to give it a look  http://www.tallerano...o_Companion.pdf






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