Cone 6 Body W/ Low Warping & Water Absorption

[mrcasey]

I've been making slab built bonsai pots for a while.  One of my biggest issues is dealing 
with warping.  I now use Laguna #75.  I chose it because of its low water absorption and color. 

1.  Can anyone suggest a cone 6 clay body with very low warping and less than 2% water absorption?  

2.  Are there any articles that explain how to formulate clay bodies to get the properties that I want?  I'm happy to buy the raw materials and mix them myself.  

3.  Could I just add some grog to the #75?  If I do, will it increase water absorption?  

[neilestrick]

Warping during construction or during firing?

[mrcasey]

Firing

 

[Pres]

@mrcasey, Could you describe your construction technique, along with your drying process, and finally how you set the pieces in the kiln?

 

best,

Pres

[mrcasey]

I use a slab roller.  Clay is rolled out at roughly the softness at which one would throw on the wheel.  Slabs are about 1/4" thick.  The clay is then placed on sheets of dry wall for about an hour and left to firm up a bit to about leather hard.  These slabs get flipped a couple of times so that they dry evenly on top and bottom.

The walls are anywhere from 6" to 14" long. The wall corners are beveled and cut in a trapezoid shape so that the walls angle out slightly from vertical at about 10 degrees.  Corners are scored, slipped, and assembled.  I use a t square to try to get all corners at 90 degrees with walls straight.  There is usually a little bit of a gap (1/16") where the corners meet, and I fill them with a bit of coil. 

I score and slip the bottom of the rectangle, score the slab for the pot floor.  I use the t square again 
before pushing the walls onto the floor.  Sometimes, the pot will have a rim; sometimes not.  The rims are made in one piece and look like picture frames before being attached.  They are scored, slipped, etc.  A coil of clay is then placed around the outside join.  The clay for 
the coil is necessarily softer than the leather hard walls.  I use an apple corer to make drainage holes.  The pot is flipped
over and feet are attached. 

The pot is put under plastic and left upside down on drywall for a day to help keep the rim flat and level.  The pot is flipped back on its feet so that the feet remain relatively level.  After a couple of days, I'll take the plastic off.  Sometimes, I will start seeing a bit of rim bowing as the pot dries.  Often, one side will remain relatively straight but the other side will suck in a bit.  Once the pots is bone dry, I will spray water on a mirror and move the pot back and forth on the mirror to even up the feet.  

I don't have any particular place I set these pots in the kiln.  I use flat kiln shelves.  We fire electric
to cone 6.

I have noticed that the thinner the walls and longer the pot, the more warping I see.  14" long rectangles with 1/4" walls have quite  a bit of "life" to them.  I've noticed that a lot of the expensive Chinese and Japanese antique bonsai pots have some warping.  Maybe unless I want to use modern mold methods, I just have to live with it?          

 

[neilestrick]

Can you post a picture, top and bottom? The feet are probably the biggest culprit, but any time you have long flat sides like that they're going to warp. On solution is to put a slight curve in the sides, which makes them less likely to bow inward.

Any clay body that vitrifies is going to want to warp in the firing. The lower the absorption, the closer the clay is getting to its melting point and the softer it gets at the peak of the firing. Have you tried firing them on a waster slab? I wold also try going thicker as they get larger.

[mrcasey]

18 minutes ago, neilestrick said:

 On solution is to put a slight curve in the sides, which makes them less likely to bow inward.

 

I've considered this and may try it out.  Never tried waster slabs.    

[dhPotter]

How do you flip the slabs while they are drying on the drywall? Do you pick them up with your hands, or use another board of drywall on top to flip them - like a sandwich of drywall with slab in the middle?

[mrcasey]

2 hours ago, dhPotter said:

How do you flip the slabs while they are drying on the drywall? Do you pick them up with your hands, or use another board of drywall on top to flip them - like a sandwich of drywall with slab in the middle?

sandwich

 

[Min]

Another thing you can do is make a slab the same thickness as the height of the feet, make it at the same time as you roll the slabs for the pot so it's the same moisture level. After attaching the feet sit the pot on the slab and then keep it there during drying, bisque and glaze fire. (assuming there is no glaze underneath the pot base) Stops the pot slumping and pulling the sides off kilter.

Support slab doesn't need to be the full size of the base but big enough to cover the centre area mostly. 

7 hours ago, mrcasey said:

I've noticed that a lot of the expensive Chinese and Japanese antique bonsai pots have some warping.  Maybe unless I want to use modern mold methods, I just have to live with it?        

I don't think there is a right or wrong way of looking at this, it's what you like it to be.

[Pres]

How many feet do you add to the bottom of this 14" long bonsai pot? Does it have enough support for the length?

 

best,

Pres

[mrcasey]

2 hours ago, Pres said:

How many feet do you add to the bottom of this 14" long bonsai pot? Does it have enough support for the length?

 

best,

Pres

It has the traditional 4 feet.  Unless I'm mistaken, the warping seems to be mostly side-to-side.    

[Callie Beller Diesel]

+1 for Min’s suggestion about making a support piece, or even building additional support feet into the design. I’d think it more unusual if a 14” long, 1/4” thick slab on feet didn’t warp.

[Pres]

Whenever I build slab pieces I usually have a foot every 4-6 inches of length. In your case that would be 3 to a side or 6 total. If the piece is more towards a square, I also include a center foot. Crazy, but for me it works. 

 

best,

Pres

[Mark C.]

I have a few ideas

clay has memory so if you bend those slabs during any part of the making ,drying assembly process you will get bending layer-maybe you are keeping them flat100% and this is a non issue

Are these glazed at all or left bare as glaze on one side will warp the walls

If the bottom is unglazed, place for supports under the Bottom exactly same thickness as feet -they can be loose (not attached )for support as they will shrink the same as the feet while supporting that 14 inch bottom. 

Use waster slab as the feet feet/and pot will shrink the the same as slab and not grab the shelve.

14 inch  bottom with 4 corner feet is asking for trouble unless you take at least some of the above tips

Edited March 17, 2023 by Mark C.

[mrcasey]

2 hours ago, Mark C. said:

I have a few ideas

clay has memory so if you bend those slabs during any part of the making ,drying assembly process you will get bending layer-maybe you are keeping them flat100% and this is a non issue

Are these glazed at all or left bare as glaze on one side will warp the walls

If the bottom is unglazed, place for supports under the Bottom exactly same thickness as feet -they can be loose (not attached )for support as they will shrink the same as the feet while supporting that 14 inch bottom. 

Use waster slab as the feet feet/and pot will shrink the the same as slab and not grab the shelve.

14 inch  bottom with 4 corner feet is asking for trouble unless you take at least some of the above tips

Some are unglazed.  Traditionally, glazed bonsai pots are not glazed on the insides. 

[Kelly in AK]

Everything mentioned so far will help reduce warpage. 

I’m glad Mark mentioned “memory,” I thought of it right away when you described how you make your work. Vince Pitelka has a great explanation of this seemingly nebulous concept:

https://www.vincepitelka.com/wp-content/uploads/2021/06/Rolling-Slabs-Platelet-Grain-Structure-and-Clay-Memory.pdf

The short version is you need to roll the slab after it’s out of the slab roller, in other directions than the roller first forms the slab. Both sides. Rolling it in only one direction gives the slab a “grain” and you’ll get different shrinkage in one direction than another.

The glaze tension and how the feet support the form certainly play a role, but eliminating the grain structure will prevent at least the initial forming tensions from being set up. 

[Mark C.]

(Some are unglazed.  Traditionally, glazed bonsai pots are not glazed on the insides.) 

 

I am pretty famliar with bonsai pots-I have seen them glazed and unglazes-I have both types

My point is glaze on one side will put stress on clay(hence warping) -glaze on two sides is equal stress

[neilestrick]

I agree with Callie, I'd be really surprised if it didn't warp. 14" is a long slab and needs more support to keep it from moving. I don't think a different clay body is going to help much nearly as much as improving the engineering of the piece. Add more feet, curve the sides, make it thicker, add a lip to the walls, etc.