Building a wet slab bonsai pot

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Most hand-built pots (not wheel thrown) are made from slabs that are leather-hard. This is a technique I've been working on to make pots from wet slabs. This thread will go step by step showing how this finished pot is constructed.

 

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As you can see a male form has been made from 2" insulation foam. A 1" and 2" sheet have been laminated for a depth of 3". I guess another lesson could be showing how to make this. Next time. Suffice to say it has been cut into quadrants and lightly taped back together with 2" packing tape.

 

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Next the form is wrapped in newspaper to act as a mold release. I take care to only tape the paper to the form on its top edge. This makes it easier when the plug is to be removed. I've gotten better at this with practice. Big wrinkles don't matter much as they're easily smoothed out later.

 

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The first slab added is the bottom. The form has some 1" spacers under it to accommodate the side slabs wrapping around the top edge.

 

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Here you can see how the side slabs have been added and small balls of clay have been wedged in the support the reverse turn of the top lip. All the surfaces are worked smooth with a urethane rib to make sure the joints are well compressed.

 

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With the form completely slab covered and smoothed, it's turned over, to ready it for the top banding.

 

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Since the shape was designed in quadrants, paper templates can be used to mark off the excess clay with a needle tool. The center-lines are used for strike-up marks.

 

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Once trimmed, strips of slabs can be scored and slipped into place. I always use a scarf joint to join the butt ends so they compress when being smoothed.

 

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Banding completed

 

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As a woodcarver, I tend to make custom ribs for the task at hand. The take only a few minute to make but saves a lot of time in the end. Using the top surface of the form as a straight edge for the rib yields a consistent lip.

 

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Now that the basic form has been built, it's time to remove the plug. This is very much a timing issue. If the clay is too soft, it may slump after removal. If you wait too long the clay will shrink so much that removal is very difficult without deforming the shape.

 

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I begin by removing the packing tape that was holding it temporarily together. Here you can see the 1" wide keys with their center-lines that will be removed. These were cut into the plug to accommodate the wrap-around side of the pot.

 

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Note that the excess newspaper and tape have been removed. This leaves the rest of the paper unattached to the foam. The long screw has driven in at an angle so it can be used to pull the key sideways, out from under the lip. The missing 1" square in the center gives the key clearance to move.

 

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The sides of the keys have been covered with packing tape before it was all put together. The smooth tape reduces the friction between the two foam surfaces which can be very tight as the clay shrinks. ALSO: Note that the ONLY tape holding the bottom of the quadrants together is one 2" square placed directly in the center of the missing 1" square. More tape than that makes it difficult to pull the keys.

 

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All of the keys removed.

 

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Once all the keys are removed the form collapses. You can see the small 2" square of tape that was holding the bottom center of the quadrants together.

 

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The newspaper/mold release is removed.

 

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You can see the score marks from where slabs were joined, as well as wrinkles from the newspaper. I smooth out as much as I dare. The clay is still pretty soft.

 

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This is a 24.5" pot, wet. Not counting building the form, it has taken approximately 5 hours to get to this point. I wrap the outside in cling-wrap to let it firm up over night.

 

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The form can now be roughly reinserted to act as a support so the bottom can be worked on.