Rules of thumb for slab thickness and support

andrewiles

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Hoping someone can provide general guidelines on two questions I have for large, rectangular slab pots. For context I'm making some pots with 26" diagonals and probably an inch or two in height. Midfire stoneware. These are for forest plantings.

1) What is an appropriate thickness for the bottom slab? I've been using 3/8" for some slightly smaller pots but that's really just a random width.
2) Does it ever make sense to add additional feet under the center of the pot, away from the edges? For a larger pot I suppose this could allow for a slightly thinner bottom slab. But I could see that being couterproductive if it warps a little and makes a pivot point.

Goal is to make a pot that doesn't way 500lbs and, just maybe, doesn't break it half when I put something in it.

Thanks for any input!!
 

mwar15

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It’s a trial and error. Personal I think the bottom should be thicker than the sides. I go with 3/8” and haven’t had a problem but I have only made a couple that large. They haven’t had trees in them yet and haven’t gone through a winter
 

andrewiles

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Thanks @mwar15. @NaoTK I just looked at your website and saw a bunch of forest pots from a little while back. What bottom thickness are you using? Do you have any support feet in the interior for the longest pots?
 

NaoTK

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3/8" is unnecessarily thick! If the floor will fail, it will fail in tension. It's counterintuitive but a thicker floor isn't going to make a difference in tension, because clay is very weak in tension at any thickness. You have to put the floor into compression, where clay is very strong. You can do this by having it dry slightly faster than the walls.

I use 3/16" thickness for pots below ~16" wide and 1/4" for anything bigger. The 28" rectangle I just posted also has a 1/4" floor. Fired ceramic is very strong and even 1/8" can support a lot of weight.

I don't add any supports below the pot. You should fire with temporary supports under the pot
 

Shibui

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You should fire with temporary supports under the pot
This is the important thing for wide pots. Clay gets close to liquid during high temp firing and large horizontal surfaces can sag. Larger pots are notoriously difficult and failure rates are higher which partly explains the exponential increase in prices for larger pots.
 

andrewiles

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Thanks both. Very helpful. I fired my first couple pots with supports during bisque, but without supports during the glaze firing. Let's just say there aren't many straight lines left. I'll fix that next time. 3/16" blows my mind a little. I'll switch to 1/4" first and see how that goes.

It's hard to get the floor to dry faster than the walls. Gotta work on that.

NaoTK, the compression only takes place from drying that occurs after the floor and walls are connected, right? If I understand the underlying mechanics properly, there's no compression advantage to further drying the floor and wall components separately, before building the pot.
 

NaoTK

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It is best to work as dry as possible to reduce the total shrinkage and internal stress. I haven't experimented with drying components separately. If you join a very wet wall to a dry floor the pot may twist in an attempt to match shrinkage.

You should be drying the pot upside down, with foam or something on the inside. You want the floor at this point to sag a little (the weight is on the rim not the floor) to keep the walls from warping. With the floor higher it will dry faster. I cover the holes with newspaper because they may dry and crack. In Tokoname they cover the pot with newspaper and change it periodically.

You should bisque upside down to keep the walls from warping. The floor will sag a lot but that is ok (antique Chinese pots are very sagged). When you final fire the floor will sag back to level and in doing so it will pull the walls in and keep them straight. In Tokoname they use custom supports that fit perfectly under each pot design, so the whole floor is level. Other potters use little squares of ceramic.
 

mwar15

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It is best to work as dry as possible to reduce the total shrinkage and internal stress. I haven't experimented with drying components separately. If you join a very wet wall to a dry floor the pot may twist in an attempt to match shrinkage.

You should be drying the pot upside down, with foam or something on the inside. You want the floor at this point to sag a little (the weight is on the rim not the floor) to keep the walls from warping. With the floor higher it will dry faster. I cover the holes with newspaper because they may dry and crack. In Tokoname they cover the pot with newspaper and change it periodically.

You should bisque upside down to keep the walls from warping. The floor will sag a lot but that is ok (antique Chinese pots are very sagged). When you final fire the floor will sag back to level and in doing so it will pull the walls in and keep them straight. In Tokoname they use custom supports that fit perfectly under each pot design, so the whole floor is level. Other potters use little squares of ceramic.
That is some great knowledge!!!
 

WNC Bonsai

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I have seen several potters who use central supports or feet. Also a lot seem to use temporary supports during construction and firing. I believe the techniques is to place a piece of newsprint between the temporary feet and the bottom of the pot and it will burn away during the firing and leave the temproary foot free. So far I haven’t been able to get my wife to make me any big ovals, just round ones. But I don’t argue as they are “free” as long as I don’t include the cost of her pottery shed, the wheel, kiln, clay, glazes, tools, etc., etc., etc.
 
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