Slab Work

[dooarts]

hi - i'm new to slab work. does anyone have a trick for keeping a round vase (with two vertical seams) from warping itself out of round? i've covered it in plastic, and let it dry for many days, but no luck. any help is appreciated. (using porcelain clay)

[Pres]

I think that folks need a little more information on you slab rolling and assembly technique. Do you use a tube form to work around, are you putting the piece together when the clay is stiffer, or when it first comes off of the roller. How tall are you working. All of these things might help to pinpoint some areas of improvement.

 

best,

Pres

[dooarts]

i'm letting the clay rest, sandwiched between wallboards (plaster covered sheets with heavy paper...used in building trades) for 2-3 hrs. then miter and score edges of seams, then use magic water for seal. using tubular form, but it's pretty flexible. piece is about 12" tall. i considered using balloon for support on inside of vase, but thought it might interfere with drying (and might also not be strong enough to keep clay from warping)

[Benzine]

It might be an issue with the clay's memory then. It is rolled and then dried flat, then you try and round it, but it wants to go back to being flat.

 

What about after rolling it out, you wrap it around your form and let it dry a bit? it could even be dried sideways around the form, so it wouldn't sag in it's more flexible state. You wouldn't even have to connect the ends at this point. Once it has dried slightly, slightly then connect the ends, via your stated method. It will be tougher to cut the miter in this manner, but it should help resolve your issue.

[bciskepottery]

I'm wondering if the amount of time the slabs spend on the wallboard isn't drying the clay out and making it less resilient and workable.

 

I do a lot of slab work, including cylinders 15+ inches. I make my slabs larger than the forms planned and I place the slab on a piece of foam and roll it with the paper form -- that gives the slab a new memory. By making the slab larger than needed, you can cut away the ends of the slab that will want to pull back and which don't get as much new memory from rolling as the interior of the slab.

 

I form my slabs around thick paper tubes -- the inside tubes found in rolls of shrink wrap; I also put the paper tube inside a cheap knee-high nylon that is tied at the top; the clay does not stick to the nylon and you don't have to keep wrapping the tube in newspaper, etc. I only use one seam, beveled, scored and slipped (I use magic water in my joining slips). I use a 60 degree bevel, not 45 degree bevel because it gives you more overlap. If I want to keep a round form, and while the cylinder is still wrapped around the form, I will add a bottom and then I will take strips of newspaper or plastic bag and wrap them around the outside and fasten down the ends with masking tape. These strips provide just enough tension at the outset that it helps keep the clay memory from wanting to pull apart at the seam; as the clay becomes leather-hard, the bands will loosen. I let the cylinder set up for a while before removing the paper tube. Overnight, I place a plastic bag (generally the ones newspapers are delivered in) over the cylinder to slow drying.

[dooarts]

thanks so much for the wonderful input. really is appreciated. will try all of your suggestions.

[Diane Puckett]

I love the knee-high stocking idea. Now I know why I should have kept those silly things when I retired.

 

It is possible the outside of your slabs feels drier than the inside is, and that your tall pieces are slumping. If I make slabs in advance, I put a sheet of plastic between the clay and each piece of Sheetrock. The clay does not dry out, and the Sheetrock does not mildew. I can uncover the slab to dry a bit and then re-cover it to reach moisture consistency. Assorted sponges are good for propping things up while they dry.

[Chris Campbell]

I too question why you are trying to dry it while it rests. Work as wet as you can handle it.

 

Also ... How on earth do you drywall lovers handle the extra weight of those things when they are wet? Where do you store them between uses? How do you move around a slab of clay sandwiched between two of them?

 

They just seem to be a 'solution' that creates several other problems ... But then, I admit to being lazy anyhow. I cannot imagine hauling drywall around in an effort to deal with slab warping problems.

[Benzine]

If they were small bits of drywall, it wouldn't be that bad Chris. A couple one by one foot pieces of drywall, even with clay, wouldn't be too difficult to move around. Now large slabs with the drywall, absolutely.

[Diane Puckett]

If I am in the mood to make slabs, I like to make several to be used in the next week or two. I have 2x3 foot sheets of drywall I use. I don't do that to prevent warpage but rather just because I don't know what else to do with them. Given my tiny studio and the weight, they are a nuisance even though they fit on the top of my wedging table, the only place I have for them.

 

My fantasy is a pottery fairy who comes during the night and leaves behind perfectly wedged clay and several nice slabs, ready for use.

[bciskepottery]

I like to make several slabs at a time; to save on space, I'll place them on a 24"x24" piece of drywall with a sheet of plastic to prevent drying out. I also have cut several pieces of wood blocks (2x4, 2x2, 1x2 - really doesn't matter) and place those at the corners of the bottom piece of drywall and then stack them on top of each other. At night, a sheet of plastic goes over the stack. By stacking you can minimize the footprint.

 

I will also make multiple slabs and store them in plastic bins with covers, with a piece of plastic between slabs. You can stack 4 or 5 without them squishing the bottom one. I place a piece of plastic large enough the cover the top of the bin before putting on the lid.