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Keeping a 22" Platter from Sagging


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22 inches is the widest slab I cab make with my slab roller, and I am beginning to become obsessed with making a 22 inch platter.  It has to be footed -- there is no way it will ever come out flat enough to sit nicely otherwise (plus that permits glazing the bottom), and it probably needs multiple feet for support, or something creative (like maybe straight line supports radiating out from the center...).  My first attempt, in which I took the flat slab and just stuffed old rags under the edge to make a lip, and then affixed many small circles underneath for support, was basically a failure -- it sagged and stuck to the kiln shelves (I cleaned it up best I could; it hangs on the wall now).  My second attempt is not looking any better -- for this one I laid the 22 inch slab over a 20 inch slab to form a lip and affixed two foot rings, but I inverted it too soon and it went into the bisque pretty close to flat.  I expect it to sag during glaze firing, which will be most unpleasant...

My latest idea is to  build a plaster hump mold using the inside of a large convex mirror.  There is a 26 inch (diameter) model available on Amazon, but it has very little "rise," and I'm concerned I'll have the same problem.  Here is a picture of it from the side:

image.png.5ae43ae2e4e4b01b1b4c257c320aae85.png

(I am assuming I will be able to detach the mirror from its backing to fill it with plaster...)

On the other hand I can get a 22 inch diameter hemispherical mirror (my slab would only drape partway down it -- there would be about 6 inches of the base of the mirror left all around), but I think that would be too bowl-like and not enough like a platter.  I can also get a larger hemispherical mirror, which might be the ticket, but the price goes way up, so I'm less inclined to experiment.

Other information: I'm pretty much set on a hump mold, as I've made smaller platters (successfully) in a handmade bisqued slump mold, and it's tricky to get the foot ring on properly -- the platter dries out some, and matching the state of dryness of the foot ring with that of the platter is difficult.  With a hump mold, you can attach the foot ring(s) right away.  Also, I know I could save myself some trouble and just use the mirror itself as a hump mold, but I think I'll have better results with plaster, as the clay will dry from both sides, and there shouldn't be a sticking issue.  (And as it dries and shrinks, the shape being spherical, it should just slide around on the mold -- so I can let it firm up pretty well before removing it.)  Finally, and maybe this is the kicker, I've been rolling my slabs about 1/4 inch thick (it sounds thin, but it looks thick...).  Maybe I just have to beef that up some.  And I tend to be using clay that has dried out (in its bags) too much to lend itself to throwing -- it seems those bags are not perfect, and there is a time limit...  I buy in quantity, and then never throw it all quickly enough.  Bisque firing to cone 04; glaze firing to cone 6.  

Thanks in advance for any thoughts.

 

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1 hour ago, LinR said:

Thrift stores are a great place to find shapes for moulds.  Lin

Yes thats where I find mine-I like plastic or metal forms-the mirror is waiting to break 

Use a large pan lid (remove the top handle) for that large curve

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A thrown platter would be relatively flat, with just the lip curving upward. The foot sits at the junction of the lip and flat part. On a really wide platter a second foot ring is often added closer to the middle to prevent the center from sagging. With this design the diameter of the platter doesn't matter. 10", 20", 30", the bulk of the piece is supported by the foot ring(s), and the size/overhang of the lip doesn't increase all that much as the diameter increases. What you have there I would call a shallow bowl, as it is curved across the entire form. Even in a thrown piece something that wide and shallow will be problematic if it's too thin or if the foot ring is placed too close to the center. I think for your piece to survive it's going to need to be made thicker than a typical 14" bowl, from a clay that doesn't deform much in firing (like a stoneware as opposed to a porcelain), and the foot ring will need to be fairly wide in order to support it well. If you were to divid the diameter into thirds and put the ring at those marks it would be a good place to start. Good compression of the slab will also help, and glaze firing it on a waster slab wouldn't hurt, either.

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Thrift shop woks are great for slump and humps, handles removed.

Great glazing vessels also.

Thicker clay.

Hump allows the footring to be added at right time.

As in another post, may have to accentvthe curve toget wshat you want after glazing

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Polyester/lycra material is great for a slump mold, I usually use a square wooden box even for a round one when I make them. Let them get slightly leather hard then add in the foot rings. finish on the wheel.

I also use a large plaster bat that I made years ago. I often will join a thrown foot ring onto the slab, after I have thrown on foot rings. Works well for a bird bath or for serving platters/fruit bowls.

 

best,

Pres

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22 inches might be ok for your slab roller, what about putting it into the kiln?   is there room for your fingers to lower it into a top loader if that is what you have?

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Thank you all.  Neil is (unsurprisingly) right -- my plan leaves me with a shallow bowl, not a platter, which I arrived at in order to try to solve some problems, but which has its own problems, and in any case is not really what I want.  So back to the drawing board.  I am pretty attached to the hump mold part of the plan, which has two big problems:

1) I want the platter to be able to dry at least to leather hard on the mold, so it doesn't deform.  But it will shrink, and the mold won't.  I'm thinking I might be able to finesse this by making the mold support just the flat part of the platter, and letting the rim just hang over, or maybe I can find a length of soft foam or some such.  

2) Construction of the mold.  Probably my best bet is to throw it on the wheel (I do have a 22 inch medex bat, purchased with this in mind), then use the bisqued form as my mold.  

Other details: Yes, I am using stoneware, but I should probably pay more attention to the precise variety -- as I said, I've been using whatever has dried out too much to easily throw on the wheel.  And I will go a little thicker.  A waster slab for the glaze firing is so obvious I can't believe I didn't think of it (thank you); of course I don't have a 22 inch waster slab lying around...  but will make one.  

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5 minutes ago, oldlady said:

22 inches might be ok for your slab roller, what about putting it into the kiln?   is there room for your fingers to lower it into a top loader if that is what you have?

Thanks, there is!  I bought my kiln from Neil a few years back (which I highly recommend, for anyone in the market), and part of his excellent advice was to go wide instead of deep -- easier on the back.  

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46 minutes ago, algebraist said:

Thank you all.  Neil is (unsurprisingly) right -- my plan leaves me with a shallow bowl, not a platter, which I arrived at in order to try to solve some problems, but which has its own problems, and in any case is not really what I want.  So back to the drawing board.  I am pretty attached to the hump mold part of the plan, which has two big problems:

1) I want the platter to be able to dry at least to leather hard on the mold, so it doesn't deform.  But it will shrink, and the mold won't.  I'm thinking I might be able to finesse this by making the mold support just the flat part of the platter, and letting the rim just hang over, or maybe I can find a length of soft foam or some such.  

2) Construction of the mold.  Probably my best bet is to throw it on the wheel (I do have a 22 inch medex bat, purchased with this in mind), then use the bisqued form as my mold.  

Other details: Yes, I am using stoneware, but I should probably pay more attention to the precise variety -- as I said, I've been using whatever has dried out too much to easily throw on the wheel.  And I will go a little thicker.  A waster slab for the glaze firing is so obvious I can't believe I didn't think of it (thank you); of course I don't have a 22 inch waster slab lying around...  but will make one.  

Speaking from experience (more than one time) leaving the platter on the mold, whether plaster or bisque, can be a problem.  The piece can crack. 

Roberta

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I've made platter rim molds/forms, thrown then bisque fired. Rim on a batt or board, slab flipped on without picking it up. The oval one is a bit sloppy, should have a slight indent where the wall meets the tabletop if no foot is added. If adding a foot it's fine as is. They can be put on the wheel to compress and add some throwing lines. If you want a foot then place a circle of upholstery foam to cover the entire base, excluding the wall and rim, then flip it upside down and add your foot/feet. You can throw a circular rim then shape it to oval when it's stiffened up a bit too or cut and join to make a square etc. The rim will dry faster than the base so keep it covered while the base catches up.

I would suggest wedging in some fine grog if your clay doesn't already contain it. Waster slab only needs to be as wide as the base or outer footring.

image.png.a75f533a19e2c20521952c424031c396.png

Edited by Min
clarity
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3 hours ago, algebraist said:

I want the platter to be able to dry at least to leather hard on the mold, so it doesn't deform.  But it will shrink, and the mold won't. 

I always work on the inside of a form, i.e., slump mold instead of hump mold.  As the clay dries it simply pulls away from the walls of the form.

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2 hours ago, Min said:

I've made platter rim molds/forms, thrown then bisque fired. Rim on a batt or board, slab flipped on without picking it up. The oval one is a bit sloppy, should have a slight indent where the wall meets the tabletop if no foot is added. If adding a foot it's fine as is. They can be put on the wheel to compress and add some throwing lines. If you want a foot then place a circle of upholstery foam to cover the entire base, excluding the wall and rim, then flip it upside down and add your foot/feet. You can throw a circular rim then shape it to oval when it's stiffened up a bit too or cut and join to make a square etc. The rim will dry faster than the base so keep it covered while the base catches up.

I would suggest wedging in some fine grog if your clay doesn't already contain it. Waster slab only needs to be as wide as the base or outer footring.

image.png.a75f533a19e2c20521952c424031c396.png

Wow, that's pretty clever -- a bottomless slump mold, which makes it possible (with some extra support -- such as the upholstery foam you suggest) to add a foot while it's still in the mold.  I will certainly be thinking some more about that.  Pretty brilliant....  Thanks.

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24 minutes ago, Piedmont Pottery said:

I always work on the inside of a form, i.e., slump mold instead of hump mold.  As the clay dries it simply pulls away from the walls of the form.

Thanks -- yeah, I have done that, and it has the obvious benefit you mention.  But I wanted to get out of the business of trying to match the foot ring to the platter in terms of degree of dryness -- that can be tricky, and although I have mostly succeeded, when I've failed, it's a total loss...

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On 6/28/2021 at 12:55 PM, Callie Beller Diesel said:

If you’re working on a slump mould, aren’t you just adding the foot while it’s still in an easily worked state?

It's a big platter.  If I take it out of the slump mold to add a foot (or multiple feet) while it is still easily worked, then it isn't going to be able to support itself, and will deform.  If I leave it in the slump mold until it's fully rigid, then it's not so easily worked anymore...  It is still possible to add a foot, but arranging for the foot to have the same degree of dryness as the platter is pretty challenging.  And if they don't match pretty closely, there is a real risk of the foot detaching during firing.

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1 hour ago, algebraist said:

It's a big platter.  If I take it out of the slump mold to add a foot (or multiple feet) while it is still easily worked, then it isn't going to be able to support itself, and will deform.  If I leave it in the slump mold until it's fully rigid, then it's not so easily worked anymore...  It is still possible to add a foot, but arranging for the foot to have the same degree of dryness as the platter is pretty challenging.  And if they don't match pretty closely, there is a real risk of the foot detaching during firing.

So fill it with towels or other supporting material. Put a board over the slump mold,flip it , and attach foot.

The towels will have enough give to allow shrinkage.

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13 hours ago, Callie Beller Diesel said:

There’s also the option of building your slump mould like a puki. Build the foot ring into the mould.

I think that might work fine for something small, but the platter has to sit in the mold until it's rigid.  That entails shrinkage, and if the foot ring is built into the mold, well, the mold is not going to shrink with the clay...  

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