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Waster Slab Question


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I've got some forms with open bottoms that I need to fire on a waster slab to keep them from warping. I can get them through the bisque without a problem, it's just the glaze firing that's an issue. I've never done this before, so do I need to bisque fire the waster slabs? There's so little shrinkage in the bisque that I don't think it'll be a problem to not bisque fire the wasters, but I wanted to hear from some folks who have actually done it.

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I always have the waster slabs be whatever the forms are so if the forms are bisque the slabs are bisque as well-that way the shrinkage is the same-it they are all green  then the slabs are green as well.

So if your forms are bisque I'd bisque the slabs.

I have done this a lot about ten years ago with large flat forms

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When you have a piece that's open on the bottom, or has a large flat bottom like a large tile, you put a slab of clay under the piece to prevent the piece from warping or cracking due to friction with the kiln shelf. The waster slab will shrink with the piece so there's no friction with it. The waster slab may crack, which is fine. It's just there to save the piece.

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In our community college studio, I get to load and unload the kilns. The sculpture students frequently send out their sculptures to be glaze-fired with a greenware (sometimes not even dry yet) waster slab to protect against the experimental glazes they find, because that is what the teacher told them to do. So I do. Every time when I open the kiln, the waster slab is in pieces. It may be true that totally dry greenware and bisque are already shrunk to the same size and will finish their further shrinkage together in the glaze firing, but the greenware waster slab simply has no structural strength to retain its integrity with the sculpture sitting on top of it. Fortunately we have not had any serious glaze runs, only some drips which mostly ended up on the chunk of waster that happened to be below it, but if so, it would have run right through the broken waster. Truly a waster...

 

If you are confident of the glaze issues and are only concerned about shelf friction, consider waxing the bottoms with a wax that has some alumina hydrate added. The alumina will be embedded in the wax as you apply it and it will dry there. In the kiln, the wax will burn out as usual, but the spherical grains of alumina will still be under the bottom edge of the ware and will act as little ball bearings for the ware to roll on as it expands and contracts.

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If you are confident of the glaze issues and are only concerned about shelf friction, consider waxing the bottoms with a wax that has some alumina hydrate added. The alumina will be embedded in the wax as you apply it and it will dry there. In the kiln, the wax will burn out as usual, but the spherical grains of alumina will still be under the bottom edge of the ware and will act as little ball bearings for the ware to roll on as it expands and contracts.

 

I respectfully disagree. I don't find that alumina wax has enough of alumina in it to do the job. Nor does silca sand work well in my situation. I'm firing open bottom porcelain cylinders, and it only take a little bit of friction to warp them. A waster slab is the only thing I have found that works, but I'm only tested with bisque fired slabs in the past, and I'd like to eliminate that bisque if possible.

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I always put either alumina and liquid wax or kiln wash (mine has a lot of alumina in it) on wasters that have been bisqued. Never occurred to me to try firing them from greenware, porcelain is so fussy so my assumption would be you have a greater chance of success if both the pot and the waster start out of the gate with the same shrinkage going into it. For cylinders I would be tempted to even trim an ever so tiny grove into the waster and have the cylinder sit snugly in that, haven't done this yet, just a thought.

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I fired a fish to cone 11 that was 39 inches long x 23 wide. I spaced that slab over my 3 12x24 inch shelves and had the slab thick eough to hang over the shelves . I fired it on the top stack in car kiln-the fish came out great the slap well it cracked and was toast but did its job well.That why they call them waster slabs. 

The alumina in wax for me does not do enough.That only keeps lids from fuzing together.

The best friction reducer I have are these small (about 1/8 fired ceramic balls) think of them as ball bearings that handle high temps. They came to me in the 80's and are from Coors Tec.-Yes its a spin off of the beer company. I tried last year to get more but they have huge minimum order. This company is in the high tech ceramic field and only sells thru a few distributors.I have about 1/8 cup of them .They are fantastic-wish I had more.

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Thanks for the info!  I learned something new!  I use cookies under pots to take care of drips, etc.  I have used some small strips made out of high fire clay under plates and large flat pieces for that whole movement issue, and now I know about the waster slabs!  great idea.  I also like the idea of the tiny ceramic balls.   That would be good also!. 

 

Roberta

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The largest slabs I have fired were 26 x 26 x 1 inch thick. They were actually made for a kitchen countertop. I used straight alumina hydrate under them and lost two out of sixteen. If you are dealing with weight, plus area: this would be my recommendation. I did not use any wadding wasters, or slats under them; just a healthy layer of ah.

Nerd

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Do both let us know how it went. I was going to suggest what Dick already posted. I would think bisque wasters would yield a better result. You may even want to do the alumina hydrate and wax with the waster.

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I've only done this a time or two but bisque the waste slab right along with the piece in the same position as it will be in the glaze kiln. thats was I was advised and it worked. Same clay same shrinkage. 

 

I've done the same thing with success. On the last firing was one of four of the waste piece was stuck on and with little effort it popped off. No alumina wax or wash on the waste piece . The pots where about 4" tall stoneware, bisqued at ^04 then fired to ^6.

 

When I first read this thread I was wondering what a waster was and thought it was pronounced was'-ter, now I know it is pronounced way'-ster

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I agree with Rakuku and Rosa. I always believed the waster slab should go through all the drying and firing processes with the piece so that they are shrinking the same. I have 5% shrinking on most of my clays in the bisque with an additional 6-7% in the higher firings.

So I would not put a green slab with a bisqued piece. INHO.

Marcia

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UPDATE:

I glaze fired a piece on a slab that hadn't been bisqued, and it did just fine. There is so little shrinkage, if any, from bone dry to bisque that not bisque firing doesn't seem to be an issue. As I said before, I did put some alumina wax on the waster so the piece wouldn't fuse, and it came right off. I rolled my waster to about 1/16", very thin so as not to waste clay and so it would dry quickly, yet it didn't crack. The piece sitting on it was not large, though. Success!

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I used to fire student work on that had large flat bottoms with an even layer of grog on the shelf. I would put this down with a pepper shaker, Know it sounds weird, but it worked.

 

 

best,

Pres

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I think with flat bottom pieces you can use alumina, grog, sand, whatever, with success. But with open cylinders I wouldn't trust that to work as well as a waster slab. Plus I think the open cylinders are more likely to settle into the material when loading them into the kiln, possibly getting the material onto the glaze at the bottom of the piece.

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Neil did you take it up on bisque schedule till above danger points?

Pres, I too use grog but can see potential for fouling glaze on open ended ware.

 

No. Regular glaze speed. The slab was so thin that there wasn't really any danger of it blowing. Plus I was sure it was totally dry.

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@Mark C. : What size are these little ceramic balls you speak of? This thread is addressing a few questions I have, such as how to fire some lantern cylinders and use my specialty glazes without ruining the kiln I rent, because the people who do the work are often worried my stuff will drip (hasn't yet that I'm aware of). But about the balls, I've been making little pie weights when I'm bored and too distracted to make actual art; they range in size from tiny peas to marbles. I'm wondering if it's worth making some of these roller-things myself. It would be fun to impress the shop by showing up with a bag of those to put under something.

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I fired a fish to cone 11 that was 39 inches long x 23 wide. I spaced that slab over my 3 12x24 inch shelves and had the slab thick eough to hang over the shelves . I fired it on the top stack in car kiln-the fish came out great the slap well it cracked and was toast but did its job well.That why they call them waster slabs. 

The alumina in wax for me does not do enough.That only keeps lids from fuzing together.

The best friction reducer I have are these small (about 1/8 fired ceramic balls) think of them as ball bearings that handle high temps. They came to me in the 80's and are from Coors Tec.-Yes its a spin off of the beer company. I tried last year to get more but they have huge minimum order. This company is in the high tech ceramic field and only sells thru a few distributors.I have about 1/8 cup of them .They are fantastic-wish I had more.

 

Mark, like these? they have them in 1/8" size, supposed to be good to 1550C. 1kg minimum order. 

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