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Hello,  I mostly do sculptural work but occasionally make flat wall pieces out of slabs.  They are usually 10 x 10.  I have had a couple of these pieces crack in two during glaze firing.  I use a low fire clay with  compatible glazes and fire them flat on the shelf.  I have had suggestions to use alumina but was wondering if I stood them up in the kiln or if using kiln stilts would help?  Thoughts?  Thank you!!!

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Hi Earthgirlart and welcome to the forum! Couple questions, how fast does your kiln heat up and cool down and do the cracked slabs have a shelf above them? Is the glaze on the cracked edges sharp or slightly smoothed over? If it's a sharp clean break then the cracking happened on the cool down, smoother glaze edge and it happened on the heating up. You can put thin coils under the tiles so they don't get hung up on the shelves. 

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I would try just one slab with coils under, I would bisque them first, put the slab in the middle of the kiln with a shelf above it and if possible some mass around the outside of it. Can just be some kiln posts lying on their sides, the idea is to try to slow down how fast the slabs cool. The other thing you can do is program in a slow cooling, between 1100F to 1000F  and also between 500F to 350F  have the kiln cool at 100F an hour. Inversion temperatures are within those ranges but the thermocouple is measuring the heat of the air inside the kiln not the kiln shelf where the slab is so there is a range.

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You could also use a shallow bed of silica sand; the coils of clay will work just like Min said, but the slab may form to the coils slightly. The bed of sand will allow the slab to move as it expands/contracts, but it will also be a heat sink like your shelf will, so you'll still want to watch your temp up/down.

You can fire slabs on their edges, but do get good results doing this you'd likely want them in a tile/plate sitter, and not just leaning against the wall of the kiln. If the slab is closer to being more perfectly vertical/plumb it wont have as much tendency to warp; leaned against the wall it will likely curl in the direction of the lean.

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6 minutes ago, Earthgirlart said:

where do you purchase silica sand?  Brand? Type? Thank you again!

Play sand from the hardware store would be just fine. It may have more organic stuff growing in it, especially if its been sitting around for a while getting mildewy or sitting on the earth, so you may have a little more smoke than normal during first fire as that stuff burns off. If its wet (even damp), I might fire off enough for your needs in a thick crucible before using it under you work so excess moisture does not steam your work and potentially blow it up. Just put some in a bowl during a bisque firing and that will be all you need to do.

You can buy it from your local clay supplier. If they dont carry silica sand, which they more than likely do, you could also use fine or medium grog; grog is essentially fired ceramic "sand". Grog will be marginally more expensive than sand from your supplier, but neither will cost you a bunch. 5# would more than likely be enough for you to build up a 1/4-1/2" thick bed on your kiln shelves. Wear a mask when dumping the sand off your kiln shelves (after the firing) back into your container, and vacuum any that spills into the bottom of the kiln. Pure silica wont melt until 3,000*+ but no need to have excess debris in your kiln. If any of the sand sticks to your vessel during the firing (the fluxes in your clay body may "pick" up some of the sand during the glaze fire) you can use a piece of sand paper to knock them loose with barely any effort. 

The suggestion from your other friend about using alumina would work in the same scenario, but alumina is much more expensive, and dusty.

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13 minutes ago, hitchmss said:

would more than likely be enough for you to build up a 1/4-1/2" thick bed on your kiln

Is this also the thickness I should use for play sand?  And thank you again for your patience and help!

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1 hour ago, Earthgirlart said:

s this also the thickness I should use for play sand?

Yes. It could be thicker if you had a very heavy, large object, but if its a relatively thin slab, then it wont compress the sand much. You just need enough so the sand acts as ball bearings.

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You really only need a dusting of sand  to allow the piece to move on the shelf. Any thicker than that and you're just increasing the insulating factor of the shelf, and creating a mess for cleanup. Silica sand has smoother edges than grog, so acts more like little ball-bearings. It allows the piece to move easily on the shelf as it expands and contracts during heating and cooling. Grog has jagged edges, so isn't as effective. At low fire temps play sand should work, but for anything hotter you'd want to test fire the sand or stick with silica sand. If the problem is cooling, then the coil method will be more effective since it makes an air gap between the shelf and the work. With low fire clay it shouldn't sag or deform from the coils. I wouldn't trust it for cone 6, though, unless the piece is thick enough to not deform, and the coils were supporting the piece very evenly.

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i have been firing flat things for many years and have learned that is is not as simple as dumping sand on a shelf inside the kiln.

 i use silica sand and a tile setting trowel with pointed teeth to smooth some sand over the center section of a shelf.  some of my pieces stretch from one side to the other across a shelf, much larger than your 10x10 pieces.   half a cup of sand is enough to let the clay slide as it heats and cools.

the biggest problem you might have is scattering sand all over the shelves below the one you are putting the flat piece on.  i use whole shelves, never half shelves.  i put the sand on the shelf before loading it into the kiln carefully avoiding the thermocouples.   if you put the sand only in the center, leaving about 3 inches around the edges without any sand, you should be able to avoid dropping sand into the work below that shelf.  

in addition, place the slab down flat and do not slide it.   if you do, you might find a rim of sand stuck to the glaze on the side that you slid.  if you pick it up after putting it on sand, some sand will stick and perhaps fall into something else on the shelf.  

there are two pieces of blue jean legs that i use to clean up the edges of the shelves, the fabric is sturdy enough to do the job easily.

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Oh, that vertical stacking is JUST what I needed to see!

I'm planning a piece with some flat slabs that are longer than my kiln is wide, but perhaps i could stack vertically...

Were those single fired? I'm not glazing the slabs, so I'd like to do one and done if possible. Did you have any problems with clay-to-clay sticking together in a single firing? 

And I saw one comment on your photo: " Flat plate or two, flat tile or 7 raise them off the shelf so the heat at top level doesn't crack 'em"  - what does that mean? Any tips? 

Thanks! 

 

 

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On 2/21/2019 at 6:19 PM, kristinanoel said:

Were those single fired? I'm not glazing the slabs, so I'd like to do one and done if possible. Did you have any problems with clay-to-clay sticking together in a single firing? 

Yes, single fired, unglazed.  Not all those photos in my gallery became "fired".  Lots were to show the lovely people here, and get their opinions.  Only the final photo layouts got fired.

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