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Firing A "large" Slab


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#1 Benzine

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Posted 20 February 2014 - 06:38 PM

I recently had a student create a relatively large relief sculpture, as an independent project.  In the standard class, I also have the students make a relief sculpture, but the slabs are only 5.5"X8.5"X.5".  So I say "large" because in relation, the independent student's work is pretty big.  It's a circular format, with a diameter of 20"X.75-1".  When planning the project, he knew he wanted it to be big, so I just had him build it on a spare kiln shelf, to avoid potential breaks, while trying to move and load it. 

It's been drying for a couple weeks now, and has no spots, that feel cool.  So I'm not worried about residual moisture, but I've heard people suggest placing slabs on a layer of silica sand(?) or clay coils, to avoid stress during the firing.  The clay is low fire, so will this be necessary in either firing?

 

If it were my project, I'd just fire it, and deal with any issues that might arise, but as it's a student's project, and they put a lot of time into it, I don't want anything to happen to it.

 

Thanks for the help folks.


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#2 Marcia Selsor

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Posted 20 February 2014 - 09:02 PM

I fire my raku slabs up right. They are larger than 20 inches.
1" thick is pretty thick. Fire slow. If not upright fire on coils to allow moisture off the bottom to escape.

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#3 Benzine

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Posted 20 February 2014 - 09:22 PM

The majority of it, is .75" or less.  There may be a couple spots around an inch, and anything over that, is hollowed out and vented.


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#4 Marcia Selsor

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Posted 21 February 2014 - 08:42 AM

Then fire flat on coils.
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#5 ChenowethArts

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Posted 21 February 2014 - 09:17 AM

I have limited experience with this, but use the silca sand layer to compensate for any shelf warping/unevenness... also, even with bone dry pieces, I stick to a slow ramp and hold schedule/program*:

  • 80 degrees per hour to 212 degrees, hold for three+ hours
  • 80 degrees per hour to 750 degrees, hold for two hours
  • 80 degrees per hour to 1100 degrees, no hold
  • 120 degrees per hour to desired bisque temperature, no hold

* From Nan Smith - Minnesota Clay Company


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#6 Benzine

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Posted 21 February 2014 - 09:57 AM

Paul, that's similar to the basic bisque program I use normally. You can never be too careful with student work!

Marcia, any suggestions for getting the slab on the coils? I'd prefer sliding it, rather than lifting, to avoid snapping a huge chunk off. But sliding it onto the shelf with the coils is going to push the coils around.

Also, would the coils be necessary for the glaze firing, or is their purpose just to add air circulation, while the remaining water is "smoked" off?
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#7 bciskepottery

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Posted 21 February 2014 - 10:08 AM

One suggestion: place soft foam on the top of the slab, then place ware board on foam. With the aid of another person, flip the sandwich over. Place bone dry coils on the back of the slab, then gently place kiln shelf on coils. Again, with the aid of your assistant, flip the sandwich over and place in kiln for firing.

#8 Mark C.

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Posted 21 February 2014 - 12:24 PM

I'd go with the coils as noted above

Next time consider having it built on a thin waster slab of clay. That way thet slab takes the abuse as it shrinks during firing and if it grabs on shelve and cracks then its ok as the work is on top of that thinner slab.

I have small ceramic (high fire ball bearings) that I place under large  work. I was given a small bag long ago of them but never found a source for more. These work great for big stuff as they are lessthan 1/8th in diameter.

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#9 Benzine

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Posted 21 February 2014 - 12:39 PM

bciske, I thought about that, and it's actually how the student smoothed all the separate slabs together into one. However, now there are some very thin elements sticking out, that will likely be snapped if we try the sandwich and flip method.

What if we carefully slid some strands of a thin rope under the slab at ,multiple points, then lift up, and place on the coils? If we used enough rope, the distributed pressure, should be lessened enough, it won't snap the slab right?

Also, for a twenty inch slab, how many coils should it rest upon, and how far should they be spaced apart?
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#10 Benzine

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Posted 21 February 2014 - 12:48 PM

Mark, yeah I thought I put in enough forethought, with having the student build it on a kiln shelf. I completely forgot to consider air flow and expansion/ contraction of the clay. Usually the projects aren't big enough for me to worry about such things.
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#11 bciskepottery

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Posted 21 February 2014 - 12:56 PM

Sounds like its time to brush up on your magic skills.



#12 bciskepottery

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Posted 21 February 2014 - 01:07 PM

I'm thinking the ropes will shift pressure to the rim as you try to lift, with undesired consequences. Instead of rope, thin -- but firm -- metal rods?

Or could you carefully (and slowly) insert a piece of sheet metal between the slab and the kiln shelf? That would give you a better way to lift the slab off the shelf. Then lay down the coils and again carefully and slowly move the slab back onto the coils. Make the coils larger than the slab so you can use pieces of clay to anchor them and keep them from rolling too much. You are going to need a fair amount of coils . . . maybe space them no more than 1 1/2 to 2 inches apart. Marcia may have a better idea on spacing since she does large tiles regularly.

Any chance of a picture to see what the slab looks like?

#13 Benzine

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Posted 21 February 2014 - 01:48 PM

Here you go.

Attached Files


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#14 bciskepottery

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Posted 21 February 2014 - 02:18 PM

That rocks. Really nice job.

#15 Benzine

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Posted 21 February 2014 - 02:42 PM

He's a great student. Give him a task, and he'll work, focused on it, stopping only to ask questions.
He's currently underglazing, and post-bisque, I suggest a black iron oxide stain, then whip off the excess, to give it an aged look.
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#16 Pres

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Posted 21 February 2014 - 04:55 PM

How about placing the shelf it is on on a higher, but level to the shelf with the coils on, use a string around the slab to help pull onto the coils rolling across on to the slab.  Similar to sliding a pot off of a wheel head onto a bat using water under the pot and a cutting wire pulled part way through. Am I making sense?


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#17 Babs

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Posted 21 February 2014 - 05:10 PM

What if you can access about 4 wide, deep paintstripper tools which poss could be slide under the piece and lifted in unison. They would go deep enough into the structure to give it support over a large area.  I've had some success just floating such a piece on a bed of fine grog on the kiln shelf. It allows it to move as it shrinks in firing. So if you had another shelf with the coils or grog set up adjacent to the piece it would allow for easy transferring with out flipping.

Is it presently sitting on a shelf or batt?  Could sacrifice the batt if it's the latter.

Good luck, don't you wish this was your piece and then....

If as you say it has been made on athe shelf I'd be running a wire under it anyway in case with the pressure of building it has been stuck to the shelf in places. If the back has never been exposed I'd be thinking htere would still be water in the  piece, Benzine.



#18 Benzine

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Posted 21 February 2014 - 06:30 PM

Pres, you are making sense, and that's kind of what I was thinking of going with.

Babs, the slab was flipped wnd smoothed multiple times, before the carving took place. I tried sliding it a bit, and it does move. I would also imagine the back is dry, as the shelf is unwashed, so somewhat absorbant.

I'll let everyone know how it goes, next week when I give it a go.
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#19 Babs

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Posted 21 February 2014 - 07:08 PM

Good luck Benzine, kiss your kiln god!



#20 Benzine

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Posted 24 February 2014 - 07:56 PM

Update:

We (my student and myself), were able to slide the slab, from the one shelf, to the other. The first shelf was set on some slats, to raise it up to the same level as the coil bed.

The next issue, was getting the loaded shelf in the kiln. It's the first time, I've ever had to have help loading a project. The only issue, was angling the shelf under/ past the thermocouple.

It will be bisque firing tomorrow.
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