Slab Warpage

[Benzine]

Good Day All,

 

I recently made a slab (approx. 16" long by 3.5" wide and .25" thick.) to use for the address number on my house.  It is made from a mid-fire brown oxidation clay body.  The house numbers and thin decorative border (also .25" thick) were attached to the main slab with magic water.

Both slabs were rolled out with a rolling pin and slats, rotating 90 degrees while rolling thinner, then flipping over and doing the same there.  Both sides were also compressed with a rubber rib/ kidney.

They were then allowed to dry between to flat boards before cutting down to size.

After attaching the numbers and border to the base, they were placed between the boards again and allowed to slowly dry for several days.

The slab was perfectly flat before going into the kiln, and was fired on waster coils to allow for expansion and contraction. It was bisque fired to Cone 04.

The warp is a bow in the middle, like a perfect arch (Abkut 5/8").

Any chance the warp goes away at the Cone 7 glaze firing?

It will will either receive an iron oxide wash only, with the possibility of putting clear over top.

So if anyone has suggestions for the glaze firing, please let me know.  If not, some suggestions for remaking it would be appreciated.  I have the templates I made to create the borders and numbers and that is where most the work was at (Math is hard).

Thanks All!

[Mark C.]

Glaze I think will pull it up more as the thats more tension on the slab.

I feel waster slab of same body is better than coils for slabs.

The letters applied also are part of the pull is also my guess .Glaze on one side is always going to distort it.

The high fire with no glaze and oxides on a waster slab is my suggestion.

Next time you could sandwich it with broken kiln shelves/ or  bricks  to keep it flat.. the weight on top would help

[Benzine]

Thanks Mark!

I figure I have nothing to lose with refiring either way.

I did think about firing it between shelves, but was worried that could cause it to crack somehow.

Oh well, we live and learn.

[Min]

6 hours ago, Benzine said:

It will will either receive an iron oxide wash only, with the possibility of putting clear over top.

Many clears will bleach out iron from the calcium that is present in most glazes. test?

6 hours ago, Benzine said:

The slab was perfectly flat before going into the kiln, and was fired on waster coils to allow for expansion and contraction. It was bisque fired to Cone 04.

 

Negligible shrinkage between bone dry and bisque temps, I agree with Mark about firing it on a thin waster slab for the glaze fire. (made from the same clay, don't need to bisque fire it)

[neilestrick]

There's a good chance it will flatten out. I vote for waster slab as well. Coils will probably cause other warping issues. Put some alumina wax or kiln wash on the slab. The slab does not need to be bisque fired first.

[Roberta12]

@Benzine If the slab doesn't flatten out after the glaze firing and you have to remake it, could you make the letters/numbers separately and attach them after all pieces are fired?  Just a thought.

Roberta

[glazenerd]

Besides the obvious issue with firing large slabs flat (dunting), the other issue is uneven heat. The side exposed to ambient kiln temperature can vary to the temperature under the slab in direct contact with the shelf; further adding to the warp issue. I have fire porcelain slabs up to 30” square by bisq firing them on edge. Use tile setters as shown, or prop them up using other wares. 

Tom

Edited June 14, 2023 by glazenerd
added pic… ooopss

[Benzine]

So a long overdue update.

 

I fired the slab, with only the oxide wash, which looked better than the samples with clear anyway.

I also fired it warp facing down to see if the warp would correct itself.

The answer, mostly yes.  The arched warp which was towards the middle, flattened back out.

It did however pick up some smaller warps on the ends.  

I need it to be significantly flatter, so I will remake it.  I still have all the templates, so no worries.

This time, along with the slow drying between boards, I will make and use some waster slabs to fire with the actual piece.

I'll give an update once I get that done.

 

 

[oldlady]

reading this late.

no update yet so i will add a suggestion.   the total thickness of your backing and the numbers seems to be 1/2 inch.   that is a very thick piece.   why not use a thinner slab and very thin numbers so you will not be asking your kiln to do something difficult.   i have an occasional piece that grows a hump in its middle but it happens only when i do not fire on a bed of sand or fine grog.