[Kabe]

Marcia Selsor, on 27 November 2012 - 07:59 AM, said:

Kabe, You are correct that glaze tension can cause warping. The coefficient of expansion and contraction is a factor in this.

Marcia

Hi Marcia. A friend of mine said someone in one of his college classes made some tile and when they were fire the tile "cupped" I guess that would be the word. They were like upside down bowls. I thought about this abit last night and maybe that would be more like a situation where if the glaze was on a pot it might of shivered and if thetile warped the oppisite way and the edges all pulled up it would be like crazing. It would be interesting to manipulate a glaze both directions and apply the test glazes to thin tiles and observe the effects. happy firing

[Marcia Selsor]

Kane,
That could be a whole new way of forming!
Marcia

[nrsmdwf]

Kabe, on 26 November 2012 - 10:50 PM, said:

I do tile work and seldom have warpage but I mix my own clay and use a lot of grog. Let me ask this as a question because I might be way off base here. Could it be that the glaze is to tight and instead of crazing you are getting shivering. Seeing that it is a flat tile and not a circular surface maybe the edges are pulled up by the difference in thermal expansion. I know that shivering can tear a pot apart. I have a glaze that will break a porcelain pot everytime I try it. I do not know what glazes you are using or if all your tiles warp or just certain ones. Maybe someone who is better informed could tell if this is possible. Happy firing

Thanks for that idea. I'd actually thought of that, but the warping does not seem to be consistent between tiles (some arch up, some get wavy, a few stay flat, etc.) and it seems to happen with several different glazes (I'm using commercial Cone 5-6 glazes: Laguna, Spectrum). Have tested several suggestions regarding handling and "dropping on the floor" =) and that lot of tiles is drying. I will definitely be looking at a groggier clay for the next batch.

How thick and what size do you make your tiles, and how do you dry them?

Thanks for all this great info!

[nrsmdwf]

An update on results of several ideas presented here, and others - :

1. Changed to a heavily grogged clay, Std 420 and Std 547 (red). The 547 seems pretty good, but have not found a glaze that looks decent on 420. Amazing shades of ugly so far. Don't like the huge sand particles, tricky to smooth edges and backs or cut hanger-hole at just the right time in the drying process when still damp enough to smooth, but firm enough to move. Really would prefer a less coarse grog.

2. Made sure when rolling out clay to have minimal movement under the fabric I was using (bedsheet cotton). I believe the surface fibers of clay were adhering to the fabric while the deeper layers were getting rolled unevenly. I'm now beginning with a slab not much thicker (1/4") than the desired thickness (3/8"), roll out, pull fabric away and flip slab/roll a couple of times so there is no 'drag' on the surface from the fabric. Using dowels for thickness guides and kitchen rolling pin. Will consider going to a local studio to use the slab roller again if I have a big order.

3. Moving, flipping slab by sliding a piece of luan plywood under the fabric and turning without distorting the slab.

4. Drying on an open wire rack with no plastic or covering. Even leaving wet tiles on the damp sheeting on the wire rack influences drying/warping.

5. The drop method suggested was not useful, at least not with wet clay. The edges slumped making an uneven thickness between center and edges of tile. Did not try after clay had firmed up a little.

6. Changed to a pizza cutter instead of fettling knife. Less distortion and drag on wet clay when cutting out tiles.

7. Tried the 'score and break apart later' method, which worked very nicely, but with the heavy grog in the clay it was too difficult to smooth the edges once dry.

8. Firing on tile racks (large size to accommodate larger thickness), and found a sale on them!

As a result of these changes (and I believe primarily #1, 2 and 4), I have only 10-15% warping, vs >60%. Better, but I still do not like the clay bodies (too gritty), and have not found a good glaze for 420. Hopefully someone can benefit from my results, and suggestions are always welcome! Thanks so much to this most generous community of artists!

[bciskepottery]

For glazing, try applying a white slip over the leatherhard tiles and then bisque fire. Glaze on top of the white slip.

[weeble]

I've never used a top layer of fabric rolling out with a chunk of round closet rod and sticks. I have a sheet of plywood with canvas permanently mounted for a base, then I just have to lift and flip the slab a few times between rolling. That'll save you some work peeling, and it results in a nice stable slab if you make sure you flip it enough times. I also dry between two sheets of drywall stacked (weight!) so they dry evenly. Once I get to the final thickness, I try to keep any manipulation to flip/flopping between boards so I'm not bending the tile, let them set up until stiff then do my final trimming.

[Diane Puckett]

Re the grog problem - I find using a rubber rib to smooth slabs works better than anything else. The yellow Mud Tools rib works very well. Anything else just seems to raise the grog. If I need to wipe something on slab work, I try to use a chamois rather than a sponge. Anything that wipes away the small clay particles will make the grog more apparent. Perhaps putting fabric over the slabs as you roll them is lifting those small particles, like the opposite of using terra sig.

[nrsmdwf]

Once again, excellent ideas!


I haven't tried the slip idea yet - concerned it will fill in my impressions/stamps. But recently, I tried drawing into the clay, and liked the ease of it, so may try the slip with that. I need to look for a slip from clay with similar shrinkage as the main clay body, right?

I had been using the two pieces of sheetrock (wall board) in the past, but I'm actually getting better results from the open racks right now. Probably because of the change in clay body. Also allows for my occasional tiles with an added on surface decoration.

Also have used a rib (wood) to smooth the slab surface (need to try the rubber one) ... but never thought about the fabric taking some clay with it .... basically it seems to be just wet, but might be contributing. I don't think I could pick up and flip slabs without deforming them ... they are usually about 20 x 6 inches (to cut out 4 - 4 1/2 inch tiles). That's where I'm using the fabric to be able to slip a piece of luan under the slab to flip it.

This forum and site has been SOOO amazing!

[Diane Puckett]

Using a sheet of plastic rather than fabric might help. The plastic would not absorb water or clay. If I need to lift a slab to drape it over a form, I leave the plastic on the slab until I have it in place. I have a box of the economy plastic drop cloths from Lowe's. they are even thinner than dry cleaning bags, can be stretched to make them perfectly smooth, and are very flexible.

I have had a lot more problems with warping of slabs made with a slab roller than slabs rolled by hand.