Making flat tiles is often a major challenge for tile artists. In my experience there are three main issues that contribute to the problem of tile warping. First is the quality of the clay itself. The second is the method of creation (rolling out, extruding, stamping, cutting or molding the tiles) and the third issue is the drying of the finished tile.
Quality of the clay: Ideally your clay body should be formulated properly for tile making. This usually means stiff clay with a lot of grog in it and a low overall shrinkage as opposed to plastic clay for modeling or throwing. You might be able to create great tile with porcelain or sculpting clay but it will be much easier if you stick to a product that is designed for tile. Most large ceramic suppliers offer at least one or two clays suitable for tile making. I like my tile clay to be old, hard and on the dry side because this seems to make warping less likely.
The method of manufacture: Most of my tiles are press-molded in plaster molds. I wedge the clay well and roll it out by hand (with a rolling pin) in several directions, flipping the clay slab over at least once or twice during this process. It is good to make sure there are no air bubbles in the blank; if you see any bubbles, pop them with a needle tool. Clay is then pressed into the mold (by hand and with a rolling pin in my studio) but options include different types of tile presses or a rubber mallet. After the clay sits in the mold a couple of hours I pop the tile right out onto a piece of sheetrock where it can sit until the next day. Avoid handling, lifting or moving the soft tile. This can stretch it and deform it and will increase the likelihood of warping.
I don't use an extruder or slab roller but these both have timesaving advantages. I've read that a slab roller may induce stresses into a clay slab if you only roll it one direction. So it may help to flip the slab over and roll it back through a second time the other direction. Slabs may be cut, sculpted, stamped, etc. for decorative effect. Always minimize handling the slab because plastic memory will cause warping in the drying and/or firing stages. Keep the wet clay flat on a board. If you need to slide it around, roll the clay out onto sheets of newsprint or a piece of cloth. Then pull the cloth or paper by the edges to move the slab. Don't bend the clay at this point or warping will almost certainly occur.
Drying the finished tile: A important step in trouble-shooting your tile-warping issues is to consider how the wet tile is drying after you have made it. You may have too much drying occurring from the top or perhaps too much from the bottom. Your tiles may be drying out too quickly in an uncontrolled manner. I have found that gradual, even drying from all sides is best.
As an example, consider a wet riverbank or mud flat. As it dries out on a warm sunny day, the topside will dry faster. The heat from the sun shrinks the top layer of mud or clay, causing it to contract. The edges of mud flat (or the tile) will pull upward, creating warping.
This mud flat shows uneven drying due to heat from the sun on the upper surface and little or no drying on the bottom.
If you see a tile with this problem, you know that it has dried unevenly, with excessive shrinkage on top due to heat or air circulation and limited drying from below.
If the reverse is true, and your tile's center is bowing upwards while the sides/edges are going down, then the tile is drying too fast from beneath. When you roll out a thin slab of clay onto sheetrock and let it sit there too long, this is what usually happens. The plaster in the sheetrock absorbs moisture faster than the tile releases water to the air, hence the bottom is too dry while the top is too moist.
I dried this tile (left) on the top of a warm kiln. There is greater shrinkage (contraction) on the bottom side and an upward curve to the top side because it dried faster from below. If your tiles exhibit either concave or convex warping traits in the green stage, it is too late to fix them in my experience. They will stay like this from bisque fire through high fire; they will never relax and flatten out again. It is best just to start over.
I mostly solve the uneven drying problem by keeping my green tiles on a piece of sheetrock for no more than 24 hours or until stiff enough to lift without bowing. Then they go right onto a rigid metal rack to air dry – where they stay until ready for bisque firing. Once on a rack, the tiles will dry evenly top and bottom.
Below::Home-made drying rack using closet organizer shelves (plastic coated wire rack) with wood framing.
Working with large tiles presents its own set of challenges. They may be loosely draped in plastic sheeting to slow down the drying. I also keep my studio on the cool side (around 60 degrees F) so things don't dry too rapidly. To create drying racks, you can visit a home improvement store and buy wire shelves for closet organizers. Also useful are steel oven racks or rigid plastic grills used to diffuse light from overhead fixtures. My local second-hand store sells the closet organizer racks for only a couple bucks each. This is a great deal compared to retail price! I recently invested in a couple of metal bakery racks on wheels that have adjustable wire shelves. These hold a lot of tiles and can be rolled around your studio.
If you are making flat tile (field tile, for example), you could sandwich the trimmed tiles between rigid boards made of sheetrock. However, this technique does not work well for tiles having raised, decorative surfaces. The high points will get squished! And it is a lot of effort to change out the boards daily to keep the tiles drying steadily.
In conclusion, it helps to experiment with different clays, drying techniques and ways of handling the clay during the creation of your tiles. Over time you should discover what works best in your own studio. I hope these tips will help other tile-makers overcome those warped tiles blues!
Cautionary note: If you choose to use pieces of sheetrock (drywall) in your studio, be sure to tape up the cut edges to prevent the plaster crumbs from going everywhere and contaminating your clay. I use duct tape to seal the edges.
Handmade Tile Association
34 Thirteenth Avenue NE
Minneapolis, MN 55413