Damp clay is made up of fine platelets which ride within a thin cushion of water. When a slab is rolled out, the pressure thins out the clay and also moves the platelets so that they are aligned with the direction of the force. In essence, the clay takes on a grain, much like the grain in a piece of wood. Unless modified, the platelets will remain aligned in that grain throughout drying and firing.
This is where problems can develop. Grain in a slab will affect the clay's shrinkage. The slab will shrink more along the width, across the grain, than it will along its length, or with the grain. If pieces are assembled so that the grains aren't aligned in the same direction, the pot can literally pull itself apart during drying and firing.
Getting Rid of the Grain
Avoiding the creation of a grain while making slabs is actually easy and quite straight forward. Simply take the time to rotate the clay after each rolling. It is also helpful to flip a slab over and roll it on both sides, rather than just one.
In order to flip a large slab, you will need two pieces of canvas, larger than the slab will be. Begin by rolling the slab on one piece of canvas. After the initial rolling, cover the slab with the second piece of canvas. Spreading out your hands as much as possible to support the clay, flip the canvas-clay-canvas sandwich over. Remove the top layer of canvas (that used to be the bottom) and continue rolling.
One of the easiest methods of making slabs is to use a rolling pin. Large, heavy ones with ball bearings at each end tend to work the best.
If you want to be certain that a slab is uniform in thickness, you can use two slats of wood on either side of the slab as depth guides. Just be sure to rotate the slab as you roll, rather like rolling out pie dough.
Making Really Thin Slabs
What if you need a very thin slab, such as for clay appliqué or to laminate onto a pot? The slab needs to be very thin, which can mean difficult to roll out successfully using normal methods.
Use plastic wrap instead of canvas. Flatten the clay between your palms, then lay it between two sheets of plastic wrap. Roll with a rolling pin, being careful that no wrinkles develop in the plastic.
Rotate and roll out again, repeating this sequence until the clay is as thin as you desire. Carefully pull the uppermost piece of wrap straight back and away from the clay.
After firmly applying the thin slab to the other clay surface, pull the second piece of wrap straight back and away. The clay slab should stick to the other clay surface.
Make a Super-Strong Slab
You may want to work with enormous slabs, very thin slabs, or you may want to use them in ways that damp clay generally isn't strong enough for. If this is the case, consider adding chopped nylon to your clay body.
To do this, add a couple of handfuls of chopped nylon per hundred pounds dry weight to the clay as you mix it. Sprinkle the nylon in while the dry clay is combining in the clay mixer, before adding the water.
You may also be able to special order a custom mixed batch of clay; check with your favorite supplier. (There's a partial list of suppliers here.)
Slab rollers are wonderful pieces of equipment if you are using large quantities of slabs in your work. They are fairly expensive, though, and do take up a lot of studio space. You may find that a large rolling pin will do the job for you just as or nearly as well.
Again, when using a slab roller care must be taken to avoid creating a grain in the clay. If the slab is too long to fit width-wise in the roller when rotated, roll across the width of the slab with a rolling pin using very positive, even aggressive, strokes.