My Slab Building Method
All potters have their own favourite procedures. The following are the slab building steps that work for me. The most important factor is knowledge and respect for the physical properties of the clay and how they affect the behaviour of the clay. Here are the main things to take care of when you want to join slabs of clay to make pots and sculptures.
Please ask me for more details if anything isn't clear.
1. Type of clay.
I like to add grog to my clay to reduce the shrinkage and to stiffen it when it is wet. Grog also gives a surface texture that picks up rubbed oxides beautifully. For smallish pieces I add between 10% and 15% by weight of grog or clean sand to the clay. For large pieces I have sometimes added as much as 25%, but I would not recommend that for beginners.
2. Wedging
I cannot overstate the importance wedging the clay well to get a homogeneous body. Wedging removes air bubbles and ensures that the tensions in the clay are removed. I always prepare a block of clay that will be enough for the whole project. My preference is for the cut and slam wedging method, which is very dramatic and noisy. A side benefit is that it allows one to show off one's physique in the studio; my record is a block of almost 40 kg. I never use slabs from different batches of clay. This YouTube video shows it well.
During the wedging process the required amount of grog or sand is added to the clay. Once the block has been prepared, wrap it in plastic, keep it in a safe place, and issue a warning that the penalty is death if anyone else uses your clay.
3. Preparing the Slabs
Never, ever, roll the clay with a rolling pin or rolling machine to make your slabs. Most newcomers, especially experienced cookie bakers, will be tempted to do this. Rolling causes uneven tensions in the clay that will cause it to warp and crack at the joints. Instead, make or buy one of these slab cutters.
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Lay a sheet of canvas on the table, place the block of clay on it, and pull the cutter through it to cut a slab on the bottom of the block. Make sure to keep the ends firmly on the surface of the canvas to ensure even slab thickness. Then lift off the block and put it on another sheet of canvas to cut a new slab. I like working on canvas, because I can then move the slabs about and turn them over with minimal stresses induced in the clay. Allow the slabs to dry slowly until they are almost leather hard before cutting them to shape with a knife or pizza wheel. I plan carefully and make templates that are larger by the correct percentage to compensate for shrinkage during drying and firing.
For smaller pieces I cut the slabs 10 or 12 mm thick. For larger pieces, around 15 mm.
4. Joining the Slabs
The clay slabs can be joined with clay paste when they are leather hard and stiff enough to handle. I prepare a clay paste "glue", called slip, from small scraps of clay and water in a lidded jar ahead of time. I like it to be smooth and buttery, the consistency of peanut butter. I then score the slabs (a fork is useful for this) where they will be joined and wet the score lines with a wet brush before applying the joining clay past to both sides. After pressing the pieces together firmly, I usually add a roll of clay on the inside of the seam and smooth it with a finger to reinforce the join. I see many people use wet sponges and cloths excessively to try and get a smooth finish on the outside, which can cause warping if overdone. I prefer to scrape the outside with a thin metal kidney to smooth the surface. That exposes the grog texture to accept oxide rubs, which is the look I like, but that may not be your preference if you want it very smooth and shiny.
5. Drying the Piece
Patience is the key element at this stage. It is important to minimize stresses by covering the piece with plastic and allowing it to dry completely, as slowly and as evenly as possible, before firing. This can take many days. Any moisture in the clay may result in an explosion in the kiln.
In my hands, the above steps have ensured a 99% success rate. I hope it will be the same for you. They prevent cracking and warpage, as shown in the straight, sharp edges of this pot I made to test my experimental vanadium pentoxide glaze. The rounded inside corners can be seen clearly.
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Keep the leftover slab scraps. They can be joined in an unplanned, spontaneous way to make bowls and vases. I will show some of those in another post, if anyone is interested.
