Here is a quick guide to slab building. Its one of the basic making methods used in pottery.
Slab building is so called because you roll out sheets (or slabs) of clay and the cut out shapes and join them to construct the finished article.
Slab building is probably best for small and medium sized pieces. Large pieces are possible but require more equipment and ingenuity.
As with everything in pottery there are different opinions and methods so this is my personal take on it.
There are broadly two main approaches to slabbing, the first I describe as architectural and is more akin to wood-working. It uses leather-hard slabs. The second is more free-style, uses softer slabs and is more like coiling or perhaps dress-making or sugar craft.
What I'm calling free-style slabbing is most often used for sculptural and figurative work. The slabs are cut or more often torn to shape and then adjusted and fitted immediately. Because the clay is soft it may need to be thicker or be supported by an armature to prevent it from collapsing.
In its extreme what I describe as architectural slab work uses leather hard slabs that can be cut and handled very much like wood. This style is best for things with flat sides because leather hard clay is hard to bend without cracking.
FusionIt is possible to work using softer slabs but using the more controlled architectural approach. This fusion of free-style and architectural methods seems the most interesting to me and can allow you to make things like teapots and other curved forms without having to wait for the clay to harden.
How to do it
Whichever style you adopt, here are some basic techniques and principles.
Step 1 - Rolling out
You need to make some slabs and if you have space then try to roll pently of clay so you don't need to keep stopping.
The quickest way is to start by cutting a very thick slab from a block of clay and then rolling it with a rolling pin until it is the right size and thickness.
You can use guide sticks to help you guage the thickness accurately. Remember to keep releasing the clay and turning it between passes of the rolling pin.
Either roll-out on a slightly absorbant surface so you can peal the slab away easily or roll on sheet plastic and peal the plastic from the clay. You can also add texture by rolling fabric or textured wallpaper into the slab or by using a patterned roller.
Air bubbles can be pricked with a pin and any unwanted texture removed with a rubber kidney.
Step 2 - Cutting
Either tear or use a needle, scalpel or pottery knife to cut the clay slab into pieces. You can make templates to cut around using paper, card or any other sheet material like wood. Templates are useful for making several sides the same (eg a square vase) or to repeat the same complete design (eg a set of cups).
Remember to allow extra material for overlaps at the joints. You may also find it helpful to cut at an angle to make mitre-joints rather than butt-joints.
Step 3 - Shaping
You may want to bend the pieces you've cut and then let them dry to stiffen up. You could use a curved former for example when making a piece with curved sides.
Step 4 - Drying to leather-hard
If you want stiffer slabs then let them dry to leather-hard. Make sure the drying is even so that the pieces dry from both sides otherwise they may curl up at the edges. Sandwiching flat slabs between two sheets of plaster board works well but wooden boards and sheets of paper work too.
Step 5 - Assembly
Assembling the slabs requires some care to ensure strong joints. The drier the clay you are using the harder it is to join the slabs well. Very soft clay can be wiped with water and pressed firmly together. Harder clay needs scoring and slipping and then pressing firmly together so that the slip oozes out along the joint. The idea is to create an area of softer tacky clay at the joint that is not so wet that it creates differential drying cracks.
To score the mating surfaces scratch fairly deeply with a serrated tool - like a fork. The ideal joining slip dries and shrinks together with the parent clay thus avoiding any drying cracks. You can make up a joining slip from a recipe or from the parent clay and apply it with a squeezy bottle or brush. Alternatively, the low tech method is to add water to the sractched surface and work the scratched surface into a slip with a toothbrush or even your finger.
Step 6 - Drying the finished work
Some percentage of losses is common in slab built work due to cracking, either at the drying or firing stages. Allowing the piece to dry evenly helps. The drying need not be especially slow so long as it is even. Force drying in the kiln for example can be very successful. A good approach is to lightly wrap the work in plastic and ensure that any condensation drips clear.
One fun variation on slab-building is drop-moulding: