Every potter needs a surface to wedge clay on, and there are many different types. The most common are plaster, plaster covered in canvas, and plywood covered in canvas. Other possibilities are concrete, granite, masonite, concrete board and hardibacker board.
The simplest wedging surface is a piece of canvas. It can be put on a table, slab roller top, or chair.
One concern about canvas wedging is that clay dust is created. However many people swear by canvas and do not find dust issues. They periodically soak and sponge it off, and sometimes lightly spritz it with water before wedging.
The main concern about plaster is that plaster chunks can break off and get into your clay, causing cracks or surface pops during firing. (Fine dust is not a problem, just chunks.) However, if a strong plaster is chosen and a frame is constructed around the plaster to protect the edges, chunks will usually not be a problem.
You can use #1 Pottery Plaster, or the stronger plasters Hydrocal or Hydrostone. If you need information on mixing plaster, see Tip 44.
Some people use canvas over plaster. Typically they make a sturdy wood frame (from 2"x2" lumber), attach wet canvas (similar to a stretching canvas for a painting), turn it over, and mix and fill the frame with plaster. Most say they get very little dust using this system.
Some wedging surfaces suck water out of the clay, and others do not. Plaster is known for drying clay out, more than most other surfaces. However, plaster can be wet slightly before use to make it less absorbent.
The fact is that sometimes you have wet clay and you want the moisture removed, while other times your clay is dry and you don’t want any further moisture removed. So how you use your wedging board most of the time, might sway you toward one material or another.
Recently many people have started using cement board or HardiBacker board (fiber cement tile underlayment) from the hardware store. This seems to work well; it is simple to make (at a minimum, just cut a piece and clamp it to a table), it doesn’t stick to the clay, it doesn’t suck too much moisture out of the clay, and it is durable.
Natural stone is also a good material, and you may be able to pick up scrap pieces of granite or other stones such as sandstone from a fabricator. The more polished the stone, the less it will absorb water (which again can be good or bad depending on your application).
Wedging boards should be relatively low. Standing straight, you should just be able to reach it with your arms hanging down, or it could be up to 6 or so inches higher. Having a lower surface makes it easier to use your body and reduces stress on your wrists.