Several years ago, I attended a workshop with Randy Johnston. I observed him using a sandbag to gently manipulate his clay into a plywood cutout slump mold. I went home and filled several tube socks with grog and began using them in a similar way—to push, stretch, and prod soft slabs of clay into forms. The sandbag provides a broad, non-marring, edgeless surface that is infinitely manipulable for shaping and forming soft slabs.
Recently, I found another use when I was pulling texture from a deeply patterned, bisque-fired slab that I had made by pressing clay onto pine-tree bark. When I used a rolling pin to impress the clay into the bisque-fired texture slab, I found I was losing the deeper textures. Switching to using the sandbag to gently pound the soft clay into the textured surface solved the problem. The nature of the sand in the bag is such that it conforms to the contours of the textured surface beneath the clay, resulting in a well-defined, textured surface.
While the tube sock is a quick and easy way to make a sandbag, the sand or grog tends to find its way through the open weave of the sock. Making pouches from denim cut into squares (the one shown in figure 1 is 6×6 inches) is a better route, as the fabric is tight enough to prevent the sand from coming through it.
Note: The beanbags from the popular Bags game also work great too, but you may find yourself having to answer where the missing bag went.