Sponging is to dampen a sponge or foam rubber with a liquid and use it in different ways, to form textures, to clean, put on colors and smooth off surfaces.
Ways to use sponges in pottery.
a) It can be cut into shapes that protrude or extrude forming patterns or textures. A sponge cut with protruding parts can be pressed into laid out slip or a color to remove it or dipped into a color and marked onto a piece of pottery.
b) When plates have to be painted they are stood up, rested against a strong object and balanced on top of a slice of synthetic sponge, between the plate and table. This is to prevent the plate from slipping and the glaze-base from being damaged. Read more about: Majolica: 5e – Preparing to paint plates
c) Good for cleaning and smoothing out clay.