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 a 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 stop the plate from slipping and the glaze-base being damaged. Read more about: Majolica: 5e – Preparing to paint plates
c) Good for cleaning and smoothing out clay.