Dry sponges are made of a rubber mixture, with soap added to keep the sponge moist in the packaging. These sponges are meant to be used dry, which makes them a great option for cleaning surfaces that cannot tolerate moisture and water. Dry sponges are frequently used to clean soot from surfaces after a fire. They can also be used to clean wallpapered walls and fabric window coverings. Store dry sponges in tightly sealed plastic bags to keep them from hardening.