There are two types of sponges, or foams; open cell foam or closed cell foam. Foam structures consist of a flexible material that is filled with voids, or bubbles. Closed cell foam, the bubbles are completely encompassed within the liquid and when it sets, the gases are entirely fixed within each individual bubble creating a structure that is impervious. Closed cell items are used in sealing applications where there is a wide tolerance to take up and the stiffness of the foam or sponge is measured in Specific Gravity. Open cell foam, the bubbles burst during the foaming process to create an interconnection cellular network. This gives rise to softer and breathable foams and commonly used in the filtration industry.