Sponges used for cleaning are made of either artificial or natural fibers. Artificially produced sponges are made of cellulose fiber or melamine fiber, while natural sponges are made from underwater animals also called sponges or from gourds known as loofahs. Most sponges in use as of 2014 are artificial.
The common kitchen sponge is typically made of cellulose fiber, which is soaked in chemicals and then blended with sodium sulphate crystals and hemp fiber. When heated, the sodium sulphate crystals dissolve, leaving the holes and gaps commonly seen in sponges. Sponges are manufactured with large holes for industrial cleaning and with very fine holes for cleaning artwork and applying makeup. After the basic sponge is made, it is softened, dyed and cut into shapes.
The "magic" erasers sold as school supplies are actually melamine foam sponges. While the melamine feels soft to the touch, at the microscopic level, it is almost as hard as glass. The tiny holes left in the melamine sponge material allow it to feel soft. When used as an eraser, the hard melamine scrapes up a stain, and the porous nature of the sponge pulls the molecules of the stain into the sponge itself.
While natural sponges made from the sea sponge are rarely sold, loofahs made from dried gourds are common. Loofah gourds grow in hot climates. After they are harvested, their skin is pulled off in one piece, revealing the spongelike material inside. The loofah "skeleton" is washed, bleached and dried in the sun before being used as a bath sponge.