For centuries, the only sponges that were available were those that grew on rocks at the bottom of the ocean. Natural sponges are still available for sale online and at some hardware stores, but since the 1940s, synthetic sponges have become the choice for most users, as they are less expensive than the natural versions. DuPont created the first cellulose sponge and sold its cellulose-sponge process to General Mills in 1952. Today, we have a choice of sponges of all shapes, sizes, colors and absorbency.
Cellulose is the most common organic compound on earth. It is found in the cell walls of plants and often obtained from wood pulp and cotton, which in its raw state is composed of 91 percent pure cellulose. We use it to make cardboard and paper, as well as sponges and other materials. Instead of dissolving in water, cellulose absorbs it, making it the ideal material for a sponge.
Cellulose, sodium sulphate crystals, cut hemp fibers and dye are mixed together in a drum. When sufficiently amalgamated, the mixture is put into large, rectangular forms to cook; the crystals melt when heated and form the "pores" of synthetic sponges. Depending on the absorption requirements of the sponge, the crystals added to the mixture may be large (to make larger holes) or small (to make smaller holes). After cooling in the mold, the material is removed, bleached, cleaned with water, dried again and cut into usable pieces. Plastic scouring pads are attached with cured polyurethane after the sponge is cut.
For hundreds of years, natural sponges were harvested primarily in the Mediterranean by divers trained from childhood to swim deep into the water without artificial breathing apparatus. Today, the natural sponges in the U.S. are most commonly harvested in Tarpon Springs, Florida, where a community founded by divers who emigrated from Greece still thrives. Using masks, oxygen tanks and wetsuits is now the norm.
If they stay wet between uses, sponges can become infected with harmful fungi or bacteria. Sometimes synthetic sponges are treated with anti-microbial solutions meant to prevent bacteria growth; however, users should be aware that bacterial growth is still possible. Kill the bacteria by soaking a cleaned sponge in water and microwaving it on high for no more than two minutes. Make sure the sponge is wet before putting it in your microwave---a dry sponge will start a fire if microwaved too long.
Unlike natural sponges, synthetic sponges can be made specifically to meet a variety of requirements. They can be small squares or large circles, thick rectangles that can absorb half of the water held by a bucket, or thin pads with large holes (meant to hold very little water) and an attached scouring pad. 3M even made a sponge featuring a "rigid polyether reinforced with an abrasive and hard-wearing polyethylene compound" (http://www.absoluteastronomy.com/topics/Sponge_(tool)) specifically for removing dried-on bugs from a car windshield.