The obvious purpose of soles on shoes is to protect the bottom of the foot of a person wearing the shoe, but different materials also serve different purposes. The sole and heel of a running shoe are naturally made of different materials than that of a ballet shoe or a hiking boot. Although the materials used in making the heels and soles of shoes have changed with new technology, some of the original materials are still used.
Wood is a traditional material used in shoe making that still is used regularly in modern shoe making. According to Every Clog Has Its Day, shoe makers use wood as varied as cherry, alder, poplar, maple, and walnut. According to Jim Barnier, a shoe maker, the weight of the wood is more important than how hard the wood is, and lighter wood is better. Shoe makers put a layer of rubber on the bottom for comfort and to protect the wood.
People have used cork in shoes since the 14th century. Cork insoles mold to the wearer's foot, making it both comfortable and protective. The flexibility of cork allows the shoe to bend and makes walking in cork shoes a more natural motion than walking on a rigid sole.
Natural rubber has been used in soles for years and is now supplemented with a variety of synthetic rubber and rubber compounds. Among the forms of rubber used are carbon rubber—a mix of synthetic rubber and carbon, solid rubber—a mix of natural and synthetic rubber, DRC—solid rubber with additional additives to increase durability, Duralon—a soft, synthetic rubber, and gum rubber—a combination of natural and synthetic rubber that provides good traction, but less durability.
Leather soles of shoes can be soft, thin Italian leather or thick, rigid leather. Leather-soled shoes occasionally have thin rubber additions to help protect the leather and increase traction. Some shoes are made of a combination of leather and rubber, where the majority of the sole is leather, but the ball of the foot has a rubber insert.