are usually wound with copper. To resist corrosion from sweat, aluminium may be used as a resistant alloy such as hydronalium. Classical guitar strings are typically nylon, with the basses being wound with either silver or bronze. Electric guitar strings are usually wound with nickel-plated steel; pure nickel and stainless steel are also used. Bass guitar strings are most commonly wound with stainless steel or nickel. Copper, gold, silver, and tungsten are used for some instruments. Silver and gold are more expensive and are used for their resistance to corrosion and hypoallergenicity. Some "historically-informed" strings use an open metal winding with a "barber pole" appearance. This practice improves the acoustic performance of heavier gauge gut strings by adding mass and making the string thinner for its tension. Specimens of such open wound strings are known from the early 18th century, in a collection of artifacts from Antonio Stradivari. "Silk and steel" guitar strings are overwound steel strings with silk filaments under the winding. Phosphor bronze Phosphor bronze was introduced by D'Addario in the early 1970s. Phosphor bronze is said to keep its "new" sound longer than other strings. Small amounts of phosphorus and zinc are added to the bronze mixture. This makes the phosphor bronze slightly more corrosion resistant than 80/20 bronze. 80/20 Bronze 80/20 bronze strings are 80 percent copper and 20 percent zinc. The zinc also gives it a brighter tone, additional hardness and slows down the aging process. With additional string coating, they are preserved even more. Although, If some of the coating is applied poorly, the strings can lose their tone in just a matter of hours, and if left in high humidity can turn a hint of green because of the copper and corrode with time. The name "80/20 Bronze" is a misnomer since bronze is by definition an alloy of copper and tin. "80/20 bronze" strings would be more correctly referred to as brass. Nickel-plated steel Some acoustic players use strings, wound with nickel-plated-steel, meant for electric guitar. The properties of the nickel-plated strings make it a good choice for flattop guitars with sound hole-mounted magnetic pickups. String corrosion and prevention All metal strings are susceptible to oxidation and corrosion. Wound strings commonly use metals such as brass or bronze in their winding. These two metals are very vulnerable to corrosion. The sebaceous gland in the player's skin produces oils that can be acidic.