McHardy, and Peter G. Fernie received a patent in 1897 for a ball with indentations; Froy played in the Open in 1900 at the Old Course at St. Andrews with the first prototype. Players were able to put additional backspin on the new wound, dimpled balls when using more lofted clubs, thus inducing the ball to stop more quickly on the green. Manufacturers soon began selling various types of golf balls with various dimple patterns to improve the length, trajectory, spin, and overall "feel" characteristics of the new wound golf balls. Wound, balatá-covered golf balls were used into the late 20th century. Modern Resin and Polyurethane Covered Balls In the mid-1960s, a new synthetic resin, an ionomer of ethylene acid named Surlyn was introduced DuPont as were new urethane blends for golf ball covers, and these new materials soon displaced balatá as they proved more durable and more resistant to cutting. Along with various other materials that came into use to replace the rubber-wound internal sphere, golf balls came to be classified as either two-piece, three-piece, or four-piece balls, according to the number of layered components. These basic materials continue to be used in modern balls, with further advances in technology creating balls that can be customized to a player's strengths and weaknesses, and even allowing for the combination of characteristics that were formerly mutually-exclusive. Liquid cores were commonly used in golf balls as early as 1917. The liquid cores in many of the early balls contained a caustic liquid, typically an alkali, causing eye injuries to children who happened to dissect a golf ball out of curiosity. By the 1920s, golf ball manufacturers had stopped using caustic liquids, but into the 1970s and 1980s golf balls were still at times exploding when dissected and were causing injuries due to the presence of crushed crystalline material present in the liquid cores. In 1967, Spalding purchased a patent for a solid golf ball from Jim Bartsch. His original patent defined a ball devoid of the layers in earlier designs, but Bartsch's patent lacked the chemical properties needed for manufacturing. Spalding's chemical engineering team developed a chemical resin that eliminated the need for the layered components entirely. Since then, the majority of non-professional golfers have transitioned to using solid core (or "2-piece") golf balls. The specifications for the golf ball continue to be governed by the ruling bodies of the game; namely,