A variety of softwood and hardwoods timbers are used as ties, oak, jarrah and karri being popular hardwoods, although increasingly difficult to obtain, especially from sustainable sources. Some lines use softwoods, including Douglas fir; while they have the advantage of accepting treatment more readily, they are more susceptible to wear but are cheaper, lighter (and therefore easier to handle) and more readily available. Softwood is treated, while creosote is the most common preservative for railway ties, preservatives are also sometimes used such as pentachlorophenol, chromated copper arsenate and a few other preservatives. Sometimes non-toxic preservatives are used, such as copper azole or micronized copper. New boron-based wood preserving technology is being employed by major US railroads in a dual treatment process in order to extend the life of wood ties in wet areas. Some timbers (such as sal, mora, jarrah or azobé) are durable enough that they can be used untreated.