Obsidian is capable of producing an edge sharper than high quality steel razor blades. Older technology used Billhooks, Kaiser blades and pruning knives. Generally short, concave blades used for heavy cutting. The highest precision clocks have pivots of 'knife' blades resting on agate plates. Controls vary the pitch of the main rotor blades cyclically throughout rotation. The rotor assembly (blades and hub) weighs . The tailboom was also lengthened, in order to accommodate the longer rotor blades. There may be many stages of rotating blades and fixed guide vanes. Older engines had stationary vanes in front of the moving blades. During the late 19th century steel blades and steel towers replaced wooden construction. BIM involved pressurizing the interior of the rotor blades with nitrogen. Materials commonly used in wind turbine blades are described below. Practices self mutilation by cutting herself with her blades. She called it her "razor blades and Prozac" album. The drawing depicted a thin, solemn man with long, sharp blades for fingers.