Urethanes are a critical accessory item for use with the Bonny Doon press. Each urethane is specifically shaped and formulated for increased cut-resistance and memory retention. Used properly, a single urethane can easily last 10 years or longer. Its purpose is to provide the matching half of the form that you create in your press; it forces the metal into every crevice of the die, precisely conforming to the design. To ensure good results and to prevent damage to the urethane, choose a piece that is thicker than the design. Designs should no more than 2/3 of the thickness of the urethane you use.