Hardeners are provided for curing epoxies at lower temperatures than currently available hardeners while retaining superior mechanical and thermal properties. A first component is selected from imidazole, dicyandiamide, or a mixture of polyamines and tertiary amines. A poiyol mixture is then selected. The first component is combined with the polyol mixture to form a hardener. The hardener is combined with epoxy resin and is cured. A method for selecting a hardener to yield an epoxy-hardener system with good properties after curing at a specified temperature and time includes selecting components for the hardener so the glass transition temperature of the cured epoxy-hardener system is not significantly lower than 45 C. above the cure temperature. The method includes steps for adjusting and controlling the ultimate glass transition temperature to maximize the mechanical properties of the epoxy-hardener system. The method regularly produces hardened epoxy systems having about 90% or more epoxy groups reactect.