As per the Newport website:
The 60Z40ER.2 Broadband Metallic Mirror is an excellent general purpose, broadband reflectors. This 6 inch (152.4 mm) diameter mirror is made from a 23.5 mm thick Zerodur substrate. Zerodur is a glass-ceramic material where thermal expansion is nominally zero. This extra stability can be critical for applications requiring diffraction limited performance. The ER.2 protected silver coating is composed of a low emissivity silver film fully encapsulated by a multilayer dielectric stack to prevent oxidation. This coating has excellent broadband reflectivity in the IR, while maintaining very good performance in the visible and near IR. Average reflectivity is greater than 96% from 480 - 1100 nm and greater than 98.5% from 1.1 - 20 µm. This mirror has a λ/20 surface flatness and a 10-5 surface quality.