Can a material other than glass be used for making telescope mirrors?
Front-surface glass mirrors weren't developed until the 1850s.
Before that, the most common material was speculum metal, a high-tin bronze.
Notable examples include Isaac Newton's 1668 prototype and William Herschel's late 18th century telescopes.
Since speculum metal tarnished easily, it was common to have a spare mirror to continue observations while the other was being reconditioned.
Reflectivity and thermal characteristics were not great either.