Horse Dung and Telescopes

I'm reading The Age of Wonder: How the Romantic Generation Discovered the Beauty and Terror of Science by Richard Holmes. It's about science and culture in the late eighteenth and early nineteen century - mostly British. I just finished the section about William Herschel, the German/English discoverer of the planet Uranus. He was an innovative telescope maker and he employed horse dung to make the moulds which he used to cast the metal mirrors for his telescopes. The book says that horse dung was used for this purpose until the early twentieth century. Herschel was also a professional musician and that's how he made his living until he became famous for discovering the planet.