Renaissance craftsmen, moreover, would have faced severe technical challenges in silvering and sealing a concave mirror, which required the application of hot tar and pitch on the outside of the glass.

• 文艺复兴时代的工匠在涂银与装置凹面镜时，还要面对更多艰难的技术挑战，那需要在玻璃表面施予滚烫的焦油与沥青。