Why were mirrors in the Hall of Mirrors a major technical achievement in the 17th century?

The Hall of Mirrors at the Palace of Versailles represented a significant technical achievement in the 17th century primarily due to the immense difficulty and prohibitive cost of manufacturing mirror glass in large sizes.1

During this period, the production of high-quality mirror glass was a complex process involving the backing of flat glass with an amalgam of tin and mercury. Creating large, clear sheets of glass was technically challenging, and the process was prone to discoloration from the silvering materials used. Because large mirrors were extremely rare and expensive, the profusion of them used to adorn the state rooms at Versailles created a sense of wonderment and luxury that was widely imitated by other European courts.21

Beyond their decorative impact, these mirrors served a functional purpose by multiplying the effect of artificial light, which was essential for illuminating grand interiors before the advent of modern lighting.1